feat(postbox): add governed content protection profiles

README.md

@@ -103,12 +103,22 @@ an independently readable copy in the next frozen function Postbox. The
 `govoplan.postbox.dispatch_routes` periodic Core worker drains due routes when
 Celery beat and a worker consuming the `postbox` queue are enabled.
 
-Postboxes support `plaintext_v1` and an optional `server_envelope_v1` profile.
-The latter stores message bodies as ciphertext through the Encryption
-capability and fails closed on reads if that capability or key is unavailable.
+Postboxes expose three configurable content-protection profiles. The recommended
+`server_envelope_v1` profile stores message bodies as ciphertext through an
+institution-controlled Encryption vault and fails closed if its capability or
+key is unavailable. `external_e2ee_v1` accepts only ciphertext, a signed
+manifest, wrapped recipient keys, and a verified content digest produced by an
+approved external client; GovOPlaN cannot decrypt that content. `plaintext_v1`
+keeps content unencrypted for deployments that explicitly accept that boundary.
 Subjects, participants, routing, attachment references, and lifecycle metadata
-remain observable. Existing externally produced ciphertext references remain
-supported, but neither path is described as end-to-end encryption.
+remain observable in every profile.
+
+Administrators may govern future-only changes or migrate retained history.
+Transitions record user-consent and/or institutional key-holder evidence,
+quorum, reason, per-message digest continuity, and completion state. Managed
+envelope changes use the Encryption migration ledger. Any transition to or from
+E2EE waits for client-supplied transforms for historical messages; the module
+does not claim or silently simulate native browser/device key custody.
 
 Run focused checks with:
 

docs/POSTBOX_CONCEPT.md

@@ -12,14 +12,16 @@ needed a role, process, portal, campaign, or service responsibility. It may look
 like an inbox for a message task or like a vault for content shared with the
 current holders of that responsibility; neither form is owned by one account.
 
-The strategic target is an encrypted administrative postbox. The current
-implementation supports ordinary persisted messages and an optional
-server-readable Encryption envelope for message bodies. The model also retains
-external ciphertext, wrapped-key, signed-manifest, external-recipient-token,
-and key-epoch metadata needed for later independently reviewed E2EE profiles.
-The server-envelope profile is not E2EE, and subjects, routing, participants,
-and attachment references remain visible. The
-cross-module target architecture is recorded in
+The strategic target is a policy-selectable administrative postbox. The current
+implementation offers unencrypted content, an institution-managed Encryption
+envelope, and a strict external E2EE boundary. E2EE messages contain only an
+external ciphertext reference, signed-manifest reference, wrapped recipient
+keys, and a verified plaintext digest; an approved producer or client owns the
+actual cryptographic operation and private-key custody. GovOPlaN cannot decrypt
+that profile. The institution-managed envelope remains server-readable by
+authorized institutional key holders. Subjects, routing, participants, and
+attachment references remain visible in every profile. The cross-module target
+architecture is recorded in
 `govoplan-core/docs/POSTBOX_E2EE_ARCHITECTURE.md`.
 
 ## Function-Organization-Bound Access
@@ -356,47 +358,80 @@ The WebUI should start as an administration and inbox surface:
 
 Campaign, files, portal, and mail behavior should arrive as optional integrations after the core postbox model is stable.
 
-### Current content-protection profile
+### Configurable content-protection profiles
 
-An exact Postbox or template revision may select `server_envelope_v1` and an
-Encryption vault. New locally authored and delivered message bodies are then
-stored in `body_ciphertext` with an owner-bound envelope reference; clear body
-text is not persisted. Reads ask the optional `encryption.content_cipher`
-capability to open the exact tenant, message, and envelope tuple. Missing
-Encryption, a lost deployment key, a destroyed vault key, ciphertext tampering,
-or a mismatched resource causes a fail-closed read.
+An exact Postbox or template revision selects one profile. The administration
+surface recommends the managed profile and requires its vault explicitly; the
+API retains the legacy plaintext default when an older integration omits these
+new fields so an upgrade cannot make an unavailable Encryption module block
+existing automation.
 
-Plaintext Postboxes continue to work without Encryption. A protected Postbox
-cannot silently fall back to plaintext. Database recovery of protected messages
-requires Postbox and Encryption tables from the same consistency point plus the
-matching provider/deployment key. Hierarchy-routed copies retain the source
-envelope reference rather than decrypting and re-encrypting during routing.
+- `server_envelope_v1` is the recommended standard. New local message bodies
+  are encrypted through the optional `encryption.content_cipher` capability,
+  stored in `body_ciphertext`, and linked to an owner-bound envelope in the
+  institution's selected vault. Authorized reads ask that capability to open
+  the exact tenant, message, and envelope tuple. Missing Encryption, unavailable
+  or destroyed keys, tampering, and resource mismatch fail closed.
+- `external_e2ee_v1` is a server-blind storage contract. Clear bodies are
+  rejected. A producer must provide a ciphertext reference, signed manifest,
+  wrapped recipient keys for the current key epoch, and `sha256` content digest.
+  The server retains and authorizes those artifacts but has no private key with
+  which to decrypt them.
+- `plaintext_v1` stores the body without content encryption. It remains
+  available for deployments that explicitly choose transport and
+  infrastructure controls only.
 
-## E2EE Readiness Checklist
+No profile hides operational metadata. Subjects, senders, participants,
+routing, timestamps, classifications, attachment references, receipts,
+retention state, and access evidence remain server-visible. Native browser or
+device enrollment, private-key custody, offline recovery, and independently
+reviewed cryptographic clients are not bundled by Postbox; an institution that
+selects E2EE must provide and govern that client/provider boundary.
 
-Before the data model is considered stable, verify that it can represent:
+### Protection and hand-over policy
 
-- message or attachment ciphertext references
-- signed manifest references
-- recipient, role, or function key wrapping records
-- key epoch and device-key references
-- key-fetch/access audit events
-- external recipient token state
-- expiry and withdrawal state separate from deletion
-- retention state that can operate without decrypting content
+Each Postbox snapshots policy for the choices that cannot safely be inferred:
 
-## E2EE decisions still to settle before implementation
+- a new incumbent sees all retained history, content since assignment, or a
+  bounded look-back period;
+- ordinary and compromise rotations select key rewrapping or full content
+  re-encryption;
+- recovery, hand-over, emergency access, export, and destruction name the
+  required user-consent, institutional key-holder, or dual-control authority
+  and quorum;
+- external retrieval requires strong identity, email plus a one-time code, or
+  may be disabled; and
+- vacancy escalation is always metadata-only and never gives an unrelated
+  personal account content access.
 
-The product direction above is selected, but the first trusted profile still
-needs bounded decisions on:
+The defaults are deliberately conservative: history since assignment,
+ordinary rewrapping, re-encryption after compromise, two-person institutional
+recovery, dual-control hand-over/emergency/export/destruction, strong external
+identity, and metadata-only vacancy escalation. These are product defaults, not
+hard-coded policy decisions; administrators can change them per template or
+exact Postbox.
 
-- whether a new incumbent receives all retained history, history from a
-  policy-defined date, or only content delivered during the assignment;
-- organizational recovery/escrow and the authority required when every holder
-  loses all registered device keys;
-- whether ordinary rotation only rewraps per-content keys or also re-encrypts
-  ciphertext, and which events require the stronger path;
-- assurance and quorum requirements for delegation, hand-over, emergency
-  access, export, and destructive retention; and
-- how attention/escalation works during a vacancy without granting plaintext
-  access to an unrelated personal account.
+### Governed profile transitions
+
+A profile change increments the Postbox key epoch and applies immediately to
+new messages. The administrator chooses whether retained history stays under
+its existing profile or is migrated. Every transition records an idempotency
+key, source and target profiles/vaults, user-consent and/or institutional
+authorization evidence, quorum, reason, immutable configuration snapshot,
+message digests, and per-message outcome.
+
+Plaintext-to-managed and managed-to-plaintext migrations can complete through
+the configured Encryption capability. Managed decrypt, export, and
+re-encryption operations are also written to the Encryption migration ledger;
+old envelopes are not merely orphaned. A transition to or from E2EE pauses each
+historical message until an approved external client supplies the ciphertext or
+plaintext transform and evidence. Postbox checks the immutable SHA-256 digest
+before committing the new representation. Leaving E2EE requires user-consent
+evidence; changing institution-managed history requires institutional
+key-holder evidence; dual control can require both. Previously viewed, copied,
+printed, or exported cleartext cannot be recalled and must be acknowledged.
+
+Database recovery of managed messages requires Postbox and Encryption tables
+from the same consistency point plus the provider/deployment key. Recovery of
+E2EE content additionally depends on the institution's external private-key
+custody and client procedures.

src/govoplan_postbox/backend/content_protection.py

@@ -25,6 +25,8 @@ def protect_message_body(
     vault_id: str,
     plaintext: str,
     actor_id: str,
+    operation_ref: str = "v1",
+    policy_decision_ref: str = "postbox:configured-server-envelope:v1",
 ) -> ProtectedContent:
     capability = encryption_content_cipher(get_registry())
     if capability is None:
@@ -43,8 +45,8 @@ def protect_message_body(
                 vault_id=vault_id,
                 ciphertext_ref=f"postbox-db://messages/{message_id}/body",
                 plaintext=plaintext.encode("utf-8"),
-                policy_decision_ref="postbox:configured-server-envelope:v1",
-                idempotency_key=f"postbox-message:{message_id}:body:v1",
+                policy_decision_ref=policy_decision_ref,
+                idempotency_key=f"postbox-message:{message_id}:body:{operation_ref}",
                 actor_id=actor_id,
                 metadata={"content_type": "text/plain;charset=utf-8"},
             ),

src/govoplan_postbox/backend/db/__init__.py

@@ -10,6 +10,8 @@ from govoplan_postbox.backend.db.models import (
     PostboxMessage,
     PostboxMessageReceipt,
     PostboxParticipant,
+    PostboxProtectionTransition,
+    PostboxProtectionTransitionItem,
     PostboxRoute,
     PostboxTemplate,
     PostboxTemplateRevision,
@@ -27,6 +29,8 @@ __all__ = [
     "PostboxMessage",
     "PostboxMessageReceipt",
     "PostboxParticipant",
+    "PostboxProtectionTransition",
+    "PostboxProtectionTransitionItem",
     "PostboxRoute",
     "PostboxTemplate",
     "PostboxTemplateRevision",

src/govoplan_postbox/backend/db/models.py

@@ -161,9 +161,7 @@ class PostboxTemplateRevision(Base, TimestampMixin):
         default="plaintext_v1",
         nullable=False,
     )
-    encryption_vault_id: Mapped[str | None] = mapped_column(
-        String(255), nullable=True
-    )
+    encryption_vault_id: Mapped[str | None] = mapped_column(String(255), nullable=True)
     history_policy: Mapped[dict[str, Any]] = mapped_column(
         JSON,
         default=dict,
@@ -323,6 +321,11 @@ class Postbox(Base, TimestampMixin):
         back_populates="postbox",
         cascade="all, delete-orphan",
     )
+    protection_transitions: Mapped[list["PostboxProtectionTransition"]] = relationship(
+        back_populates="postbox",
+        cascade="all, delete-orphan",
+        order_by="PostboxProtectionTransition.created_at",
+    )
 
     @property
     def strong_etag(self) -> str:
@@ -439,9 +442,7 @@ class PostboxMessage(Base, TimestampMixin):
     )
     subject: Mapped[str] = mapped_column(String(1000), nullable=False)
     body_text: Mapped[str | None] = mapped_column(Text, nullable=True)
-    body_ciphertext: Mapped[bytes | None] = mapped_column(
-        LargeBinary, nullable=True
-    )
+    body_ciphertext: Mapped[bytes | None] = mapped_column(LargeBinary, nullable=True)
     status: Mapped[str] = mapped_column(
         String(30),
         default="delivered",
@@ -558,6 +559,121 @@ class PostboxMessage(Base, TimestampMixin):
     )
 
