alknet/docs/architecture/crates/vault/README.md

---
status: stable
last_updated: 2026-06-23
---

# alknet-vault

Local key vault: BIP39 mnemonic generation, SLIP-0010 Ed25519 HD key
derivation, BIP-0032 secp256k1 derivation (feature-gated), and AES-256-GCM
encryption. Holds the master seed — the root of trust for all derived keys
and encrypted credentials in the alknet system.

## What This Crate Is

alknet-vault is a **standalone crate** with zero alknet crate dependencies
(ADR-018) and zero RPC framework dependencies (ADR-025). It provides the
cryptographic primitives and runtime API for managing the root of trust.
The CLI binary (the `alknet` crate) is the sole component that talks to the
vault directly (ADR-019) — handlers receive derived/decrypted material
through capabilities, never through a vault reference.

The vault is **not a network service**. It has no ALPN, no
`ProtocolHandler` implementation, no operations registered in the call
protocol (ADR-008, ADR-014), and no remote dispatch capability (ADR-025).
The vault is **local-only by construction** — direct method calls on
`VaultServiceHandle`, no actor, no message enum, no wire format. The master
seed and derived private keys never cross the network.

## Documents

| Document | Status | Description |
|----------|--------|-------------|
| [mnemonic-derivation.md](mnemonic-derivation.md) | stable | BIP39, SLIP-0010, BIP-0032, derivation paths, key types |
| [encryption.md](encryption.md) | stable | AES-256-GCM, EncryptedData, key versioning, HD derivation (ADR-020) |
| [service.md](service.md) | stable | VaultServiceHandle lifecycle, direct dispatch, cache, error model |
| [protocol.md](protocol.md) | stable | DerivedKey redaction, KeyType, serialization behavior |

## Applicable ADRs

| ADR | Title | Relevance |
|-----|-------|-----------|
| [003](../../decisions/003-crate-decomposition.md) | Crate Decomposition | alknet-vault's standalone position |
| [008](../../decisions/008-secret-service-integration.md) | Vault Integration Point | CLI-embedded, capability source |
| [010](../../decisions/010-alpn-router-and-endpoint.md) | ALPN Router and Endpoint | Ed25519 as default curve for TLS raw key identity |
| [014](../../decisions/014-secret-material-flow-and-capability-injection.md) | Secret Material Flow and Capability Injection | Capabilities carry vault-derived material |
| [018](../../decisions/018-vault-standalone-crate.md) | Vault as Standalone Crate | Zero alknet crate dependencies |
| [019](../../decisions/019-vault-assembly-layer-only.md) | Vault Assembly-Layer-Only Access | The assembly layer is the sole caller |
| [020](../../decisions/020-hd-derivation-for-encryption-keys.md) | HD Derivation for Encryption Keys | SLIP-0010 derivation, not PBKDF2; salt unused in v2 |
| [021](../../decisions/021-key-rotation-via-version-indexed-paths.md) | Key Rotation via Version-Indexed Paths | Version-indexed paths; `rotate` re-encrypts |
| [025](../../decisions/025-vault-local-only-dispatch.md) | Vault Local-Only Dispatch | Dropped irpc; direct method calls; local-only by construction |
| [026](../../decisions/026-vault-key-model-hd-derivation.md) | Vault Key Model — HD Derivation | HD derivation from BIP39 seed; `74'` coin type; AES-256-GCM |

## Relevant Open Questions

| OQ | Title | Status | Relevance |
|----|-------|--------|-----------|
| OQ-20 | Encryption key derivation | resolved (ADR-020) | HD derivation from seed; salt field unused in v2 |
| OQ-21 | Remote vault access | resolved (ADR-025) | Vault is local-only by construction; remote access requires a separate vault-server crate with its own ADR |
| OQ-22 | Key rotation mechanism | resolved (ADR-021) | Version-indexed paths; `rotate` method |

## Key Design Principles

1. **Standalone**: The vault depends on no alknet crate and no RPC framework.
   It defines its own types and errors. External crates depend on the vault;
   the vault depends on nothing in alknet.
2. **Assembly-layer only**: The vault's API is consumed by the CLI binary,
   not by handlers. Handlers receive material through capabilities
   (ADR-014). The vault is not on the wire.
3. **Local-only by construction**: The vault has no remote dispatch
   capability. Direct method calls on `VaultServiceHandle` — no actor, no
   message enum, no wire format (ADR-025). Remote access, if ever needed,
   requires a separate crate with its own ADR.
4. **Zeroize everything sensitive**: The mnemonic, seed, derived private
   keys, encryption keys, and cached keys all implement `Zeroize` and
   `ZeroizeOnDrop`. Secret material does not linger in freed heap memory.
5. **Deterministic derivation**: The same mnemonic + passphrase + path
   always produces the same key. Derivation is reproducible across runs
   and across nodes.
6. **OsRng for nonces**: AES-GCM IVs and any cryptographic nonces use
   `OsRng` (or equivalent CSPRNG), never `rand::random()`. IV reuse under
   the same key is catastrophic for GCM.
7. **No `unwrap()` or `expect()` outside tests**: vault operations
   propagate errors. A poisoned lock is recovered with
   `unwrap_or_else(|e| e.into_inner())`, not `unwrap()`. A panic in one
   vault operation must not brick the vault for all other operations.

## Security Constraints

These are security-critical implementation requirements, not architectural
decisions (the architecture is locked by the ADRs above). They are
documented here so implementation agents don't miss them. See
[service.md → Security Constraints](service.md#security-constraints) for
the full list.

- **OsRng for IVs**: AES-GCM IVs must use `OsRng`, not `rand::random()`.
- **Zeroized drop**: `Seed`, `Mnemonic`, `ExtendedPrivKey`,
  `Secp256k1ExtendedPrivKey`, `EncryptionKey`, `CachedKey`, and
  `DerivedKey` all derive `Zeroize` and `ZeroizeOnDrop`. The cache must
  clear on drop, not just on explicit `lock()`.
- **No `unwrap()` outside tests**: poisoned lock recovery uses
  `unwrap_or_else(|e| e.into_inner())` or explicit error propagation.
- **DerivedKey redaction in serialization**: `DerivedKey` serializes the
  `private_key` as `"[REDACTED]"` in all formats (ADR-025 dropped the
  postcard/remote path that previously preserved bytes in binary formats).
  Deserialization rejects `"[REDACTED]"` with an error (resolves review
  #002 W8). The redaction is a defense-in-depth measure for logging safety,
  not the primary control — the primary control is that `DerivedKey` never
  crosses the call protocol wire (ADR-014).

## Public API

The vault re-exports its primary types from the crate root:

```rust
// Mnemonic and seed
pub use mnemonic::{Language, Mnemonic, Seed};

// Derivation
pub use derivation::{DerivationError, ExtendedPrivKey, PATHS};
// Derivation helpers (derive_path_from_seed, parse_derivation_path,
// device_path, encryption_path_for_version) are accessible as
// alknet_vault::derivation::* — not re-exported at crate root to avoid
// clutter, but fully public.

// Encryption
pub use encryption::{EncryptedData, EncryptionError, EncryptionKey};
pub use encryption::CURRENT_KEY_VERSION;

// Key types (DerivedKey, KeyType)
pub use protocol::{DerivedKey, KeyType};

// Service (runtime)
pub use service::{VaultServiceError, VaultServiceHandle};

// Cache
pub use cache::CacheConfig;
```

The `secp256k1` feature flag gates Ethereum (BIP-0032) derivation:

```rust
#[cfg(feature = "secp256k1")]
pub mod ethereum;
```