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128 lines
3.5 KiB
Go
128 lines
3.5 KiB
Go
// Package chain provides chain keys used in double ratchet sessions.
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package chain
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import (
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"crypto/hmac"
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"crypto/sha256"
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"go.mau.fi/libsignal/kdf"
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"go.mau.fi/libsignal/keys/message"
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)
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var messageKeySeed = []byte{0x01}
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var chainKeySeed = []byte{0x02}
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// NewKey returns a new chain key with the given kdf, key, and index
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func NewKey(kdf kdf.HKDF, key []byte, index uint32) *Key {
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chainKey := Key{
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kdf: kdf,
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key: key,
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index: index,
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}
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return &chainKey
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}
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// NewKeyFromStruct will return a chain key built from the given structure.
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func NewKeyFromStruct(structure *KeyStructure, kdf kdf.HKDF) *Key {
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return NewKey(
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kdf,
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structure.Key,
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structure.Index,
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)
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}
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// NewStructFromKey will return a chain key structure for serialization.
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func NewStructFromKey(key *Key) *KeyStructure {
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return &KeyStructure{
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Key: key.key,
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Index: key.index,
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}
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}
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// KeyStructure is a serializeable structure for chain keys.
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type KeyStructure struct {
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Key []byte
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Index uint32
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}
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// Key is used for generating message keys. This key "ratchets" every time a
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// message key is generated. Every time the chain key ratchets forward, its index
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// increases by one.
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type Key struct {
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kdf kdf.HKDF
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key []byte
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index uint32 // Index's maximum size: 4,294,967,295
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}
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// Current returns the current ChainKey struct.
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func (c *Key) Current() *Key {
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return c
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}
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// Key returns the ChainKey's key material.
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func (c *Key) Key() []byte {
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return c.key
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}
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// SetKey will set the ChainKey's key material.
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func (c *Key) SetKey(key []byte) {
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c.key = key
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}
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// Index returns how many times the ChainKey has been "ratcheted" forward.
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func (c *Key) Index() uint32 {
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return c.index
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}
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// SetIndex sets how many times the ChainKey has been "ratcheted" forward.
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func (c *Key) SetIndex(index uint32) {
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c.index = index
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}
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// NextKey uses the key derivation function to generate a new ChainKey.
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func (c *Key) NextKey() *Key {
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nextKey := c.BaseMaterial(chainKeySeed)
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return NewKey(c.kdf, nextKey, c.index+1)
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}
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// MessageKeys returns message keys, which includes the cipherkey, mac, iv, and index.
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func (c *Key) MessageKeys() *message.Keys {
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inputKeyMaterial := c.BaseMaterial(messageKeySeed)
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keyMaterialBytes, _ := c.kdf(inputKeyMaterial, nil, []byte(message.KdfSalt), message.DerivedSecretsSize)
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keyMaterial := newKeyMaterial(keyMaterialBytes)
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// Use the key material returned from the key derivation function for our cipherkey, mac, and iv.
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messageKeys := message.NewKeys(
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keyMaterial.CipherKey, // Use the first 32 bytes of the key material for the CipherKey
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keyMaterial.MacKey, // Use bytes 32-64 of the key material for the MacKey
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keyMaterial.IV, // Use the last 16 bytes for the IV.
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c.Index(), // Attach the chain key's index to the message keys.
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)
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return messageKeys
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}
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// BaseMaterial uses hmac to derive the base material used in the key derivation function for a new key.
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func (c *Key) BaseMaterial(seed []byte) []byte {
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mac := hmac.New(sha256.New, c.key[:])
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mac.Write(seed)
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return mac.Sum(nil)
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}
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// NewKeyMaterial takes an 80-byte slice derived from a key derivation function and splits
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// it into the cipherkey, mac, and iv.
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func newKeyMaterial(keyMaterialBytes []byte) *kdf.KeyMaterial {
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cipherKey := keyMaterialBytes[:32] // Use the first 32 bytes of the key material for the CipherKey
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macKey := keyMaterialBytes[32:64] // Use bytes 32-64 of the key material for the MacKey
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iv := keyMaterialBytes[64:80] // Use the last 16 bytes for the IV.
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keyMaterial := kdf.KeyMaterial{
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CipherKey: cipherKey,
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MacKey: macKey,
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IV: iv,
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}
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return &keyMaterial
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}
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