nips/44.md

NIP-44

Encrypted Payloads (Versioned)

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The NIP introduces a new data format for keypair-based encryption. This NIP is versioned to allow multiple algorithm choices to exist simultaneously.

Nostr is a key directory. Every nostr user has their own public key, which solves key distribution problems present in other solutions. The goal of this NIP is to have a simple way to send messages between nostr accounts that cannot be read by everyone.

The scheme has a number of important shortcomings:

• No deniability: it is possible to prove the event was signed by a particular key
• No forward secrecy: when a user key is compromised, it is possible to decrypt all previous conversations
• No post-compromise security: when a user key is compromised, it is possible to decrypt all future conversations
• No post-quantum security: a powerful quantum computer would be able to decrypt the messages
• IP address leak: user IP may be seen by relays and all intermediaries between user and relay
• Date leak: the message date is public, since it is a part of NIP 01 event
• Limited message size leak: padding only partially obscures true message length
• No attachments: they are not supported

Lack of forward secrecy is partially mitigated by these two factors:

1. the messages should only be stored on relays, specified by the user, instead of a set of all public relays.
2. the relays are supposed to regularly delete older messages.

For risky situations, users should chat in specialized E2EE messaging software and limit use of nostr to exchanging contacts.

Dependence on NIP-01

It's not enough to use NIP-44 for encryption: the output must also be signed.

In nostr case, the payload is serialized and signed as per NIP-01 rules.

The same event can be serialized in two different ways, resulting in two distinct signatures. So, it's important to ensure serialization rules, which are defined in NIP-01, are the same across different NIP-44 implementations.

After serialization, the event is signed by Schnorr signature over secp256k1, defined in BIP340. It's important to ensure the key and signature validity as per BIP340 rules.

Versions

Currently defined encryption algorithms:

• 0x00 - Reserved
• 0x01 - Deprecated and undefined
• 0x02 - secp256k1 ECDH, HKDF, padding, ChaCha20, HMAC-SHA256, base64

Version 2

The algorithm choices are justified in a following way:

• Encrypt-then-mac-then-sign instead of encrypt-then-sign-then-mac: only events wrapped in NIP-01 signed envelope are currently accepted by nostr.
• ChaCha instead of AES: it's faster and has better security against multi-key attacks
• ChaCha instead of XChaCha: XChaCha has not been standardized. Also, we don't need xchacha's improved collision resistance of nonces: every message has a new (key, nonce) pair.
• HMAC-SHA256 instead of Poly1305: polynomial MACs are much easier to forge SHA256 instead of SHA3 or BLAKE: it is already used in nostr. Also blake's speed advantage is smaller in non-parallel environments - Custom padding instead of padmé: better leakage reduction for small messages
• Base64 encoding instead of an other compression algorithm: it is widely available, and is already used in nostr

Functions and operations

• Cryptographic methods
  • secure_random_bytes(length) fetches randomness from CSPRNG
  • hkdf(IKM, salt, info, L) represents HKDF (RFC 5869) with SHA256 hash function, comprised of methods hkdf_extract(IKM, salt) and hkdf_expand(OKM, info, L)
  • chacha20(key, nonce, data) is ChaCha20 (RFC 8439), with starting counter set to 0
  • hmac_sha256(key, message) is HMAC (RFC 2104)
  • secp256k1_ecdh(priv_a, pub_b) is multiplication of point B by scalar a (a ⋅ B), defined in BIP340. The operation produces shared point, and we encode the shared point's 32-byte x coordinate, using method bytes(P) from BIP340. Private and public keys must be validated as per BIP340: pubkey must be a valid, on-curve point, and private key must be a scalar in range [1, secp256k1_order - 1]
• Operators
  • x[i:j], where x is a byte array and i, j <= 0, returns a (j - i)-byte array with a copy of the i-th byte (inclusive) to the j-th byte (exclusive) of x
• Constants c:
  • min_plaintext_size is 1. 1b msg is padded to 32b.
  • max_plaintext_size is 65535 (64kb - 1). It is padded to 65536.
• Functions
  • base64_encode(string) and base64_decode(bytes) are Base64 (RFC 4648, with padding)
  • concat refers to byte array concatenation
  • is_equal_ct(a, b) is constant-time equality check of 2 byte arrays
  • utf8_encode(string) and utf8_decode(bytes) transform string to byte array and back
  • write_u8(number) restricts number to values 0..255 and encodes into Big-Endian uint8 byte array
  • write_u16_be(number) restricts number to values 0..65535 and encodes into Big-Endian uint16 byte array
  • zeros(length) creates byte array of length length >= 0, filled with zeros
  • floor(number) and log2(number) are well-known mathematical methods