 
+class PostboxProtectionTransition(Base, TimestampMixin):
+    __tablename__ = "postbox_protection_transitions"
+    __table_args__ = (
+        Index(
+            "ix_postbox_protection_transition_state",
+            "tenant_id",
+            "postbox_id",
+            "state",
+        ),
+        UniqueConstraint(
+            "tenant_id",
+            "postbox_id",
+            "idempotency_key",
+            name="uq_postbox_protection_transition_idem",
+        ),
+    )
+
+    id: Mapped[str] = mapped_column(String(36), primary_key=True, default=new_uuid)
+    tenant_id: Mapped[str] = mapped_column(String(36), nullable=False, index=True)
+    postbox_id: Mapped[str] = mapped_column(
+        ForeignKey("postboxes.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    idempotency_key: Mapped[str] = mapped_column(String(255), nullable=False)
+    source_profile: Mapped[str] = mapped_column(String(80), nullable=False)
+    target_profile: Mapped[str] = mapped_column(String(80), nullable=False)
+    source_vault_id: Mapped[str | None] = mapped_column(String(255), nullable=True)
+    target_vault_id: Mapped[str | None] = mapped_column(String(255), nullable=True)
+    history_mode: Mapped[str] = mapped_column(String(30), nullable=False)
+    authority_mode: Mapped[str] = mapped_column(String(40), nullable=False)
+    required_quorum: Mapped[int] = mapped_column(Integer, default=1, nullable=False)
+    evidence_refs: Mapped[list[str]] = mapped_column(JSON, default=list, nullable=False)
+    reason: Mapped[str] = mapped_column(Text, nullable=False)
+    state: Mapped[str] = mapped_column(
+        String(30), default="pending", nullable=False, index=True
+    )
+    message_count: Mapped[int] = mapped_column(Integer, default=0, nullable=False)
+    completed_count: Mapped[int] = mapped_column(Integer, default=0, nullable=False)
+    failed_count: Mapped[int] = mapped_column(Integer, default=0, nullable=False)
+    requested_by: Mapped[str | None] = mapped_column(String(255), nullable=True)
+    activated_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+    completed_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+    resource_revision: Mapped[int] = mapped_column(Integer, default=1, nullable=False)
+    configuration_snapshot: Mapped[dict[str, Any]] = mapped_column(
+        JSON, default=dict, nullable=False
+    )
+
+    postbox: Mapped[Postbox] = relationship(back_populates="protection_transitions")
+    items: Mapped[list["PostboxProtectionTransitionItem"]] = relationship(
+        back_populates="transition",
+        cascade="all, delete-orphan",
+        order_by="PostboxProtectionTransitionItem.created_at",
+    )
+
+    @property
+    def strong_etag(self) -> str:
+        return strong_resource_etag(
+            "postbox_protection_transition",
+            self.id,
+            self.resource_revision,
+        )
+
+
+class PostboxProtectionTransitionItem(Base, TimestampMixin):
+    __tablename__ = "postbox_protection_transition_items"
+    __table_args__ = (
+        UniqueConstraint(
+            "transition_id",
+            "message_id",
+            name="uq_postbox_protection_transition_message",
+        ),
+        Index(
+            "ix_postbox_protection_transition_item_state",
+            "tenant_id",
+            "transition_id",
+            "state",
+        ),
+    )
+
+    id: Mapped[str] = mapped_column(String(36), primary_key=True, default=new_uuid)
+    tenant_id: Mapped[str] = mapped_column(String(36), nullable=False, index=True)
+    transition_id: Mapped[str] = mapped_column(
+        ForeignKey("postbox_protection_transitions.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    message_id: Mapped[str] = mapped_column(
+        ForeignKey("postbox_messages.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    source_profile: Mapped[str] = mapped_column(String(80), nullable=False)
+    target_profile: Mapped[str] = mapped_column(String(80), nullable=False)
+    state: Mapped[str] = mapped_column(
+        String(30), default="pending", nullable=False, index=True
+    )
+    source_digest: Mapped[str | None] = mapped_column(String(255), nullable=True)
+    target_digest: Mapped[str | None] = mapped_column(String(255), nullable=True)
+    completed_by: Mapped[str | None] = mapped_column(String(255), nullable=True)
+    completed_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+    error_code: Mapped[str | None] = mapped_column(String(100), nullable=True)
+    evidence: Mapped[dict[str, Any]] = mapped_column(JSON, default=dict, nullable=False)
+
+    transition: Mapped[PostboxProtectionTransition] = relationship(
+        back_populates="items"
+    )
+
+
 class PostboxParticipant(Base, TimestampMixin):
     __tablename__ = "postbox_participants"
     __table_args__ = (

src/govoplan_postbox/backend/manifest.py

@@ -71,6 +71,7 @@ MODULE_ID = "postbox"
 MODULE_NAME = "Postbox"
 MODULE_VERSION = "0.1.18"
 
+
 def _permission(scope: str, label: str, description: str) -> PermissionDefinition:
     module_id, resource, action = scope.split(":", 2)
     return PermissionDefinition(
@@ -209,6 +210,8 @@ _OWNED_TABLES = (
     postbox_models.PostboxDelivery,
     postbox_models.PostboxAttachmentReference,
     postbox_models.PostboxParticipant,
+    postbox_models.PostboxProtectionTransitionItem,
+    postbox_models.PostboxProtectionTransition,
     postbox_models.PostboxMessage,
     postbox_models.PostboxBinding,
     postbox_models.Postbox,
@@ -505,7 +508,10 @@ manifest = ModuleManifest(
                     ),
                 }
             },
-            metadata={"kind": "reference", "help_contexts": ["postbox.quick_access.messages"]},
+            metadata={
+                "kind": "reference",
+                "help_contexts": ["postbox.quick_access.messages"],
+            },
             order=33,
         ),
         DocumentationTopic(
@@ -525,6 +531,70 @@ manifest = ModuleManifest(
             related_modules=("search", "idm", "encryption"),
             order=34,
         ),
+        DocumentationTopic(
+            id="postbox.content-protection-policy",
+            title="Choose and change Postbox content protection",
+            summary="Configure plaintext, institution-managed envelope, or externally managed E2EE content with governed hand-over and migration evidence.",
+            body=(
+                "Every exact Postbox and immutable template revision selects a content-protection profile. "
+                "Institution-managed server envelopes are the recommended standard and require an Encryption vault; authorized institutional key holders can decrypt them. "
+                "External E2EE rejects clear message bodies and requires an approved producer or client to supply ciphertext, a signed manifest, wrapped recipient keys, and a verified digest; GovOPlaN cannot decrypt that content. "
+                "Plaintext remains an explicit deployment choice. Subjects, routing, participants, attachment references, lifecycle state, and other operational metadata remain visible in every profile. "
+                "The accompanying policy selects history for new incumbents, ordinary and compromise rotation, recovery, hand-over, emergency access, export, destruction, external-recipient assurance, and metadata-only vacancy escalation. "
+                "A profile transition applies to future messages immediately and may retain or migrate history. It records user-consent and/or institutional key-holder evidence, quorum, reason, digest continuity, and per-message outcome. "
+                "Transitions to or from E2EE wait for approved client transformations; native device key custody and cryptographic clients are not supplied by Postbox. Previously decrypted, copied, printed, or exported content cannot be recalled."
+            ),
+            layer="configured",
+            documentation_types=("admin", "user"),
+            audience=("administrator", "user", "auditor"),
+            related_modules=("encryption", "identity_trust", "policy", "audit"),
+            links=(
+                DocumentationLink(
+                    label="Postbox administration",
+                    href="/admin?section=postbox",
+                    kind="runtime",
+                ),
+                DocumentationLink(
+                    label="Protection profile catalog API",
+                    href="/api/v1/postbox/admin/protection-profiles",
+                    kind="api",
+                ),
+                DocumentationLink(
+                    label="Postbox protection concept",
+                    href="docs/POSTBOX_CONCEPT.md",
+                    kind="source",
+                ),
+            ),
+            translations={
+                "de": {
+                    "title": "Inhaltsschutz für Postfächer wählen und ändern",
+                    "summary": "Klartext, institutionell verwaltete Umschlagverschlüsselung oder extern verwaltete Ende-zu-Ende-Verschlüsselung mit geregelten Übergabe- und Migrationsnachweisen konfigurieren.",
+                    "body": (
+                        "Jedes exakte Postfach und jede unveränderliche Vorlagenrevision wählt ein Inhaltsschutzprofil. "
+                        "Institutionell verwaltete Server-Umschläge sind der empfohlene Standard und benötigen einen Encryption-Tresor; berechtigte institutionelle Schlüsselverantwortliche können sie entschlüsseln. "
+                        "Externe Ende-zu-Ende-Verschlüsselung weist Klartextnachrichten ab und verlangt von einem zugelassenen Erzeuger oder Client Chiffrat, signiertes Manifest, umhüllte Empfängerschlüssel und einen geprüften Digest; GovOPlaN kann diesen Inhalt nicht entschlüsseln. "
+                        "Klartext bleibt eine ausdrückliche Wahl. Betreff, Routing, Beteiligte, Anlagenverweise, Lebenszyklus und weitere Betriebsmetadaten bleiben bei allen Profilen sichtbar. "
+                        "Die begleitende Richtlinie regelt den Verlauf für neue Stelleninhaber, normale Rotation und Kompromittierung, Wiederherstellung, Übergabe, Notfallzugriff, Export, Vernichtung, externe Empfängerprüfung und rein metadatenbasierte Vakanzeskalation. "
+                        "Ein Profilwechsel gilt sofort für neue Nachrichten und kann den Bestand beibehalten oder migrieren. Er protokolliert Einwilligungen und/oder institutionelle Freigaben, Quorum, Grund, Digest-Kontinuität und Ergebnis je Nachricht. "
+                        "Wechsel zu oder von E2EE warten auf freigegebene Client-Transformationen; Geräte-Schlüsselverwahrung und Kryptografie-Clients liefert Postbox nicht mit. Bereits entschlüsselte, kopierte, gedruckte oder exportierte Inhalte können nicht zurückgerufen werden."
+                    ),
+                }
+            },
+            metadata={
+                "kind": "guide",
+                "help_contexts": [
+                    "postbox.admin.templates",
+                    "postbox.field.protection-profile",
+                    "postbox.action.protection-transition",
+                ],
+                "privacy_notes": [
+                    "E2EE protects content, not operational metadata.",
+                    "Managed envelopes are decryptable by authorized institutional key holders.",
+                    "Profile transitions preserve authority evidence and content-digest continuity.",
+                ],
+            },
+            order=35,
+        ),
         DocumentationTopic(
             id="postbox.function-bound-containers",
             title="Function-bound Postboxes",
@@ -538,12 +608,11 @@ manifest = ModuleManifest(
                 "reassignment. Current access combines a generic Postbox "
                 "permission with effective IDM assignment context. Templates "
                 "can lazily materialize unit-specific addresses, while exact "
-                "postboxes cover exceptional responsibilities. Plaintext "
-                "Postboxes remain available without Encryption. A "
-                "server-envelope profile stores message bodies as ciphertext and "
-                "uses the optional Encryption capability for authorized reads. "
-                "External ciphertext profiles retain producer-managed references "
-                "and keys; neither profile is described as end-to-end encryption. "
+                "postboxes cover exceptional responsibilities. Administrators "
+                "choose plaintext, the recommended institution-managed Encryption "
+                "envelope, or a strict externally produced E2EE contract. Managed "
+                "envelopes remain decryptable by authorized institutional key holders; "
+                "E2EE rejects plaintext and GovOPlaN has no private decryption key. "
                 "When Tasks is enabled, currently readable unread messages also appear "
                 "in the common work inbox and disappear when the personal read receipt is recorded."
             ),
@@ -587,7 +656,7 @@ manifest = ModuleManifest(
                     "Subjects, participants, routing facts, and attachment references remain observable metadata.",
                 ],
             },
-            order=35,
+            order=36,
         ),
         DocumentationTopic(
             id="postbox.reference.fields-and-consequences",
@@ -660,7 +729,7 @@ manifest = ModuleManifest(
                     "withdraw_or_expire": "Blocks future content access while retaining permitted audit metadata.",
                 },
             },
-            order=36,
+            order=37,
         ),
     ),
     architecture=declared_module_architecture(
@@ -672,10 +741,22 @@ manifest = ModuleManifest(
         known_limits=(
             "Subjects, routing metadata, participants, and attachment references remain plaintext metadata.",
             "Server-envelope protection is server-decryptable and is not end-to-end encryption.",
-            "External ciphertext profiles require a separately governed producer and client key-custody profile.",
+            "External E2EE requires a separately governed producer/client, private-key custody, device enrollment, and independent cryptographic review.",
+        ),
+        owned_concepts=(
+            "postbox",
+            "postbox address",
+            "postbox message",
+            "delivery receipt",
+            "access event",
+            "postbox protection transition",
+        ),
+        non_owned_concepts=(
+            "identity",
+            "function assignment",
+            "campaign",
+            "cryptographic key custody",
         ),
-        owned_concepts=("postbox", "postbox address", "postbox message", "delivery receipt", "access event"),
-        non_owned_concepts=("identity", "function assignment", "campaign", "cryptographic key custody"),
         recovery_docs=("docs/POSTBOX_CONCEPT.md",),
         security_docs=("docs/POSTBOX_CONCEPT.md",),
         operations_docs=("README.md",),

src/govoplan_postbox/backend/migrations/versions/a7c1e4f8b2d6_v016_protection_transitions.py

