def encrypt(message: bytes, pk_receiver: EncryptionPublicKey) -> bytes:
"""
Encrypts a string message under a ec25519 public key by using a custom
dhaes-based scheme. See: https://eprint.iacr.org/1999/007
"""
assert \
len(message) == NOTE_LENGTH_BYTES, \
f"expected message length {NOTE_LENGTH_BYTES}, saw {len(message)}"
# Generate ephemeral keypair
eph_keypair = generate_encryption_keypair()
# Compute shared secret and eph key
shared_key = _exchange(eph_keypair.k_sk, pk_receiver)
pk_sender_bytes = encode_encryption_public_key(eph_keypair.k_pk)
# Generate key material
sym_key, mac_key = _kdf(pk_sender_bytes, shared_key)
# Generate symmetric ciphertext
# Chacha encryption
algorithm = algorithms.ChaCha20(sym_key, _SYM_NONCE_VALUE)
cipher = Cipher(algorithm, mode=None, backend=default_backend())
encryptor = cipher.encryptor()
sym_ciphertext = encryptor.update(message)
# Generate mac
mac = poly1305.Poly1305(mac_key)
mac.update(sym_ciphertext)
tag = mac.finalize()
# Arrange ciphertext
return pk_sender_bytes + sym_ciphertext + tag
def generate_poly1305(key, nonce_or_iv):
p = poly1305.Poly1305(key)
def do_computation(msg: bytes):
p.update(msg)
# p.finalize()
return do_computation
def decrypt(encrypted_message: bytes,
sk_receiver: EncryptionSecretKey) -> bytes:
"""
Decrypts a NOTE_LENGTH-byte message by using valid ec25519 private key
objects. See: https://pynacl.readthedocs.io/en/stable/public/
"""
assert \
len(encrypted_message) == ENCRYPTED_NOTE_LENGTH_BYTES, \
"encrypted_message byte-length must be: "+str(ENCRYPTED_NOTE_LENGTH_BYTES)
assert(isinstance(sk_receiver, X25519PrivateKey)), \
f"PrivateKey: {sk_receiver} ({type(sk_receiver)})"
# Compute shared secret
pk_sender_bytes = encrypted_message[:EC_PUBLIC_KEY_LENGTH_BYTES]
pk_sender = decode_encryption_public_key(pk_sender_bytes)
shared_key = _exchange(sk_receiver, pk_sender)
# Generate key material and recover keys
sym_key, mac_key = _kdf(pk_sender_bytes, shared_key)
# ct_sym and mac
ct_sym = encrypted_message[
EC_PUBLIC_KEY_LENGTH_BYTES:EC_PUBLIC_KEY_LENGTH_BYTES +
NOTE_LENGTH_BYTES]
tag = encrypted_message[EC_PUBLIC_KEY_LENGTH_BYTES +
NOTE_LENGTH_BYTES:EC_PUBLIC_KEY_LENGTH_BYTES +
NOTE_LENGTH_BYTES + _TAG_LENGTH_BYTES]
# Verify the mac
mac = poly1305.Poly1305(mac_key)
mac.update(ct_sym)
mac.verify(tag)
# Decrypt sym ciphertext
algorithm = algorithms.ChaCha20(sym_key, _SYM_NONCE_VALUE)
cipher = Cipher(algorithm, mode=None, backend=default_backend())
decryptor = cipher.decryptor()
message = decryptor.update(ct_sym)
return message