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// Package root provides root keys which are used to derive new chain and
// root keys in a ratcheting session.
package root
import (
"go.mau.fi/libsignal/ecc"
"go.mau.fi/libsignal/kdf"
"go.mau.fi/libsignal/keys/chain"
"go.mau.fi/libsignal/keys/session"
)
// DerivedSecretsSize is the size of the derived secrets for root keys.
const DerivedSecretsSize = 64
// KdfInfo is used as the info for message keys to derive secrets using a Key Derivation Function
const KdfInfo string = "WhisperRatchet"
// NewKey returns a new RootKey given the key derivation function and bytes.
func NewKey(kdf kdf.HKDF, key []byte) *Key {
rootKey := Key{
kdf: kdf,
key: key,
}
return &rootKey
}
// Key is a structure for RootKeys, which are used to derive a new set of chain and root
// keys for every round trip of messages.
type Key struct {
kdf kdf.HKDF
key []byte
}
// Bytes returns the RootKey in bytes.
func (k *Key) Bytes() []byte {
return k.key
}
// CreateChain creates a new RootKey and ChainKey from the recipient's ratchet key and our private key.
func (k *Key) CreateChain(theirRatchetKey ecc.ECPublicKeyable, ourRatchetKey *ecc.ECKeyPair) (*session.KeyPair, error) {
theirPublicKey := theirRatchetKey.PublicKey()
ourPrivateKey := ourRatchetKey.PrivateKey().Serialize()
// Use our key derivation function to calculate a shared secret.
sharedSecret := kdf.CalculateSharedSecret(theirPublicKey, ourPrivateKey)
derivedSecretBytes, err := kdf.DeriveSecrets(sharedSecret[:], k.key, []byte(KdfInfo), DerivedSecretsSize)
if err != nil {
return nil, err
}
// Split the derived secret bytes in half, using one half for the root key and the second for the chain key.
derivedSecrets := session.NewDerivedSecrets(derivedSecretBytes)
// Create new root and chain key structures from the derived secrets.
rootKey := NewKey(k.kdf, derivedSecrets.RootKey())
chainKey := chain.NewKey(k.kdf, derivedSecrets.ChainKey(), 0)
// Create a session keypair with the generated root and chain keys.
keyPair := session.NewKeyPair(
rootKey,
chainKey,
)
return keyPair, nil
}
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