@@ -0,0 +1,112 @@
+"""v0.1.18 governed Postbox protection transitions.
+
+Revision ID: a7c1e4f8b2d6
+Revises: f2a5c8e1b4d7
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+
+revision = "a7c1e4f8b2d6"
+down_revision = "f2a5c8e1b4d7"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "postbox_protection_transitions",
+        sa.Column("id", sa.String(36), nullable=False),
+        sa.Column("tenant_id", sa.String(36), nullable=False),
+        sa.Column("postbox_id", sa.String(36), nullable=False),
+        sa.Column("idempotency_key", sa.String(255), nullable=False),
+        sa.Column("source_profile", sa.String(80), nullable=False),
+        sa.Column("target_profile", sa.String(80), nullable=False),
+        sa.Column("source_vault_id", sa.String(255), nullable=True),
+        sa.Column("target_vault_id", sa.String(255), nullable=True),
+        sa.Column("history_mode", sa.String(30), nullable=False),
+        sa.Column("authority_mode", sa.String(40), nullable=False),
+        sa.Column("required_quorum", sa.Integer(), nullable=False),
+        sa.Column("evidence_refs", sa.JSON(), nullable=False),
+        sa.Column("reason", sa.Text(), nullable=False),
+        sa.Column("state", sa.String(30), nullable=False),
+        sa.Column("message_count", sa.Integer(), nullable=False),
+        sa.Column("completed_count", sa.Integer(), nullable=False),
+        sa.Column("failed_count", sa.Integer(), nullable=False),
+        sa.Column("requested_by", sa.String(255), nullable=True),
+        sa.Column("activated_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("completed_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("resource_revision", sa.Integer(), nullable=False),
+        sa.Column("configuration_snapshot", sa.JSON(), nullable=False),
+        sa.Column("created_at", sa.DateTime(timezone=True), nullable=False),
+        sa.Column("updated_at", sa.DateTime(timezone=True), nullable=False),
+        sa.ForeignKeyConstraint(["postbox_id"], ["postboxes.id"], ondelete="CASCADE"),
+        sa.PrimaryKeyConstraint("id"),
+        sa.UniqueConstraint(
+            "tenant_id",
+            "postbox_id",
+            "idempotency_key",
+            name="uq_postbox_protection_transition_idem",
+        ),
+    )
+    op.create_index(
+        "ix_postbox_protection_transition_state",
+        "postbox_protection_transitions",
+        ["tenant_id", "postbox_id", "state"],
+    )
+    for column in ("tenant_id", "postbox_id", "state"):
+        op.create_index(
+            f"ix_postbox_protection_transitions_{column}",
+            "postbox_protection_transitions",
+            [column],
+        )
+
+    op.create_table(
+        "postbox_protection_transition_items",
+        sa.Column("id", sa.String(36), nullable=False),
+        sa.Column("tenant_id", sa.String(36), nullable=False),
+        sa.Column("transition_id", sa.String(36), nullable=False),
+        sa.Column("message_id", sa.String(36), nullable=False),
+        sa.Column("source_profile", sa.String(80), nullable=False),
+        sa.Column("target_profile", sa.String(80), nullable=False),
+        sa.Column("state", sa.String(30), nullable=False),
+        sa.Column("source_digest", sa.String(255), nullable=True),
+        sa.Column("target_digest", sa.String(255), nullable=True),
+        sa.Column("completed_by", sa.String(255), nullable=True),
+        sa.Column("completed_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("error_code", sa.String(100), nullable=True),
+        sa.Column("evidence", sa.JSON(), nullable=False),
+        sa.Column("created_at", sa.DateTime(timezone=True), nullable=False),
+        sa.Column("updated_at", sa.DateTime(timezone=True), nullable=False),
+        sa.ForeignKeyConstraint(
+            ["transition_id"],
+            ["postbox_protection_transitions.id"],
+            ondelete="CASCADE",
+        ),
+        sa.ForeignKeyConstraint(
+            ["message_id"], ["postbox_messages.id"], ondelete="CASCADE"
+        ),
+        sa.PrimaryKeyConstraint("id"),
+        sa.UniqueConstraint(
+            "transition_id",
+            "message_id",
+            name="uq_postbox_protection_transition_message",
+        ),
+    )
+    op.create_index(
+        "ix_postbox_protection_transition_item_state",
+        "postbox_protection_transition_items",
+        ["tenant_id", "transition_id", "state"],
+    )
+    for column in ("tenant_id", "transition_id", "message_id", "state"):
+        op.create_index(
+            f"ix_postbox_protection_transition_items_{column}",
+            "postbox_protection_transition_items",
+            [column],
+        )
+
+
+def downgrade() -> None:
+    op.drop_table("postbox_protection_transition_items")
+    op.drop_table("postbox_protection_transitions")

src/govoplan_postbox/backend/protection_profiles.py

@@ -0,0 +1,106 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Literal
+
+
+POSTBOX_PLAINTEXT_PROFILE = "plaintext_v1"
+POSTBOX_MANAGED_ENVELOPE_PROFILE = "server_envelope_v1"
+POSTBOX_EXTERNAL_E2EE_PROFILE = "external_e2ee_v1"
+POSTBOX_LEGACY_EXTERNAL_ENVELOPE_PROFILE = "external_envelope_v1"
+POSTBOX_STANDARD_PROFILE = POSTBOX_MANAGED_ENVELOPE_PROFILE
+
+PostboxProtectionProfile = Literal[
+    "plaintext_v1",
+    "server_envelope_v1",
+    "external_e2ee_v1",
+]
+
+SUPPORTED_POSTBOX_PROTECTION_PROFILES = frozenset(
+    {
+        POSTBOX_PLAINTEXT_PROFILE,
+        POSTBOX_MANAGED_ENVELOPE_PROFILE,
+        POSTBOX_EXTERNAL_E2EE_PROFILE,
+    }
+)
+
+
+def normalize_postbox_protection_policy(
+    policy: dict[str, object] | None = None,
+) -> dict[str, object]:
+    return {
+        "new_incumbent_history": "since_assignment",
+        "history_days": None,
+        "ordinary_rotation": "rewrap",
+        "compromise_rotation": "reencrypt",
+        "recovery_authority": "institutional_key_holders",
+        "recovery_quorum": 2,
+        "handover_authority": "dual_control",
+        "handover_quorum": 2,
+        "emergency_access": "dual_control",
+        "emergency_quorum": 2,
+        "export_authority": "dual_control",
+        "export_quorum": 2,
+        "destruction_authority": "dual_control",
+        "destruction_quorum": 2,
+        "external_recipient_assurance": "strong_identity",
+        "vacancy_escalation_content_access": "metadata_only",
+        **(policy or {}),
+    }
+
+
+@dataclass(frozen=True, slots=True)
+class PostboxProtectionProfileDefinition:
+    id: PostboxProtectionProfile
+    label: str
+    description: str
+    server_can_decrypt: bool
+    requires_encryption_module: bool
+    requires_external_client: bool
+    standard: bool = False
+
+
+POSTBOX_PROTECTION_PROFILE_DEFINITIONS = (
+    PostboxProtectionProfileDefinition(
+        id=POSTBOX_MANAGED_ENVELOPE_PROFILE,
+        label="Institution-managed envelope",
+        description=(
+            "The Encryption provider protects content and authorized institutional "
+            "key holders can govern recovery. This is the standard profile."
+        ),
+        server_can_decrypt=True,
+        requires_encryption_module=True,
+        requires_external_client=False,
+        standard=True,
+    ),
+    PostboxProtectionProfileDefinition(
+        id=POSTBOX_EXTERNAL_E2EE_PROFILE,
+        label="External end-to-end envelope",
+        description=(
+            "A reviewed client or producer supplies ciphertext, a signed manifest, "
+            "and recipient-wrapped keys. GovOPlaN stores and routes them but cannot "
+            "decrypt the content."
+        ),
+        server_can_decrypt=False,
+        requires_encryption_module=False,
+        requires_external_client=True,
+    ),
+    PostboxProtectionProfileDefinition(
+        id=POSTBOX_PLAINTEXT_PROFILE,
+        label="No application-layer encryption",
+        description=(
+            "Postbox stores readable message content. Transport and storage controls "
+            "may still apply, but this profile is not encrypted by Postbox."
+        ),
+        server_can_decrypt=True,
+        requires_encryption_module=False,
+        requires_external_client=False,
+    ),
+)
+
+
+def is_e2ee_profile(profile: str) -> bool:
+    return profile in {
+        POSTBOX_EXTERNAL_E2EE_PROFILE,
+        POSTBOX_LEGACY_EXTERNAL_ENVELOPE_PROFILE,
+    }

src/govoplan_postbox/backend/router.py

@@ -28,6 +28,7 @@ from govoplan_core.core.files import (
     PostboxFileReferenceRequest,
     postbox_file_reference_provider,
 )
+from govoplan_core.core.encryption import encryption_content_cipher
 from govoplan_core.db.session import get_session
 from govoplan_postbox.backend.manifest import (
     ACKNOWLEDGE_SCOPE,
@@ -60,6 +61,14 @@ from govoplan_postbox.backend.schemas import (
     PostboxMessageStateRequest,
     PostboxOrganizationTargetsResponse,
     PostboxMutationRequest,
+    PostboxProtectionProfileItem,
+    PostboxProtectionProfileListResponse,
+    PostboxProtectionPolicyUpdateRequest,
+    PostboxProtectionTransformRequest,
+    PostboxProtectionTransitionCreateRequest,
+    PostboxProtectionTransitionItemResponse,
+    PostboxProtectionTransitionListResponse,
+    PostboxProtectionTransitionResponse,
     PostboxRouteDryRunRequest,
     PostboxRouteDryRunResponse,
     PostboxTemplateCreateRequest,
@@ -71,6 +80,10 @@ from govoplan_postbox.backend.schemas import (
     PostboxTemplateReviseRequest,
 )
 from govoplan_postbox.backend.service import PostboxError
+from govoplan_postbox.backend.protection_profiles import (
+    POSTBOX_PROTECTION_PROFILE_DEFINITIONS,
+    POSTBOX_STANDARD_PROFILE,
+)
 from govoplan_postbox.backend.principals import (
     PostboxPrincipalError,
     actor_from_principal,
@@ -122,6 +135,8 @@ def _http_error(exc: PostboxError) -> HTTPException:
         "revision_not_found",
         "grouping_not_found",
         "target_not_found",
+        "protection_transition_not_found",
+        "transition_item_not_found",
     }:
         code = status.HTTP_404_NOT_FOUND
     elif exc.code in {"access_denied", "grouping_source_denied"}:
@@ -195,6 +210,11 @@ def _authoring_request(
         idempotency_key=payload.idempotency_key,
         subject=payload.subject,
         body_text=payload.body_text,
+        ciphertext_ref=payload.ciphertext_ref,
+        signed_manifest_ref=payload.signed_manifest_ref,
+        wrapped_keys=tuple(
+            PostboxWrappedKeyRef(**item.model_dump()) for item in payload.wrapped_keys
+        ),
         classification=payload.classification,
         participants=tuple(
             PostboxParticipantRef(**participant.model_dump())
@@ -236,9 +256,7 @@ def _template_item(template) -> PostboxTemplateItem:
                 "scope_kind": revision.scope_kind,
                 "scope_id": revision.scope_id,
                 "scope_structure_id": revision.scope_structure_id,
-                "scope_relation_type_ids": list(
-                    revision.scope_relation_type_ids or []
-                ),
+                "scope_relation_type_ids": list(revision.scope_relation_type_ids or []),
                 "name_pattern": revision.name_pattern,
                 "address_pattern": revision.address_pattern,
                 "classification": revision.classification,
@@ -246,6 +264,7 @@ def _template_item(template) -> PostboxTemplateItem:
                 "portal_visible": revision.portal_visible,
                 "encryption_profile": revision.encryption_profile,
                 "encryption_vault_id": revision.encryption_vault_id,
+                "protection_policy": dict(revision.history_policy or {}),
                 "history_policy": dict(revision.history_policy or {}),
                 "routing_policy": dict(revision.routing_policy or {}),
                 "retention_policy": dict(revision.retention_policy or {}),
@@ -291,6 +310,48 @@ def _grouping_item(
     )
 
