feat(threshold): add ceremony HPKE dealings; Details: TC-2220

tinycloud-threshold/Cargo.toml

@@ -14,6 +14,7 @@ zeroize = { version = "1.8", features = ["derive"] }
 subtle = "2.6"
 rand_core = "0.6"
 thiserror = "2"
+hpke = { version = "=0.12.0", default-features = false, features = ["x25519"] }
 
 [dev-dependencies]
 ed25519-dalek = "=2.2.0"

tinycloud-threshold/SPEC.md

@@ -190,6 +190,10 @@ Secrets that must become unrecoverable, and when:
 
 Public ciphertext transcripts may be kept once the matching recipient keys are unrecoverable. In memory the crate zeroizes on drop and avoids hidden copies (blst `Copy` types are wiped explicitly where they hold secrets); `ZeroizeOnDrop` is an in-memory operation; durable storage is handled by integration. Durable erasure, snapshots, swap and core dumps are gate A's acceptance (§12).
 
+The `seal` module's lifecycle behavior covers only accessible owned storage. `CeremonyHpkeSecretKey` clears its owned 32-byte array on explicit zeroization or drop. Internal plaintext/candidate buffers use `Zeroizing`, including error paths; a successful seal transfers ciphertext out of that buffer. Opened plaintext is returned in `Zeroizing<Vec<u8>>`. The borrowed sealing input belongs to the caller and is not cleared by the module.
+
+HPKE 0.12's `PrivateKey`, `AeadCtxS` and `AeadCtxR` are opaque dependency-managed values. This owned-buffer description makes no claim about zeroization of their internal storage.
+
 ### 3.12 Activation boundary (contract fixed now; durable state record is integration)
 
 An owner signature on a new certificate is **not** an erasure trigger. Transition states:

tinycloud-threshold/src/error.rs

@@ -34,6 +34,10 @@ pub enum Error {
     DegenerateRandomness,
     #[error("invalid IBE ciphertext")]
     InvalidCiphertext,
+    #[error("invalid HPKE key")]
+    InvalidHpkeKey,
+    #[error("invalid sealed dealing")]
+    InvalidSealedDealing,
     #[error("derived key does not match the request")]
     InvalidDerivedKey,
     #[error("invalid signature")]

tinycloud-threshold/src/lib.rs

@@ -29,6 +29,14 @@ pub mod ibe;
 pub mod identity;
 pub mod ids;
 mod poly;
+#[cfg_attr(
+    not(test),
+    allow(
+        dead_code,
+        reason = "Crate-private dealing helpers await DKG/reshare callers."
+    )
+)]
+pub mod seal;
 
 pub use activation::{
     activation_preimage, readiness_preimage, verify_activation, ReadinessReceipt, ReadyQuorum,
@@ -47,6 +55,7 @@ pub use fault::{verify_fault_proof, FaultProof, FaultVerdict, NotAFaultReason};
 pub use ibe::*;
 pub use identity::IbeIdentity;
 pub use ids::*;
+pub use seal::{CeremonyHpkePublicKey, CeremonyHpkeSecretKey, SealedDealing};
 
 #[cfg(test)]
 mod checkpoint;