An easy-to-host PDS on the ATProtocol, iPhone and MacOS. Maintain control of your keys and data, always.
docs(crypto): update CLAUDE.md with build_did_plc_genesis_op contracts (MM-89)
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crates/crypto/CLAUDE.md
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crates/crypto/CLAUDE.md
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# Crypto Crate
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Last verified: 2026-03-12
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Last verified: 2026-03-13
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## Purpose
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Provides cryptographic primitives for the ezpds workspace: P-256 key generation,
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This is a pure functional core -- no I/O, no database, no config.
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## Contracts
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- **Exposes**: `generate_p256_keypair() -> Result<P256Keypair, CryptoError>`, `encrypt_private_key(&[u8; 32], &[u8; 32]) -> Result<String, CryptoError>`, `decrypt_private_key(&str, &[u8; 32]) -> Result<Zeroizing<[u8; 32]>, CryptoError>`, `split_secret(&[u8; 32]) -> Result<[ShamirShare; 3], CryptoError>`, `combine_shares(&ShamirShare, &ShamirShare) -> Result<Zeroizing<[u8; 32]>, CryptoError>`, `P256Keypair`, `ShamirShare`, `CryptoError`
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- **P256Keypair fields**: `key_id` (full `did:key:z...` URI), `public_key` (multibase base58btc compressed point, no did:key: prefix), `private_key_bytes` (`Zeroizing<[u8; 32]>` -- zeroized on drop)
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- **ShamirShare fields**: `index` (u8, 1/2/3 -- not secret), `data` (`Zeroizing<[u8; 32]>` -- zeroized on drop)
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- **Encryption format**: `base64(nonce(12) || ciphertext(32) || tag(16))` = 80 base64 chars. Fresh 12-byte nonce from OS RNG per call.
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- **did:key format**: P-256 multicodec varint `[0x80, 0x24]` + compressed public key, multibase base58btc encoded
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- **CryptoError variants**: `KeyGeneration`, `Encryption`, `Decryption`, `SecretSharing`, `SecretReconstruction`
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### Public API functions
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**`generate_p256_keypair`**
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```rust
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pub fn generate_p256_keypair() -> Result<P256Keypair, CryptoError>
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```
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- Generates a fresh P-256 keypair from OS RNG
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- Returns `key_id` (full `did:key:z...` URI), `public_key` (multibase base58btc, no prefix), `private_key_bytes` (zeroized)
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**`encrypt_private_key`**
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```rust
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pub fn encrypt_private_key(&[u8; 32], &[u8; 32]) -> Result<String, CryptoError>
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```
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- Encrypts a 32-byte secret with a 32-byte master key using AES-256-GCM
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- Fresh 12-byte nonce per call; returns `base64(nonce(12) || ciphertext(32) || tag(16))` (80 base64 chars)
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**`decrypt_private_key`**
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```rust
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pub fn decrypt_private_key(&str, &[u8; 32]) -> Result<Zeroizing<[u8; 32]>, CryptoError>
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```
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- Decrypts a base64-encoded ciphertext with a master key
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- Returns opaque `CryptoError::Decryption` on all failure modes (no oracle)
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**`split_secret`**
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```rust
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pub fn split_secret(&[u8; 32]) -> Result<[ShamirShare; 3], CryptoError>
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```
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- Shamir secret sharing (2-of-3 scheme) with fresh OS RNG polynomial coefficients
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- Information-theoretic security: a single share reveals nothing
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**`combine_shares`**
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```rust
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pub fn combine_shares(&ShamirShare, &ShamirShare) -> Result<Zeroizing<[u8; 32]>, CryptoError>
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```
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- Reconstructs secret from 2 distinct shares (indices [1,3])
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- Returns `CryptoError::SecretReconstruction` if indices are duplicate or out of range
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**`build_did_plc_genesis_op`** (new, MM-89)
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```rust
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pub fn build_did_plc_genesis_op(
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rotation_key: &DidKeyUri, // user's root rotation key (rotationKeys[0])
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signing_key: &DidKeyUri, // relay's signing key (rotationKeys[1] + verificationMethods.atproto)
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signing_private_key: &[u8; 32], // raw P-256 private key scalar for signing_key
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handle: &str, // e.g. "alice.example.com"
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service_endpoint: &str, // e.g. "https://relay.example.com"
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) -> Result<PlcGenesisOp, CryptoError>
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```
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- Constructs a signed did:plc genesis operation
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- Returns `PlcGenesisOp { did, signed_op_json }`
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- `did` matches `^did:plc:[a-z2-7]{24}$`; derived from SHA-256 of CBOR-encoded signed op, base32-lowercase, first 24 chars
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- `signed_op_json` is ready to POST to `https://plc.directory/{did}`
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- Deterministic: same inputs → same DID (RFC 6979 ECDSA + SHA-256 + base32)
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- Errors: `CryptoError::PlcOperation` if `signing_private_key` is an invalid P-256 scalar
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### Public types
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**`P256Keypair`**
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- `key_id`: full `did:key:z...` URI
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- `public_key`: multibase base58btc compressed point (no prefix)
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- `private_key_bytes`: `Zeroizing<[u8; 32]>` (zeroized on drop)
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**`PlcGenesisOp`** (new, MM-89)
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- `did`: `"did:plc:xxxx..."` (28 chars total)
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- `signed_op_json`: contains `type`, `rotationKeys`, `verificationMethods`, `alsoKnownAs`, `services`, `prev` (null), `sig`
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**`ShamirShare`**
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- `index`: u8 in [1, 3] (not secret)
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- `data`: `Zeroizing<[u8; 32]>` (zeroized on drop)
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**`CryptoError`** variants:
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- `KeyGeneration`, `Encryption`, `Decryption`, `SecretSharing`, `SecretReconstruction`, `PlcOperation` (new, MM-89)
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### Format guarantees
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- **did:key**: P-256 multicodec varint `[0x80, 0x24]` + compressed point, multibase base58btc encoded
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- **Encryption**: `base64(nonce(12) || ciphertext(32) || tag(16))` = 80 base64 chars; fresh nonce per call
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- **did:plc genesis op sig**: base64url (no padding) decoding to exactly 64 bytes (r‖s, big-endian, low-S canonical)
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## Dependencies
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- **Uses**: p256 (ECDSA/key generation), aes-gcm (AES-256-GCM), multibase (base58btc encoding), rand_core (OS RNG), base64 (storage encoding), zeroize (secret cleanup)
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- **Used by**: `crates/relay/` (key generation endpoint)
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- **Uses**: p256 (ECDSA/key generation), aes-gcm (AES-256-GCM), multibase (base58btc encoding), rand_core (OS RNG), base64 (storage encoding), zeroize (secret cleanup), ciborium (CBOR serialization for did:plc), data-encoding (base32-lowercase), sha2 (SHA-256), serde/serde_json (struct serialization)
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- **Used by**: `crates/relay/` (key generation, did:plc genesis endpoint)
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## Invariants
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- Private key bytes are always wrapped in `Zeroizing` -- callers must not copy them into non-zeroizing storage
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## Key Files
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- `src/lib.rs` - Re-exports public API
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- `src/keys.rs` - P-256 key generation, AES-256-GCM encrypt/decrypt
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- `src/plc.rs` - did:plc genesis operation builder (MM-89)
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- `src/shamir.rs` - Shamir Secret Sharing (split/combine, GF(2^8) arithmetic)
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- `src/error.rs` - CryptoError enum