 
+def _protection_transition_item(value) -> PostboxProtectionTransitionResponse:
+    return PostboxProtectionTransitionResponse(
+        id=value.id,
+        postbox_id=value.postbox_id,
+        source_profile=value.source_profile,
+        target_profile=value.target_profile,
+        source_vault_id=value.source_vault_id,
+        target_vault_id=value.target_vault_id,
+        history_mode=value.history_mode,
+        authority_mode=value.authority_mode,
+        required_quorum=value.required_quorum,
+        evidence_refs=list(value.evidence_refs or []),
+        reason=value.reason,
+        state=value.state,
+        message_count=value.message_count,
+        completed_count=value.completed_count,
+        failed_count=value.failed_count,
+        requested_by=value.requested_by,
+        activated_at=value.activated_at,
+        completed_at=value.completed_at,
+        resource_revision=value.resource_revision,
+        etag=value.strong_etag,
+        configuration_snapshot=dict(value.configuration_snapshot or {}),
+        items=[
+            PostboxProtectionTransitionItemResponse(
+                id=item.id,
+                message_id=item.message_id,
+                source_profile=item.source_profile,
+                target_profile=item.target_profile,
+                state=item.state,
+                source_digest=item.source_digest,
+                target_digest=item.target_digest,
+                completed_by=item.completed_by,
+                completed_at=item.completed_at,
+                error_code=item.error_code,
+                evidence=dict(item.evidence or {}),
+            )
+            for item in value.items
+        ],
+    )
+
+
 @router.get("/directory", response_model=PostboxDirectoryResponse)
 def api_postbox_directory(
     assignment_context_id: str | None = None,
@@ -523,7 +584,13 @@ def api_resolve_postbox_message_attachments(
         )
 
     provider = postbox_file_reference_provider(get_registry())
-    file_types = {"file", "file_asset", "files:file", "file_version", "files:file_version"}
+    file_types = {
+        "file",
+        "file_asset",
+        "files:file",
+        "file_version",
+        "files:file_version",
+    }
     requests = tuple(
         PostboxFileReferenceRequest(
             reference_type=attachment.reference_type,
@@ -544,9 +611,7 @@ def api_resolve_postbox_message_attachments(
         if provider is not None and requests
         else ()
     )
-    by_reference = {
-        (item.reference_type, item.reference_id): item for item in resolved
-    }
+    by_reference = {(item.reference_type, item.reference_id): item for item in resolved}
     items: list[PostboxAttachmentResolutionItem] = []
     for attachment in message.attachments:
         resolution = by_reference.get(
@@ -667,8 +732,7 @@ def api_deliver_to_postbox(
         ciphertext_ref=payload.ciphertext_ref,
         signed_manifest_ref=payload.signed_manifest_ref,
         wrapped_keys=tuple(
-            PostboxWrappedKeyRef(**item.model_dump())
-            for item in payload.wrapped_keys
+            PostboxWrappedKeyRef(**item.model_dump()) for item in payload.wrapped_keys
         ),
         external_recipient_tokens=tuple(
             PostboxExternalRecipientTokenRef(**item.model_dump())
@@ -853,13 +917,9 @@ def api_postbox_organization_targets(
 ) -> PostboxOrganizationTargetsResponse:
     _require_any(principal, BINDING_ADMIN_SCOPE, TEMPLATE_ADMIN_SCOPE)
     return PostboxOrganizationTargetsResponse(
-        units=list(
-            get_service().organization_targets(tenant_id=principal.tenant_id)
-        ),
+        units=list(get_service().organization_targets(tenant_id=principal.tenant_id)),
         structures=list(
-            get_service().organization_hierarchy_targets(
-                tenant_id=principal.tenant_id
-            )
+            get_service().organization_hierarchy_targets(tenant_id=principal.tenant_id)
         ),
     )
 
@@ -881,6 +941,187 @@ def api_admin_postboxes(
     )
 
 
+@router.get(
+    "/admin/protection-profiles",
+    response_model=PostboxProtectionProfileListResponse,
+)
+def api_postbox_protection_profiles(
+    principal: ApiPrincipal = Depends(get_api_principal),
+) -> PostboxProtectionProfileListResponse:
+    _require_any(principal, BINDING_ADMIN_SCOPE, TEMPLATE_ADMIN_SCOPE)
+    managed_available = encryption_content_cipher(get_registry()) is not None
+    return PostboxProtectionProfileListResponse(
+        standard_profile=POSTBOX_STANDARD_PROFILE,
+        profiles=[
+            PostboxProtectionProfileItem(
+                **asdict(profile),
+                available=(
+                    managed_available if profile.requires_encryption_module else True
+                ),
+            )
+            for profile in POSTBOX_PROTECTION_PROFILE_DEFINITIONS
+        ],
+    )
+
+
+@router.get(
+    "/admin/postboxes/{postbox_id}/protection-transitions",
+    response_model=PostboxProtectionTransitionListResponse,
+)
+def api_list_postbox_protection_transitions(
+    postbox_id: str,
+    session: Session = Depends(get_session),
+    principal: ApiPrincipal = Depends(get_api_principal),
+) -> PostboxProtectionTransitionListResponse:
+    _require(principal, BINDING_ADMIN_SCOPE)
+    try:
+        values = get_service().list_protection_transitions(
+            session,
+            tenant_id=principal.tenant_id,
+            postbox_id=postbox_id,
+        )
+    except PostboxError as exc:
+        raise _http_error(exc) from exc
+    return PostboxProtectionTransitionListResponse(
+        transitions=[_protection_transition_item(value) for value in values]
+    )
+
+
+@router.put(
+    "/admin/postboxes/{postbox_id}/protection-policy",
+    response_model=PostboxDirectoryItem,
+)
+def api_update_postbox_protection_policy(
+    postbox_id: str,
+    payload: PostboxProtectionPolicyUpdateRequest,
+    response: Response,
+    if_match: str | None = Header(default=None, alias="If-Match"),
+    session: Session = Depends(get_session),
+    principal: ApiPrincipal = Depends(get_api_principal),
+) -> PostboxDirectoryItem:
+    _require(principal, BINDING_ADMIN_SCOPE)
+    _require_mutation_precondition(
+        if_match,
+        resource_type="postbox",
+        resource_id=postbox_id,
+        base_revision=payload.base_revision,
+    )
+    try:
+        get_service().update_protection_policy(
+            session,
+            tenant_id=principal.tenant_id,
+            postbox_id=postbox_id,
+            protection_policy=payload.protection_policy.model_dump(),
+            actor_id=principal.account_id,
+            expected_revision=payload.base_revision,
+        )
+    except PostboxError as exc:
+        session.rollback()
+        raise _http_error(exc) from exc
+    except ConcurrencyError as exc:
+        session.rollback()
+        raise _concurrency_http_error(exc) from exc
+    session.commit()
+    item = _directory_item(
+        get_service().resolve_postbox(
+            session,
+            tenant_id=principal.tenant_id,
+            target=PostboxTargetRef(postbox_id=postbox_id),
+        )
+    )
+    _set_etag(response, item.etag)
+    return item
+
+
+@router.post(
+    "/admin/postboxes/{postbox_id}/protection-transitions",
+    response_model=PostboxProtectionTransitionResponse,
+    status_code=status.HTTP_201_CREATED,
+)
+def api_create_postbox_protection_transition(
+    postbox_id: str,
+    payload: PostboxProtectionTransitionCreateRequest,
+    response: Response,
+    if_match: str | None = Header(default=None, alias="If-Match"),
+    session: Session = Depends(get_session),
+    principal: ApiPrincipal = Depends(get_api_principal),
+) -> PostboxProtectionTransitionResponse:
+    _require(principal, BINDING_ADMIN_SCOPE)
+    _require_mutation_precondition(
+        if_match,
+        resource_type="postbox",
+        resource_id=postbox_id,
+        base_revision=payload.base_revision,
+    )
+    try:
+        value = get_service().create_protection_transition(
+            session,
+            tenant_id=principal.tenant_id,
+            postbox_id=postbox_id,
+            expected_revision=payload.base_revision,
+            actor_id=principal.account_id,
+            **payload.model_dump(
+                exclude={"base_revision", "acknowledge_irreversibility"}
+            ),
+        )
+    except PostboxError as exc:
+        session.rollback()
+        raise _http_error(exc) from exc
+    except ConcurrencyError as exc:
+        session.rollback()
+        raise _concurrency_http_error(exc) from exc
+    session.commit()
+    item = _protection_transition_item(value)
+    _set_etag(response, item.etag)
+    return item
+
+
+@router.post(
+    "/admin/postboxes/{postbox_id}/protection-transitions/{transition_id}/transform",
+    response_model=PostboxProtectionTransitionResponse,
+)
+def api_apply_postbox_protection_transform(
+    postbox_id: str,
+    transition_id: str,
+    payload: PostboxProtectionTransformRequest,
+    response: Response,
+    if_match: str | None = Header(default=None, alias="If-Match"),
+    session: Session = Depends(get_session),
+    principal: ApiPrincipal = Depends(get_api_principal),
+) -> PostboxProtectionTransitionResponse:
+    _require(principal, BINDING_ADMIN_SCOPE)
+    _require_mutation_precondition(
+        if_match,
+        resource_type="postbox_protection_transition",
+        resource_id=transition_id,
+        base_revision=payload.base_revision,
+    )
+    try:
+        value = get_service().apply_client_protection_transform(
+            session,
+            tenant_id=principal.tenant_id,
+            postbox_id=postbox_id,
+            transition_id=transition_id,
+            expected_revision=payload.base_revision,
+            actor_id=principal.account_id,
+            wrapped_keys=tuple(
+                PostboxWrappedKeyRef(**item.model_dump())
+                for item in payload.wrapped_keys
+            ),
+            **payload.model_dump(exclude={"base_revision", "wrapped_keys"}),
+        )
+    except PostboxError as exc:
+        session.rollback()
+        raise _http_error(exc) from exc
+    except ConcurrencyError as exc:
+        session.rollback()
+        raise _concurrency_http_error(exc) from exc
+    session.commit()
+    item = _protection_transition_item(value)
+    _set_etag(response, item.etag)
+    return item
+
+
 @router.post(
     "/admin/postboxes",
     response_model=PostboxDirectoryItem,

src/govoplan_postbox/backend/schemas.py

@@ -5,6 +5,12 @@ from typing import Any, Literal
 
 from pydantic import BaseModel, Field, model_validator
 
+from govoplan_postbox.backend.protection_profiles import (
+    POSTBOX_MANAGED_ENVELOPE_PROFILE,
+    POSTBOX_PLAINTEXT_PROFILE,
+    PostboxProtectionProfile,
+)
+
 
 PostboxClassification = Literal[
     "public",
@@ -48,6 +54,10 @@ class PostboxDirectoryItem(BaseModel):
     template_revision_id: str | None = None
     holder_count: int = 0
     vacant: bool = True
+    encryption_profile: str = POSTBOX_PLAINTEXT_PROFILE
+    key_epoch: int = Field(default=1, ge=1)
+    encryption_vault_id: str | None = None
+    protection_policy: dict[str, Any] = Field(default_factory=dict)
     access: PostboxAccessDecisionResponse | None = None
     resource_revision: int = Field(default=1, ge=1)
     etag: str | None = None
@@ -156,11 +166,32 @@ class PostboxMessageAuthoringPayload(BaseModel):
     idempotency_key: str = Field(min_length=1, max_length=255)
     subject: str = Field(min_length=1, max_length=1000)
     body_text: str | None = None
+    ciphertext_ref: str | None = Field(default=None, max_length=1000)
+    signed_manifest_ref: str | None = Field(default=None, max_length=1000)
+    wrapped_keys: list[PostboxWrappedKeyPayload] = Field(default_factory=list)
     classification: PostboxClassification = "internal"
     participants: list[PostboxParticipantPayload] = Field(default_factory=list)
     attachments: list[PostboxAttachmentPayload] = Field(default_factory=list)
     metadata: dict[str, Any] = Field(default_factory=dict)
 
+    @model_validator(mode="after")
+    def validate_content_source(self) -> "PostboxMessageAuthoringPayload":
+        if self.body_text is not None and self.ciphertext_ref:
+            raise ValueError(
+                "Provide plaintext or an external ciphertext envelope, not both."
+            )
+        if self.ciphertext_ref and (
+            not self.signed_manifest_ref or not self.wrapped_keys
+        ):
+            raise ValueError(
+                "External E2EE content requires a signed manifest and wrapped keys."
+            )
+        if not self.ciphertext_ref and (self.signed_manifest_ref or self.wrapped_keys):
+            raise ValueError(
+                "A signed manifest and wrapped keys require an external ciphertext reference."
+            )
+        return self
+
 
 class PostboxMessageCreateRequest(PostboxMessageAuthoringPayload):
     postbox_id: str = Field(min_length=1, max_length=36)
@@ -178,9 +209,7 @@ class PostboxTargetPayload(BaseModel):
     def validate_target(self) -> "PostboxTargetPayload":
         direct = bool(self.postbox_id or self.address_key)
         templated = bool(
-            self.template_id
-            and self.organization_unit_id
-            and self.function_id
+            self.template_id and self.organization_unit_id and self.function_id
         )
         if direct == templated:
             raise ValueError(
@@ -210,6 +239,24 @@ class PostboxDeliveryCreateRequest(BaseModel):
     )
     metadata: dict[str, Any] = Field(default_factory=dict)
 
