Struct EncryptionLayer 

pub struct EncryptionLayer { /* private fields */ }

Zero-knowledge encryption layer with per-tenant key derivation.

Holds a derived encryption key (zeroized on drop), a Keyring of master keys for multi-key decrypt during rotation, and the ZeroKnowledgeEncryptor from cachekit-core for AES-256-GCM operations.

L1 stores ciphertext, not plaintext — the zero-knowledge property is preserved across all cache layers.

Key rotation

Writes always encrypt under the current master key. Reads decrypt via the keyring: sequential attempts, current key first, then each decrypt-only previous key in order, rebuilding the identical AAD per attempt (cachekit-rs entries carry no per-entry key identity — sequential attempts are the spec-assigned branch, protocol/spec/encryption.md → “Key Rotation (Keyring)”). Reads served by a previous key are counted — see Self::previous_key_hits for the rotation drain workflow.

Implementations

impl EncryptionLayer

pub fn new( master_key_bytes: &[u8], tenant_id: &str, ) -> Result<Self, CachekitError>

Create a new encryption layer with HKDF-derived tenant keys.

Equivalent to Self::with_previous_keys with no previous keys.

Arguments

• master_key_bytes — Raw master key (minimum 32 bytes for AES-256)
• tenant_id — Tenant identifier for cryptographic isolation

Errors

• Master key too short (< 32 bytes)
• HKDF derivation failure
• Encryptor initialization failure

pub fn with_previous_keys( master_key_bytes: &[u8], previous_keys: &[&[u8]], tenant_id: &str, ) -> Result<Self, CachekitError>

Create an encryption layer with decrypt-only previous master keys.

previous_keys are retained during a key-rotation grace window, in attempt order. Writes always use master_key_bytes; reads attempt it first, then each previous key sequentially.

Errors

• Any key too short (< 32 bytes)
• More than 3 previous keys (CachekitError::Config — rejected, never truncated)
• The current master key also present in previous_keys (CachekitError::Config — rotation is forward-only)
• HKDF derivation failure
• Encryptor initialization failure

pub fn encrypt( &self, plaintext: &[u8], cache_key: &str, ) -> Result<Vec<u8>, CachekitError>

Encrypt plaintext with AAD bound to the cache key.

Output format: [nonce(12)][ciphertext + auth_tag(16)]

pub fn decrypt( &self, ciphertext: &[u8], cache_key: &str, ) -> Result<Vec<u8>, CachekitError>

Decrypt ciphertext with AAD bound to the cache key.

Returns the original plaintext. Fails if the cache key does not match the one used during encryption (ciphertext substitution protection).

Decryption goes through the keyring: the current master key is attempted first, then each decrypt-only previous key in order, with the identical AAD per attempt. Entries written before a key rotation stay readable as long as their key remains in the previous list.

A read served by a previous key is counted in Self::previous_key_hits (the rotation drain signal).

pub fn previous_key_hits(&self) -> Vec<u64>

Reads decrypted by each previous key, by position in the previous-key list (hits[i] ↔ previous_keys[i]); empty when there are none. Reads served by the current key are not counted.

This is the rotation drain signal. It confirms that a grace window has drained; it does not shorten one. Follow the protocol’s scheduled-rotation runbook: audit for non-expiring entries first, add the incoming key as decrypt-only fleet-wide, then promote it. The window clock starts only when that promotion deploy has completed on every instance — until then a lagging instance still writes fresh ciphertext under the retiring key and reads it silently as its current key (index 0). From that point, wait at least the longest TTL in use (per-entry TTLs passed to set_with_ttl count, not just the default), aggregating counts across every instance — they are per process and reset on restart. Only when the retiring key’s count has stayed flat over that whole window has every live entry aged out or been re-encrypted on write, and the key can be dropped from the previous list without a hard cut-over. Positions are comparable across instances only once they all run the same keyring configuration. The signal carries no key material — positions and counts only.

use cachekit::EncryptionLayer;

let k1 = [0x11u8; 32]; // retiring master key
let k2 = [0x22u8; 32]; // current master key after rotation

// Encrypted under k1, before the rotation...
let ciphertext = EncryptionLayer::new(&k1, "tenant-123")?.encrypt(b"cached value", "user:1")?;

// ...a layer [current=k2, previous=[k1]] serves it from previous[0]:
// the retiring key is still draining, not yet safe to drop.
let layer = EncryptionLayer::with_previous_keys(&k2, &[&k1], "tenant-123")?;
assert_eq!(layer.previous_key_hits(), vec![0]);
assert_eq!(layer.decrypt(&ciphertext, "user:1")?, b"cached value");
assert_eq!(layer.previous_key_hits(), vec![1]);

pub fn hardware_acceleration_enabled(&self) -> bool

Whether cachekit-core detected AES hardware support on this host.

Forwards ZeroKnowledgeEncryptor::hardware_acceleration_enabled(). Informational only — ring/aes-gcm dispatch independently of it; the per-architecture semantics are core’s (README → Zero-Knowledge Encryption).

pub fn tenant_id(&self) -> &str

Return the tenant ID used for key derivation.

pub fn build_aad(&self, cache_key: &str, compressed: bool) -> Vec<u8>

Build AAD v0x03 for a given cache key and compression flag.

Format: [0x03][len][tenant_id][len][cache_key][len]["msgpack"][len]["True"/"False"]

All lengths are 4-byte big-endian u32 to prevent boundary-confusion attacks.

Invariant: compressed is always false in production

encrypt and decrypt pass the literal false, and that is normative, not a gap: this SDK’s only cross-SDK encrypted surface is interop mode, and protocol/spec/interop-mode.md (“Encryption in Interop Mode”) mandates the AAD components format = "msgpack", compressed = "False" — there is no compression in interop mode.

Why a reader cannot recover from a wrong flag here: in the general flow a reader rebuilds the AAD from the stored cleartext metadata written alongside the ciphertext (protocol/spec/encryption.md, encrypt step 6 / decrypt step 3). cachekit-rs stores no such per-entry metadata — it writes plain nonce ‖ ciphertext ‖ tag with no header — so its readers have nothing to read the flag from and must reconstruct the AAD from the interop-pinned constants alone. Combined with the no-retry rule (a reader MUST NOT retry decryption with any alternative AAD input, protocol/spec/encryption.md), emitting compressed = "True" here would produce ciphertext that no conformant peer can authenticate, and none may probe for.

So do not flip this literal to thread a live compression flag through encrypt/decrypt. If this SDK ever does compress, the flag must be stored per entry and threaded from that stored value — protocol/spec/encryption.md requires compressed to describe the actual plaintext, and claiming "False" over compressed bytes is the cachekit-py#166 conformance bug (round-trips in-process, fails authentication for every correct second reader). A compressed cross-SDK profile is a versioned protocol change (interop/v2), not an SDK flag.

The "True"/"False" tokens are frozen byte-level protocol constants — normative byte table in protocol/spec/encryption.md, section “compressed tokens”; decided in protocol#12 (resolved 2026-07-19: a spec correction, not a wire change). The true branch stays, exercised by conformance tests, so those bytes remain pinned.

Trait Implementations

impl Debug for EncryptionLayer

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.