+    @model_validator(mode="after")
+    def validate_content_source(self) -> "PostboxDeliveryCreateRequest":
+        if self.body_text is not None and self.ciphertext_ref:
+            raise ValueError(
+                "Provide plaintext or an external ciphertext envelope, not both."
+            )
+        if self.ciphertext_ref and (
+            not self.signed_manifest_ref or not self.wrapped_keys
+        ):
+            raise ValueError(
+                "External E2EE content requires a signed manifest and wrapped keys."
+            )
+        if not self.ciphertext_ref and (self.signed_manifest_ref or self.wrapped_keys):
+            raise ValueError(
+                "A signed manifest and wrapped keys require an external ciphertext reference."
+            )
+        return self
+
 
 class PostboxDeliveryResponse(BaseModel):
     delivery_id: str
@@ -248,14 +295,10 @@ class PostboxLinkedCopyPolicyPayload(BaseModel):
     def validate_enabled_policy(self) -> "PostboxLinkedCopyPolicyPayload":
         self.relation_type_ids = list(dict.fromkeys(self.relation_type_ids))
         self.allowed_classifications = list(
-            dict.fromkeys(
-                value.strip() for value in self.allowed_classifications
-            )
+            dict.fromkeys(value.strip() for value in self.allowed_classifications)
         )
         self.allowed_producer_modules = list(
-            dict.fromkeys(
-                value.strip() for value in self.allowed_producer_modules
-            )
+            dict.fromkeys(value.strip() for value in self.allowed_producer_modules)
         )
         if any(not value for value in self.relation_type_ids):
             raise ValueError("Relation type IDs must not be empty.")
@@ -316,16 +359,10 @@ class PostboxRoutingPolicyPayload(BaseModel):
 
     @model_validator(mode="after")
     def validate_semantics(self) -> "PostboxRoutingPolicyPayload":
-        if (
-            self.attention.mode == "vacancy_escalation"
-            and (
-                not self.linked_copy.enabled
-                or self.linked_copy.fanout != "nearest"
-            )
+        if self.attention.mode == "vacancy_escalation" and (
+            not self.linked_copy.enabled or self.linked_copy.fanout != "nearest"
         ):
-            raise ValueError(
-                "Vacancy escalation requires nearest linked-copy routing."
-            )
+            raise ValueError("Vacancy escalation requires nearest linked-copy routing.")
         return self
 
 
@@ -361,6 +398,73 @@ class PostboxRouteDryRunResponse(BaseModel):
     diagnostics: list[str] = Field(default_factory=list)
 
 
+class PostboxProtectionPolicyPayload(BaseModel):
+    new_incumbent_history: Literal[
+        "all_retained",
+        "since_assignment",
+        "bounded_days",
+    ] = "since_assignment"
+    history_days: int | None = Field(default=None, ge=1, le=36500)
+    ordinary_rotation: Literal["rewrap", "reencrypt"] = "rewrap"
+    compromise_rotation: Literal["rewrap", "reencrypt"] = "reencrypt"
+    recovery_authority: Literal[
+        "disabled",
+        "user_consent",
+        "institutional_key_holders",
+        "dual_control",
+    ] = "institutional_key_holders"
+    recovery_quorum: int = Field(default=2, ge=1, le=20)
+    handover_authority: Literal[
+        "user_consent",
+        "institutional_key_holders",
+        "dual_control",
+    ] = "dual_control"
+    handover_quorum: int = Field(default=2, ge=1, le=20)
+    emergency_access: Literal["disabled", "dual_control"] = "dual_control"
+    emergency_quorum: int = Field(default=2, ge=1, le=20)
+    export_authority: Literal[
+        "user_consent",
+        "institutional_key_holders",
+        "dual_control",
+    ] = "dual_control"
+    export_quorum: int = Field(default=2, ge=1, le=20)
+    destruction_authority: Literal[
+        "institutional_key_holders",
+        "dual_control",
+    ] = "dual_control"
+    destruction_quorum: int = Field(default=2, ge=1, le=20)
+    external_recipient_assurance: Literal[
+        "disabled",
+        "email_otp",
+        "strong_identity",
+    ] = "strong_identity"
+    vacancy_escalation_content_access: Literal["metadata_only"] = "metadata_only"
+
+    @model_validator(mode="after")
+    def validate_history_policy(self) -> "PostboxProtectionPolicyPayload":
+        if self.new_incumbent_history == "bounded_days" and self.history_days is None:
+            raise ValueError("Bounded incumbent history requires a day limit.")
+        if self.new_incumbent_history != "bounded_days":
+            self.history_days = None
+        if self.handover_authority == "dual_control" and self.handover_quorum < 2:
+            raise ValueError(
+                "Dual-control hand-over requires a quorum of at least two."
+            )
+        if self.emergency_access == "dual_control" and self.emergency_quorum < 2:
+            raise ValueError(
+                "Emergency dual control requires a quorum of at least two."
+            )
+        if self.recovery_authority == "dual_control" and self.recovery_quorum < 2:
+            raise ValueError("Dual-control recovery requires a quorum of at least two.")
+        if self.export_authority == "dual_control" and self.export_quorum < 2:
+            raise ValueError("Dual-control export requires a quorum of at least two.")
+        if self.destruction_authority == "dual_control" and self.destruction_quorum < 2:
+            raise ValueError(
+                "Dual-control destruction requires a quorum of at least two."
+            )
+        return self
+
+
 class PostboxExactCreateRequest(BaseModel):
     name: str = Field(min_length=1, max_length=500)
     description: str | None = None
@@ -369,25 +473,31 @@ class PostboxExactCreateRequest(BaseModel):
     address_key: str | None = Field(default=None, max_length=120)
     classification: PostboxClassification = "internal"
     portal_visible: bool = False
-    encryption_profile: Literal["plaintext_v1", "server_envelope_v1"] = (
-        "plaintext_v1"
-    )
+    encryption_profile: PostboxProtectionProfile = POSTBOX_PLAINTEXT_PROFILE
     encryption_vault_id: str | None = Field(default=None, max_length=255)
+    protection_policy: PostboxProtectionPolicyPayload = Field(
+        default_factory=PostboxProtectionPolicyPayload
+    )
 
     @model_validator(mode="after")
     def validate_encryption(self) -> "PostboxExactCreateRequest":
-        if self.encryption_profile == "server_envelope_v1":
+        if self.encryption_profile == POSTBOX_MANAGED_ENVELOPE_PROFILE:
             if not str(self.encryption_vault_id or "").strip():
                 raise ValueError(
                     "Server-envelope Postboxes require an encryption vault."
                 )
         elif self.encryption_vault_id:
             raise ValueError(
-                "A plaintext Postbox cannot select an encryption vault."
+                "Only an institution-managed Postbox can select an encryption vault."
             )
         return self
 
 
+class PostboxProtectionPolicyUpdateRequest(BaseModel):
+    base_revision: int = Field(ge=1)
+    protection_policy: PostboxProtectionPolicyPayload
+
+
 class PostboxTemplateRevisionPayload(BaseModel):
     function_type_id: str | None = Field(default=None, max_length=36)
     scope_kind: Literal["tenant", "unit", "subtree", "unit_type"] = "tenant"
@@ -407,10 +517,11 @@ class PostboxTemplateRevisionPayload(BaseModel):
     classification: PostboxClassification = "internal"
     allow_vacant_delivery: bool = True
     portal_visible: bool = False
-    encryption_profile: Literal["plaintext_v1", "server_envelope_v1"] = (
-        "plaintext_v1"
-    )
+    encryption_profile: PostboxProtectionProfile = POSTBOX_PLAINTEXT_PROFILE
     encryption_vault_id: str | None = Field(default=None, max_length=255)
+    protection_policy: PostboxProtectionPolicyPayload = Field(
+        default_factory=PostboxProtectionPolicyPayload
+    )
     routing_policy: PostboxRoutingPolicyPayload = Field(
         default_factory=PostboxRoutingPolicyPayload
     )
@@ -426,18 +537,169 @@ class PostboxTemplateRevisionPayload(BaseModel):
 
     @model_validator(mode="after")
     def validate_encryption(self) -> "PostboxTemplateRevisionPayload":
-        if self.encryption_profile == "server_envelope_v1":
+        if self.encryption_profile == POSTBOX_MANAGED_ENVELOPE_PROFILE:
             if not str(self.encryption_vault_id or "").strip():
                 raise ValueError(
                     "Server-envelope Postbox templates require an encryption vault."
                 )
         elif self.encryption_vault_id:
             raise ValueError(
-                "A plaintext Postbox template cannot select an encryption vault."
+                "Only an institution-managed Postbox template can select an encryption vault."
             )
         return self
 
 
+class PostboxProtectionProfileItem(BaseModel):
+    id: PostboxProtectionProfile
+    label: str
+    description: str
+    server_can_decrypt: bool
+    requires_encryption_module: bool
+    requires_external_client: bool
+    available: bool
+    standard: bool = False
+
+
+class PostboxProtectionProfileListResponse(BaseModel):
+    standard_profile: PostboxProtectionProfile
+    profiles: list[PostboxProtectionProfileItem]
+
+
+class PostboxProtectionTransitionCreateRequest(BaseModel):
+    idempotency_key: str = Field(min_length=1, max_length=255)
+    base_revision: int = Field(ge=1)
+    target_profile: PostboxProtectionProfile
+    target_vault_id: str | None = Field(default=None, max_length=255)
+    history_mode: Literal["future_only", "migrate_history"] = "future_only"
+    authority_mode: Literal[
+        "user_consent",
+        "institutional_key_holders",
+        "dual_control",
+    ]
+    required_quorum: int = Field(default=1, ge=1, le=20)
+    user_consent_refs: list[str] = Field(default_factory=list, max_length=50)
+    institutional_authorization_refs: list[str] = Field(
+        default_factory=list, max_length=50
+    )
+    reason: str = Field(min_length=1, max_length=2000)
+    acknowledge_irreversibility: bool
+
+    @model_validator(mode="after")
+    def validate_transition(self) -> "PostboxProtectionTransitionCreateRequest":
+        self.user_consent_refs = list(
+            dict.fromkeys(
+                item.strip() for item in self.user_consent_refs if item.strip()
+            )
+        )
+        self.institutional_authorization_refs = list(
+            dict.fromkeys(
+                item.strip()
+                for item in self.institutional_authorization_refs
+                if item.strip()
+            )
+        )
+        evidence_count = len(
+            set(self.user_consent_refs + self.institutional_authorization_refs)
+        )
+        if evidence_count < self.required_quorum:
+            raise ValueError("The evidence set does not satisfy the selected quorum.")
+        if self.authority_mode in {"user_consent", "dual_control"} and not (
+            self.user_consent_refs
+        ):
+            raise ValueError(
+                "The selected authority mode requires user consent evidence."
+            )
+        if (
+            self.authority_mode
+            in {
+                "institutional_key_holders",
+                "dual_control",
+            }
+            and not self.institutional_authorization_refs
+        ):
+            raise ValueError(
+                "The selected authority mode requires institutional authorization evidence."
+            )
+        if self.authority_mode == "dual_control" and self.required_quorum < 2:
+            raise ValueError("Dual control requires a quorum of at least two.")
+        if not self.acknowledge_irreversibility:
+            raise ValueError(
+                "Confirm that previously decrypted, copied, or exported content cannot be recalled."
+            )
+        if self.target_profile == POSTBOX_MANAGED_ENVELOPE_PROFILE:
+            if not str(self.target_vault_id or "").strip():
+                raise ValueError("Institution-managed envelopes require a vault.")
+        elif self.target_vault_id:
+            raise ValueError("Only institution-managed envelopes select a vault.")
+        return self
+
+
+class PostboxProtectionTransformRequest(BaseModel):
+    base_revision: int = Field(ge=1)
+    message_id: str = Field(min_length=1, max_length=36)
+    plaintext: str | None = None
+    ciphertext_ref: str | None = Field(default=None, max_length=1000)
+    signed_manifest_ref: str | None = Field(default=None, max_length=1000)
+    wrapped_keys: list[PostboxWrappedKeyPayload] = Field(default_factory=list)
+    content_digest: str = Field(pattern=r"^sha256:[0-9a-f]{64}$")
+    transformation_evidence_ref: str = Field(min_length=1, max_length=1000)
+
+    @model_validator(mode="after")
+    def validate_target_payload(self) -> "PostboxProtectionTransformRequest":
+        if self.plaintext is not None and self.ciphertext_ref:
+            raise ValueError("Provide transformed plaintext or ciphertext, not both.")
+        if self.ciphertext_ref and (
+            not self.signed_manifest_ref or not self.wrapped_keys
+        ):
+            raise ValueError(
+                "E2EE transformation requires a signed manifest and wrapped keys."
+            )
+        return self
+
+
+class PostboxProtectionTransitionItemResponse(BaseModel):
+    id: str
+    message_id: str
+    source_profile: str
+    target_profile: str
+    state: str
+    source_digest: str | None = None
+    target_digest: str | None = None
+    completed_by: str | None = None
+    completed_at: datetime | None = None
+    error_code: str | None = None
+    evidence: dict[str, Any] = Field(default_factory=dict)
+
+
+class PostboxProtectionTransitionResponse(BaseModel):
+    id: str
+    postbox_id: str
+    source_profile: str
+    target_profile: str
+    source_vault_id: str | None = None
+    target_vault_id: str | None = None
+    history_mode: str
+    authority_mode: str
+    required_quorum: int
+    evidence_refs: list[str]
+    reason: str
+    state: str
+    message_count: int
+    completed_count: int
+    failed_count: int
+    requested_by: str | None = None
+    activated_at: datetime | None = None
+    completed_at: datetime | None = None
+    resource_revision: int = Field(ge=1)
+    etag: str
+    configuration_snapshot: dict[str, Any] = Field(default_factory=dict)
+    items: list[PostboxProtectionTransitionItemResponse] = Field(default_factory=list)
+
+
+class PostboxProtectionTransitionListResponse(BaseModel):
+    transitions: list[PostboxProtectionTransitionResponse]
+
+
 def _validate_template_write_scope(
     payload: PostboxTemplateRevisionPayload,
 ) -> None:
@@ -586,9 +848,7 @@ class PostboxOrganizationStructureItem(BaseModel):
 
 class PostboxOrganizationTargetsResponse(BaseModel):
     units: list[PostboxOrganizationUnitItem]
-    structures: list[PostboxOrganizationStructureItem] = Field(
-        default_factory=list
-    )
+    structures: list[PostboxOrganizationStructureItem] = Field(default_factory=list)
 
 
 class PostboxGroupingPayload(BaseModel):

tests/test_migration.py

@@ -38,6 +38,10 @@ class PostboxMigrationTests(unittest.TestCase):
             "govoplan_postbox.backend.migrations.versions."
             "f2a5c8e1b4d7_v015_portal_visibility"
         )
+        transition_migration = importlib.import_module(
+            "govoplan_postbox.backend.migrations.versions."
+            "a7c1e4f8b2d6_v016_protection_transitions"
+        )
         engine = create_engine("sqlite:///:memory:")
         try:
             with engine.begin() as connection:
@@ -49,6 +53,7 @@ class PostboxMigrationTests(unittest.TestCase):
                 protection_original = protection_migration.op
                 scope_original = scope_migration.op
                 portal_original = portal_migration.op
+                transition_original = transition_migration.op
                 migration.op = operations
                 route_migration.op = operations
                 occ_migration.op = operations
@@ -56,6 +61,7 @@ class PostboxMigrationTests(unittest.TestCase):
                 protection_migration.op = operations
                 scope_migration.op = operations
                 portal_migration.op = operations
+                transition_migration.op = operations
                 try:
                     migration.upgrade()
                     route_migration.upgrade()
@@ -64,11 +70,14 @@ class PostboxMigrationTests(unittest.TestCase):
                     protection_migration.upgrade()
                     scope_migration.upgrade()
                     portal_migration.upgrade()
+                    transition_migration.upgrade()
                     tables = set(inspect(connection).get_table_names())
                     self.assertIn("postboxes", tables)
                     self.assertIn("postbox_messages", tables)
                     self.assertIn("postbox_deliveries", tables)
                     self.assertIn("postbox_access_events", tables)
+                    self.assertIn("postbox_protection_transitions", tables)
+                    self.assertIn("postbox_protection_transition_items", tables)
                     message_columns = {
                         column["name"]
                         for column in inspect(connection).get_columns(
@@ -120,15 +129,12 @@ class PostboxMigrationTests(unittest.TestCase):
                         )
                     route_columns = {
                         column["name"]
-                        for column in inspect(connection).get_columns(
-                            "postbox_routes"
-                        )
+                        for column in inspect(connection).get_columns("postbox_routes")
                     }
                     self.assertTrue(
-                        {"execute_after", "processed_at"}.issubset(
-                            route_columns
-                        )
+                        {"execute_after", "processed_at"}.issubset(route_columns)
                     )
+                    transition_migration.downgrade()
                     portal_migration.downgrade()
                     scope_migration.downgrade()
                     protection_migration.downgrade()
@@ -151,6 +157,7 @@ class PostboxMigrationTests(unittest.TestCase):
                     protection_migration.op = protection_original
                     scope_migration.op = scope_original
                     portal_migration.op = portal_original
+                    transition_migration.op = transition_original
         finally:
             engine.dispose()
 

tests/test_service.py

@@ -1,6 +1,8 @@
 from __future__ import annotations
 
+import hashlib
 import unittest
+from dataclasses import replace
 from datetime import timedelta
 from types import SimpleNamespace
 from unittest.mock import patch
@@ -49,6 +51,8 @@ from govoplan_postbox.backend.db.models import (
     PostboxMessage,
     PostboxMessageReceipt,
     PostboxParticipant,
+    PostboxProtectionTransition,
+    PostboxProtectionTransitionItem,
     PostboxRoute,
     PostboxTemplate,
     PostboxTemplateRevision,
@@ -64,6 +68,8 @@ POSTBOX_TABLES = (
     Postbox.__table__,
     PostboxBinding.__table__,
     PostboxMessage.__table__,
+    PostboxProtectionTransition.__table__,
+    PostboxProtectionTransitionItem.__table__,
     PostboxParticipant.__table__,
     PostboxAttachmentReference.__table__,
     PostboxDelivery.__table__,
@@ -87,7 +93,9 @@ class FakeIdentityDirectory:
         )
 
     def identities_for_accounts(self, account_ids):
-        return tuple(self.identity_for_account(account_id) for account_id in account_ids)
+        return tuple(
+            self.identity_for_account(account_id) for account_id in account_ids
+        )
 
     def accounts_for_identity(self, identity_id: str):
         return ()
@@ -335,8 +343,7 @@ class FakeOrganizationDirectory:
                 if function.function_type_id == function_type_id
                 and (
                     not organization_unit_ids
-                    or function.organization_unit_id
-                    in organization_unit_ids
+                    or function.organization_unit_id in organization_unit_ids
                 )
             ),
         )
@@ -355,8 +362,7 @@ class FakeOrganizationDirectory:
             matches=tuple(
                 unit
                 for unit in self.units.values()
-                if unit.tenant_id == tenant_id
-                and unit.unit_type_id == unit_type_id
+                if unit.tenant_id == tenant_id and unit.unit_type_id == unit_type_id
             ),
         )
 
@@ -457,9 +463,7 @@ class FakeOrganizationDirectory:
                     root=root,
                     matches=matches,
                     cycle_detected=self.cycle_detected,
-                    depth_limited=(
-                        structure_id == self.structure.id and max_depth < 2
-                    ),
+                    depth_limited=(structure_id == self.structure.id and max_depth < 2),
                 )
             )
         return tuple(results)
@@ -529,15 +533,20 @@ class PostboxServiceTests(unittest.TestCase):
         self.actor = PostboxActorRef(
             account_id="account-1",
             identity_id="identity-1",
-            authorized_actions=frozenset(
-                {"discover", "read", "send", "acknowledge"}
-            ),
+            authorized_actions=frozenset({"discover", "read", "send", "acknowledge"}),
         )
 
     def tearDown(self) -> None:
         self.engine.dispose()
 
-    def _create_exact(self, session: Session) -> Postbox:
+    def _create_exact(
+        self,
+        session: Session,
+        *,
+        encryption_profile: str = "plaintext_v1",
+        encryption_vault_id: str | None = None,
+        protection_policy: dict[str, object] | None = None,
+    ) -> Postbox:
         return self.service.create_exact_postbox(
             session,
             tenant_id="tenant-1",
@@ -548,6 +557,9 @@ class PostboxServiceTests(unittest.TestCase):
             description=None,
             classification="internal",
             actor_id="admin-1",
+            encryption_profile=encryption_profile,
+            encryption_vault_id=encryption_vault_id,
+            protection_policy=protection_policy,
         )
 
     def _routing_policy(
@@ -575,11 +587,7 @@ class PostboxServiceTests(unittest.TestCase):
                 "max_retention_days": 30,
             },
             "attention": {
-                "mode": (
-                    "vacancy_escalation"
-                    if vacancy_escalation
-                    else "none"
-                ),
+                "mode": ("vacancy_escalation" if vacancy_escalation else "none"),
                 "delay_minutes": 1 if vacancy_escalation else None,
             },
             "shared_visibility": {"mode": "none"},
@@ -830,7 +838,9 @@ class PostboxServiceTests(unittest.TestCase):
                     ),
                 )
 
-    def test_template_revision_is_immutable_and_materialization_idempotent(self) -> None:
+    def test_template_revision_is_immutable_and_materialization_idempotent(
+        self,
+    ) -> None:
         with Session(self.engine) as session:
             template = self.service.create_template(
                 session,
@@ -957,6 +967,7 @@ class PostboxServiceTests(unittest.TestCase):
                 routing_policy={},
                 encryption_profile="plaintext_v1",
                 encryption_vault_id=None,
+                protection_policy={},
                 context_key=None,
                 limit=200,
             )
@@ -985,6 +996,7 @@ class PostboxServiceTests(unittest.TestCase):
                 routing_policy={},
                 encryption_profile="plaintext_v1",
                 encryption_vault_id=None,
+                protection_policy={},
                 context_key=None,
                 limit=200,
             )
@@ -1014,6 +1026,7 @@ class PostboxServiceTests(unittest.TestCase):
                 routing_policy={},
                 encryption_profile="plaintext_v1",
                 encryption_vault_id=None,
+                protection_policy={},
                 context_key=None,
                 limit=200,
             )
@@ -1067,9 +1080,7 @@ class PostboxServiceTests(unittest.TestCase):
                 [item.slug for item in catalog.templates],
             )
             north = next(
-                unit
-                for unit in catalog.organization_units
-                if unit.id == "unit-1"
+                unit for unit in catalog.organization_units if unit.id == "unit-1"
             )
             self.assertEqual(
                 ["function-1"],
@@ -1309,7 +1320,11 @@ class PostboxServiceTests(unittest.TestCase):
         self.idm.assignments.append(self.assignment)
         expires_at = utc_now() + timedelta(days=1)
         with Session(self.engine) as session:
-            postbox = self._create_exact(session)
+            postbox = self._create_exact(
+                session,
+                encryption_profile="external_e2ee_v1",
+                protection_policy={"external_recipient_assurance": "email_otp"},
+            )
             delivered = self.service.deliver(
                 session,
                 PostboxDeliveryRequest(
@@ -1340,6 +1355,7 @@ class PostboxServiceTests(unittest.TestCase):
                             assurance_profile="email-otp",
                         ),
                     ),
+                    metadata={"content_digest": "sha256:" + "a" * 64},
                 ),
             )
 
@@ -1361,9 +1377,7 @@ class PostboxServiceTests(unittest.TestCase):
             assert message.external_recipient_tokens[0].expires_at is not None
             self.assertEqual(
                 expires_at.replace(microsecond=0),
-                message.external_recipient_tokens[0].expires_at.replace(
-                    microsecond=0
-                ),
+                message.external_recipient_tokens[0].expires_at.replace(microsecond=0),
             )
 
     def test_server_envelope_body_is_not_persisted_in_plaintext_and_fails_closed(
@@ -1445,14 +1459,330 @@ class PostboxServiceTests(unittest.TestCase):
                         actor=self.actor,
                     )
 
+    def test_future_only_protection_transition_changes_new_message_policy(self) -> None:
+        with Session(self.engine) as session:
+            postbox = self._create_exact(
+                session,
+                protection_policy={
+                    "handover_authority": "user_consent",
+                    "handover_quorum": 1,
+                },
+            )
+            transition = self.service.create_protection_transition(
+                session,
+                tenant_id="tenant-1",
+                postbox_id=postbox.id,
+                idempotency_key="future-e2ee",
+                expected_revision=postbox.resource_revision,
+                target_profile="external_e2ee_v1",
+                target_vault_id=None,
+                history_mode="future_only",
+                authority_mode="user_consent",
+                required_quorum=1,
+                user_consent_refs=("consent:user-1",),
+                institutional_authorization_refs=(),
+                reason="Use client-held keys for future messages.",
+                actor_id="admin-1",
+            )
+
+            self.assertEqual("completed", transition.state)
+            self.assertEqual(0, transition.message_count)
+            self.assertEqual("external_e2ee_v1", postbox.encryption_profile)
+            self.assertEqual(2, postbox.key_epoch)
+
+    def test_client_transform_completes_plaintext_to_e2ee_history(self) -> None:
+        self.idm.assignments.append(self.assignment)
+        plaintext = "A governed message"
+        digest = "sha256:" + hashlib.sha256(plaintext.encode()).hexdigest()
+        with Session(self.engine) as session:
+            postbox = self._create_exact(
+                session,
+                protection_policy={
+                    "handover_authority": "user_consent",
+                    "handover_quorum": 1,
+                },
+            )
+            delivered = self.service.deliver(
+                session,
+                PostboxDeliveryRequest(
+                    tenant_id="tenant-1",
+                    target=PostboxTargetRef(postbox_id=postbox.id),
+                    producer_module="campaigns",
+                    producer_resource_type="campaign_recipient",
+                    producer_resource_id="recipient-1",
+                    idempotency_key="plaintext-history",
+                    subject="Governed history",
+                    body_text=plaintext,
+                ),
+            )
+            transition = self.service.create_protection_transition(
+                session,
+                tenant_id="tenant-1",
+                postbox_id=postbox.id,
+                idempotency_key="migrate-e2ee",
+                expected_revision=postbox.resource_revision,
+                target_profile="external_e2ee_v1",
+                target_vault_id=None,
+                history_mode="migrate_history",
+                authority_mode="user_consent",
+                required_quorum=1,
+                user_consent_refs=("consent:user-1",),
+                institutional_authorization_refs=(),
+                reason="Move retained content to client-held keys.",
+                actor_id="admin-1",
+            )
+
+            self.assertEqual("awaiting_client", transition.state)
+            self.assertEqual(1, transition.message_count)
+            completed = self.service.apply_client_protection_transform(
+                session,
+                tenant_id="tenant-1",
+                postbox_id=postbox.id,
+                transition_id=transition.id,
+                message_id=delivered.message_id,
+                expected_revision=transition.resource_revision,
+                plaintext=None,
+                ciphertext_ref="files:ciphertext-transition-1",
+                signed_manifest_ref="files:manifest-transition-1",
+                wrapped_keys=(
+                    PostboxWrappedKeyRef(
+                        recipient_type="function_postbox",
+                        recipient_id=postbox.id,
+                        key_epoch=postbox.key_epoch,
+                        wrapped_key_ref="trust:wrapped-transition-1",
+                        algorithm="HPKE-v1",
+                    ),
+                ),
+                content_digest=digest,
+                transformation_evidence_ref="client:evidence-1",
+                actor_id="admin-1",
+            )
+
+            self.assertEqual("completed", completed.state)
+            self.assertEqual(1, completed.completed_count)
+            stored = session.get(PostboxMessage, delivered.message_id)
+            assert stored is not None
+            self.assertIsNone(stored.body_text)
+            self.assertEqual("external_e2ee_v1", stored.encryption_profile)
+            self.assertEqual("files:ciphertext-transition-1", stored.ciphertext_ref)
+            self.assertEqual(digest, stored.metadata_["content_digest"])
+
+    def test_plaintext_history_migrates_to_managed_envelopes_automatically(
+        self,
+    ) -> None:
+        protected = SimpleNamespace(
+            ciphertext=b"managed-history",
+            envelope=SimpleNamespace(
+                envelope_id="transition-envelope-1",
+                ciphertext_ref="postbox-db://messages/history/body",
+                algorithm_suite="AES-256-GCM",
+                wrapped_key_refs=("wrapped-history-1",),
+                vault_id="vault-1",
+                key_version=4,
+            ),
+        )
+        with Session(self.engine) as session:
+            postbox = self._create_exact(session)
+            delivered = self.service.deliver(
+                session,
+                PostboxDeliveryRequest(
+                    tenant_id="tenant-1",
+                    target=PostboxTargetRef(postbox_id=postbox.id),
+                    producer_module="campaigns",
+                    producer_resource_type="campaign_recipient",
+                    producer_resource_id="recipient-managed-history",
+                    idempotency_key="managed-history",
+                    subject="Managed history",
+                    body_text="Move this content",
+                ),
+            )
+            with patch(
+                "govoplan_postbox.backend.content_protection.protect_message_body",
+                return_value=protected,
+            ):
+                transition = self.service.create_protection_transition(
+                    session,
+                    tenant_id="tenant-1",
+                    postbox_id=postbox.id,
+                    idempotency_key="migrate-managed",
+                    expected_revision=postbox.resource_revision,
+                    target_profile="server_envelope_v1",
+                    target_vault_id="vault-1",
+                    history_mode="migrate_history",
+                    authority_mode="dual_control",
+                    required_quorum=2,
+                    user_consent_refs=("consent:incumbent-1",),
+                    institutional_authorization_refs=("approval:key-holder-1",),
+                    reason="Adopt the managed standard.",
+                    actor_id="admin-1",
+                )
+
+            self.assertEqual("completed", transition.state)
+            stored = session.get(PostboxMessage, delivered.message_id)
+            assert stored is not None
+            self.assertIsNone(stored.body_text)
+            self.assertEqual(b"managed-history", stored.body_ciphertext)
+            self.assertEqual("transition-envelope-1", stored.encryption_envelope_id)
+            self.assertEqual("server_envelope_v1", stored.encryption_profile)
+            self.assertEqual("vault-1", stored.wrapped_keys[0]["recipient_id"])
+
+    def test_new_incumbent_history_policy_filters_lists_counts_and_direct_reads(
+        self,
+    ) -> None:
+        boundary = utc_now() - timedelta(days=1)
+        self.idm.assignments.append(replace(self.assignment, valid_from=boundary))
+        with Session(self.engine) as session:
+            postbox = self._create_exact(
+                session,
+                protection_policy={"new_incumbent_history": "since_assignment"},
+            )
+            old_delivery = self.service.deliver(
+                session,
+                PostboxDeliveryRequest(
+                    tenant_id="tenant-1",
+                    target=PostboxTargetRef(postbox_id=postbox.id),
+                    producer_module="campaigns",
+                    producer_resource_type="campaign_recipient",
+                    producer_resource_id="recipient-old",
+                    idempotency_key="history-old",
+                    subject="Before assignment",
+                    body_text="Old content",
+                ),
+            )
+            recent_delivery = self.service.deliver(
+                session,
+                PostboxDeliveryRequest(
+                    tenant_id="tenant-1",
+                    target=PostboxTargetRef(postbox_id=postbox.id),
+                    producer_module="campaigns",
+                    producer_resource_type="campaign_recipient",
+                    producer_resource_id="recipient-recent",
+                    idempotency_key="history-recent",
+                    subject="After assignment",
+                    body_text="Recent content",
+                ),
+            )
+            old = session.get(PostboxMessage, old_delivery.message_id)
+            recent = session.get(PostboxMessage, recent_delivery.message_id)
+            assert old is not None and recent is not None
+            old.delivered_at = boundary - timedelta(hours=1)
+            recent.delivered_at = boundary + timedelta(hours=1)
+            session.flush()
+
+            messages = self.service.list_messages(
+                session,
+                tenant_id="tenant-1",
+                postbox_ids=(postbox.id,),
+                actor=self.actor,
+            )
+
+            self.assertEqual([recent.id], [message.id for message in messages])
+            self.assertEqual(
+                1,
+                self.service.count_messages(
+                    session,
+                    tenant_id="tenant-1",
+                    postbox_ids=(postbox.id,),
+                    actor=self.actor,
+                ),
+            )
+            self.assertIsNone(
+                self.service.get_message(
+                    session,
+                    tenant_id="tenant-1",
+                    message_id=old.id,
+                    actor=self.actor,
+                )
+            )
+            self.assertFalse(
+                self.service.can_read_message(
+                    session,
+                    tenant_id="tenant-1",
+                    message_id=old.id,
+                    actor=self.actor,
+                )
+            )
+
+    def test_leaving_e2ee_history_requires_user_consent_authority(self) -> None:
+        with Session(self.engine) as session:
+            postbox = self._create_exact(
+                session,
+                encryption_profile="external_e2ee_v1",
+            )
+            with self.assertRaisesRegex(PostboxError, "user consent"):
+                self.service.create_protection_transition(
+                    session,
+                    tenant_id="tenant-1",
+                    postbox_id=postbox.id,
+                    idempotency_key="leave-e2ee-without-user",
+                    expected_revision=postbox.resource_revision,
+                    target_profile="plaintext_v1",
+                    target_vault_id=None,
+                    history_mode="future_only",
+                    authority_mode="institutional_key_holders",
+                    required_quorum=1,
+                    user_consent_refs=(),
+                    institutional_authorization_refs=("approval:key-holder-1",),
+                    reason="Leave E2EE without holder consent.",
+                    actor_id="admin-1",
+                )
+
+    def test_protection_policy_update_is_revisioned_and_enforces_assurance(
+        self,
+    ) -> None:
+        with Session(self.engine) as session:
+            postbox = self._create_exact(session)
+            original_revision = postbox.resource_revision
+            updated = self.service.update_protection_policy(
+                session,
+                tenant_id="tenant-1",
+                postbox_id=postbox.id,
+                protection_policy={
+                    "new_incumbent_history": "all_retained",
+                    "external_recipient_assurance": "disabled",
+                },
+                actor_id="admin-1",
+                expected_revision=original_revision,
+            )
+
+            self.assertEqual(original_revision + 1, updated.resource_revision)
+            self.assertEqual(
+                "disabled",
+                updated.settings["protection_policy"]["external_recipient_assurance"],
+            )
+            self.assertEqual(
+                "rewrap",
+                updated.settings["protection_policy"]["ordinary_rotation"],
+            )
+            with self.assertRaisesRegex(PostboxError, "does not permit"):
+                self.service.deliver(
+                    session,
+                    PostboxDeliveryRequest(
+                        tenant_id="tenant-1",
+                        target=PostboxTargetRef(postbox_id=postbox.id),
+                        producer_module="campaigns",
+                        producer_resource_type="campaign_recipient",
+                        producer_resource_id="recipient-external-disabled",
+                        idempotency_key="external-disabled",
+                        subject="External retrieval",
+                        body_text="Content",
+                        external_recipient_tokens=(
+                            PostboxExternalRecipientTokenRef(
+                                token_id="grant-disabled",
+                                state="available",
+                                one_time=True,
+                                assurance_profile="strong_identity",
+                            ),
+                        ),
+                    ),
+                )
+
     def test_hierarchy_linked_copy_snapshots_path_and_independent_state(
         self,
     ) -> None:
         self.idm.assignments.append(self.assignment)
         with Session(self.engine) as session:
-            service, source, _target_template = (
-                self._create_routing_source(session)
-            )
+            service, source, _target_template = self._create_routing_source(session)
             result = service.deliver(
                 session,
                 PostboxDeliveryRequest(
@@ -1478,10 +1808,7 @@ class PostboxServiceTests(unittest.TestCase):
             self.assertNotEqual(result.message_id, route.target_message_id)
             self.assertEqual(
                 ["edge-child-parent"],
-                [
-                    edge["edge_id"]
-                    for edge in route.policy_snapshot["target"]["path"]
-                ],
+                [edge["edge_id"] for edge in route.policy_snapshot["target"]["path"]],
             )
             self.assertEqual(
                 route.id,
@@ -1496,7 +1823,9 @@ class PostboxServiceTests(unittest.TestCase):
             )
             session.commit()
             receipts = session.query(PostboxMessageReceipt).all()
-            self.assertEqual([route.target_message_id], [item.message_id for item in receipts])
+            self.assertEqual(
+                [route.target_message_id], [item.message_id for item in receipts]
+            )
             summary = service.delivery_receipt_summaries(
                 session,
                 tenant_id="tenant-1",
@@ -1512,8 +1841,8 @@ class PostboxServiceTests(unittest.TestCase):
         self,
     ) -> None:
         with Session(self.engine) as session:
-            service, source, _target_template = (
-                self._create_routing_source(session, max_depth=1)
+            service, source, _target_template = self._create_routing_source(
+                session, max_depth=1
             )
             blocked = service.preview_hierarchy_routes(
                 session,
@@ -1573,9 +1902,7 @@ class PostboxServiceTests(unittest.TestCase):
         self.organizations.duplicate_parent_match = True
         self.organizations.cycle_detected = True
         with Session(self.engine) as session:
-            service, source, _target_template = (
-                self._create_routing_source(session)
-            )
+            service, source, _target_template = self._create_routing_source(session)
             preview = service.preview_hierarchy_routes(
                 session,
                 tenant_id="tenant-1",
@@ -1588,10 +1915,7 @@ class PostboxServiceTests(unittest.TestCase):
             self.assertIn("hierarchy_cycle_bounded", preview["diagnostics"])
             self.assertEqual(
                 1,
-                sum(
-                    route["status"] == "duplicate"
-                    for route in preview["routes"]
-                ),
+                sum(route["status"] == "duplicate" for route in preview["routes"]),
             )
             with self.assertRaisesRegex(
                 ValueError,
@@ -1639,11 +1963,7 @@ class PostboxServiceTests(unittest.TestCase):
                 ),
             )
             session.commit()
-            routes = (
-                session.query(PostboxRoute)
-                .order_by(PostboxRoute.depth)
-                .all()
-            )
+            routes = session.query(PostboxRoute).order_by(PostboxRoute.depth).all()
             self.assertEqual(
                 ["accepted_vacant", "pending_vacancy_escalation"],
                 [route.status for route in routes],
@@ -1687,10 +2007,7 @@ class PostboxServiceTests(unittest.TestCase):
             session.commit()
             pending = (
                 session.query(PostboxRoute)
-                .filter(
-                    PostboxRoute.status
-                    == "pending_vacancy_escalation"
-                )
+                .filter(PostboxRoute.status == "pending_vacancy_escalation")
                 .one()
             )
             pending.execute_after = utc_now() - timedelta(seconds=1)

webui/src/api/postbox.ts

@@ -41,11 +41,84 @@ export type PostboxDirectoryItem = {
   template_revision_id?: string | null;
   holder_count: number;
   vacant: boolean;
+  encryption_profile: PostboxProtectionProfileId;
+  key_epoch: number;
+  encryption_vault_id?: string | null;
+  protection_policy: PostboxProtectionPolicy;
   access?: PostboxAccessDecision | null;
   resource_revision: number;
   etag: string;
 };
 
+export type PostboxProtectionProfileId =
+  | "plaintext_v1"
+  | "server_envelope_v1"
+  | "external_e2ee_v1";
+
+export type PostboxProtectionPolicy = {
+  new_incumbent_history: "all_retained" | "since_assignment" | "bounded_days";
+  history_days?: number | null;
+  ordinary_rotation: "rewrap" | "reencrypt";
+  compromise_rotation: "rewrap" | "reencrypt";
+  recovery_authority: "disabled" | "user_consent" | "institutional_key_holders" | "dual_control";
+  recovery_quorum: number;
+  handover_authority: "user_consent" | "institutional_key_holders" | "dual_control";
+  handover_quorum: number;
+  emergency_access: "disabled" | "dual_control";
+  emergency_quorum: number;
+  export_authority: "user_consent" | "institutional_key_holders" | "dual_control";
+  export_quorum: number;
+  destruction_authority: "institutional_key_holders" | "dual_control";
+  destruction_quorum: number;
+  external_recipient_assurance: "disabled" | "email_otp" | "strong_identity";
+  vacancy_escalation_content_access: "metadata_only";
+};
+
+export type PostboxProtectionProfile = {
+  id: PostboxProtectionProfileId;
+  label: string;
+  description: string;
+  server_can_decrypt: boolean;
+  requires_encryption_module: boolean;
+  requires_external_client: boolean;
+  available: boolean;
+  standard: boolean;
+};
+
+export type PostboxProtectionTransition = {
+  id: string;
+  postbox_id: string;
+  source_profile: string;
+  target_profile: string;
+  source_vault_id?: string | null;
+  target_vault_id?: string | null;
+  history_mode: string;
+  authority_mode: string;
+  required_quorum: number;
+  evidence_refs: string[];
+  reason: string;
+  state: string;
+  message_count: number;
+  completed_count: number;
+  failed_count: number;
+  requested_by?: string | null;
+  activated_at?: string | null;
+  completed_at?: string | null;
+  resource_revision: number;
+  etag: string;
+  configuration_snapshot: Record<string, unknown>;
+  items: Array<{
+    id: string;
+    message_id: string;
+    source_profile: string;
+    target_profile: string;
+    state: string;
+    source_digest?: string | null;
+    target_digest?: string | null;
+    error_code?: string | null;
+  }>;
+};
+
 export type PostboxParticipant = {
   kind: string;
   reference_type: string;
@@ -93,7 +166,7 @@ export type PostboxMessage = {
   producer_resource_id?: string | null;
   in_reply_to_message_id?: string | null;
   replaces_message_id?: string | null;
-  encryption_profile: string;
+  encryption_profile: PostboxProtectionProfileId;
   key_epoch: number;
   ciphertext_ref?: string | null;
   signed_manifest_ref?: string | null;
@@ -213,6 +286,8 @@ export type PostboxTemplateRevision = {
   allow_vacant_delivery: boolean;
   portal_visible: boolean;
   encryption_profile: string;
+  encryption_vault_id?: string | null;
+  protection_policy: PostboxProtectionPolicy;
   history_policy: Record<string, unknown>;
   routing_policy: PostboxRoutingPolicy;
   retention_policy: Record<string, unknown>;
@@ -248,6 +323,9 @@ export type PostboxTemplateRevisionPayload = Pick<
   | "classification"
   | "allow_vacant_delivery"
   | "portal_visible"
+  | "encryption_profile"
+  | "encryption_vault_id"
+  | "protection_policy"
   | "routing_policy"
 >;
 
@@ -290,12 +368,18 @@ export type PostboxExactCreatePayload = {
   address_key?: string | null;
   classification: string;
   portal_visible: boolean;
+  encryption_profile: PostboxProtectionProfileId;
+  encryption_vault_id?: string | null;
+  protection_policy: PostboxProtectionPolicy;
 };
 
 export type PostboxMessageAuthoringPayload = {
   idempotency_key: string;
   subject: string;
   body_text?: string | null;
+  ciphertext_ref?: string | null;
+  signed_manifest_ref?: string | null;
+  wrapped_keys?: PostboxMessage["wrapped_keys"];
   classification: string;
   participants: PostboxParticipant[];
   attachments: PostboxAttachment[];
@@ -469,6 +553,66 @@ export async function listAdminPostboxes(settings: ApiSettings): Promise<Postbox
   return response.postboxes;
 }
 
+export async function listPostboxProtectionProfiles(
+  settings: ApiSettings
+): Promise<{ standard_profile: PostboxProtectionProfileId; profiles: PostboxProtectionProfile[] }> {
+  return apiFetch(settings, "/api/v1/postbox/admin/protection-profiles");
+}
+
+export async function listPostboxProtectionTransitions(
+  settings: ApiSettings,
+  postboxId: string
+): Promise<PostboxProtectionTransition[]> {
+  const response = await apiFetch<{ transitions: PostboxProtectionTransition[] }>(
+    settings,
+    `/api/v1/postbox/admin/postboxes/${encodeURIComponent(postboxId)}/protection-transitions`
+  );
+  return response.transitions;
+}
+
+export function createPostboxProtectionTransition(
+  settings: ApiSettings,
+  postbox: PostboxDirectoryItem,
+  payload: {
+    idempotency_key: string;
+    target_profile: PostboxProtectionProfileId;
+    target_vault_id?: string | null;
+    history_mode: "future_only" | "migrate_history";
+    authority_mode: "user_consent" | "institutional_key_holders" | "dual_control";
+    required_quorum: number;
+    user_consent_refs: string[];
+    institutional_authorization_refs: string[];
+    reason: string;
+    acknowledge_irreversibility: boolean;
+  }
+): Promise<PostboxProtectionTransition> {
+  return apiPostJson(
+    settings,
+    `/api/v1/postbox/admin/postboxes/${encodeURIComponent(postbox.id)}/protection-transitions`,
+    { ...payload, base_revision: postbox.resource_revision },
+    { headers: { "If-Match": postbox.etag } }
+  );
+}
+
+export function updatePostboxProtectionPolicy(
+  settings: ApiSettings,
+  postbox: PostboxDirectoryItem,
+  protectionPolicy: PostboxProtectionPolicy
+): Promise<PostboxDirectoryItem> {
+  return apiFetch(
+    settings,
+    `/api/v1/postbox/admin/postboxes/${encodeURIComponent(postbox.id)}/protection-policy`,
+    {
+      method: "PUT",
+      headers: { "If-Match": postbox.etag },
+      body: JSON.stringify({
+        base_revision: postbox.resource_revision,
+        protection_policy: protectionPolicy
+      })
+    }
+  );
+}
+
 export async function listPostboxOrganizationTargets(
   settings: ApiSettings
 ): Promise<PostboxOrganizationTargets> {

webui/src/features/postbox/PostboxPage.tsx

@@ -164,6 +164,10 @@ export default function PostboxPage({
     () => groupings.find((grouping) => grouping.id === selectedScope) ?? null,
     [groupings, selectedScope]
   );
+  const composeTarget = useMemo(
+    () => postboxes.find((postbox) => postbox.id === messageDraft.postbox_id) ?? null,
+    [messageDraft.postbox_id, postboxes]
+  );
   const scopePostboxIds = useMemo(() => {
     if (selectedPostboxId) return [selectedPostboxId];
     if (selectedGrouping) {
@@ -179,6 +183,9 @@ export default function PostboxPage({
   const replyDisabledReason = postboxBusyReason(false, busy)
     ?? (!canReply ? POSTBOX_INTERFACE_I18N.noReplyReason : undefined)
     ?? (!selectedMessage ? POSTBOX_INTERFACE_I18N.noMessage : undefined)
+    ?? (selectedMessage?.encryption_profile === "external_e2ee_v1"
+      ? "Replies to E2EE messages must be created by an approved encryption client."
+      : undefined)
     ?? (selectedMessage?.availability !== "available"
       ? POSTBOX_INTERFACE_I18N.unavailableMessage
       : undefined);
@@ -509,7 +516,10 @@ export default function PostboxPage({
   }
 
   function openCompose() {
-    const postbox = selectedPostbox ?? postboxes[0] ?? null;
+    const postbox = selectedPostbox
+      ?? postboxes.find((item) => item.encryption_profile !== "external_e2ee_v1")
+      ?? postboxes[0]
+      ?? null;
     if (!postbox) return;
     openComposeFor(postbox);
   }
@@ -543,7 +553,11 @@ export default function PostboxPage({
   }
 
   async function submitMessage(): Promise<boolean> {
-    if (!messageDraft.postbox_id || !messageDraft.subject.trim()) return false;
+    if (
+      !messageDraft.postbox_id
+      || !messageDraft.subject.trim()
+      || composeTarget?.encryption_profile === "external_e2ee_v1"
+    ) return false;
     setBusy(true);
     setError("");
     const participants = messageDraft.recipients
@@ -1034,9 +1048,10 @@ export default function PostboxPage({
               disabled={
                 busy ||
                 !messageDraft.postbox_id ||
-                !messageDraft.subject.trim()
+                !messageDraft.subject.trim() ||
+                composeTarget?.encryption_profile === "external_e2ee_v1"
               }
-              disabledReason={postboxBusyReason(false, busy) ?? ((!messageDraft.postbox_id || !messageDraft.subject.trim()) ? POSTBOX_INTERFACE_I18N.incompleteDraft : undefined)}
+              disabledReason={postboxBusyReason(false, busy) ?? (composeTarget?.encryption_profile === "external_e2ee_v1" ? "This browser editor has no E2EE private-key custody. Use an approved encryption client for this Postbox." : ((!messageDraft.postbox_id || !messageDraft.subject.trim()) ? POSTBOX_INTERFACE_I18N.incompleteDraft : undefined))}
             >
               <Send size={16} /> Send
             </Button>
@@ -1044,6 +1059,13 @@ export default function PostboxPage({
         }
       >
         <FormGrid columns={2} gap="small" collapseAt="narrow" className="postbox-compose-grid">
+          {composeTarget?.encryption_profile === "external_e2ee_v1" ? (
+            <div className="postbox-compose-wide">
+              <DismissibleAlert tone="info" compact resetKey={composeTarget.id}>
+                This Postbox requires externally produced E2EE. Use an approved client that supplies ciphertext, a signed manifest, wrapped keys, and a verified content digest; this browser editor never asks for or stores the private key.
+              </DismissibleAlert>
+            </div>
+          ) : null}
           <FormField label="Postbox" documentation={POSTBOX_FIELD_DOCUMENTATION}>
             <select
               value={messageDraft.postbox_id}
@@ -1204,7 +1226,19 @@ function MessageDetail({
         </dl>
       </section>
       <section className="postbox-body">
-        <p>{message.body_text || "No plaintext body is available for this message."}</p>
+        {message.encryption_profile === "external_e2ee_v1" ? (
+          <>
+            <DismissibleAlert tone="info" compact resetKey={message.id}>
+              This message is end-to-end encrypted. GovOPlaN stores and authorizes its envelope but cannot decrypt the content; open it with the institution's approved client.
+            </DismissibleAlert>
+            <dl className="postbox-provenance">
+              <div><dt>Ciphertext</dt><dd>{message.ciphertext_ref || "Not recorded"}</dd></div>
+              <div><dt>Signed manifest</dt><dd>{message.signed_manifest_ref || "Not recorded"}</dd></div>
+            </dl>
+          </>
+        ) : (
+          <p>{message.body_text || "No plaintext body is available for this message."}</p>
+        )}
       </section>
       {message.participants.length ? (
         <section className="postbox-participants">