User-defined functions:

# Calculates length of the padded byte array.
def calc_padded_len(unpadded_len):
  next_power = 1 << (floor(log2(unpadded_len - 1))) + 1
  if next_power <= 256:
    chunk = 32
  else:
    chunk = next_power / 8
  if unpadded_len <= 32:
    return 32
  else:
    return chunk * (floor((len - 1) / chunk) + 1)

# Converts unpadded plaintext to padded bytearray
def pad(plaintext):
  unpadded = utf8_encode(plaintext)
  unpadded_len = len(plaintext)
  if (unpadded_len < c.min_plaintext_size or
      unpadded_len > c.max_plaintext_size): raise Exception('invalid plaintext length')
  prefix = write_u16_be(unpadded_len)
  suffix = zeros(calc_padded_len(unpadded_len) - unpadded_len)
  return concat(prefix, unpadded, suffix)

# Converts padded bytearray to unpadded plaintext
def unpad(padded):
  unpadded_len = read_uint16_be(padded[0:2])
  unpadded = padded[2:2+unpadded_len]
  if (unpadded_len == 0 or
      len(unpadded) != unpadded_len or
      len(padded) != 2 + calc_padded_len(unpadded_len)): raise Exception('invalid padding')
  return utf8_decode(unpadded)

# metadata: always 65b (version: 1b, nonce: 32b, max: 32b)
# plaintext: 1b to 0xffff
# padded plaintext: 32b to 0xffff
# ciphertext: 32b+2 to 0xffff+2
# raw payload: 99 (65+32+2) to 65603 (65+0xffff+2)
# compressed payload (base64): 132b to 87472b
def decode_payload(payload):
  plen = len(payload)
  if plen == 0 or payload[0] == '#': raise Exception('unknown version')
  if plen < 132 or plen > 87472: raise Exception('invalid payload size')
  data = base64_decode(payload)
  dlen = len(d)
  if dlen < 99 or dlen > 65603: raise Exception('invalid data size');
  vers = data[0]
  if vers != 2: raise Exception('unknown version ' + vers)
  nonce = data[1:33]
  ciphertext = data[33:dlen - 32]
  mac = data[dlen - 32:dlen]
  return (nonce, ciphertext, mac)

def hmac_aad(key, message, aad):
  if len(aad) != 32: raise Exception('AAD associated data must be 32 bytes');
  return hmac(sha256, key, concat(aad, message));

# Calculates long-term key between users A and B: `get_key(Apriv, Bpub) == get_key(Bpriv, Apub)`
def get_conversation_key(private_key_a, public_key_b):
  shared_x = secp256k1_ecdh(private_key_a, public_key_b)
  return hkdf_extract(IKM=shared_x, salt=utf8_encode('nip44-v2'))

# Calculates unique per-message key
def get_message_keys(conversation_key, nonce):
  if len(conversation_key) != 32: raise Exception('invalid conversation_key length')
  if len(nonce) != 32: raise Exception('invalid nonce length')
  keys = hkdf_expand(OKM=conversation_key, info=nonce, L=76)
  chacha_key = keys[0:32]
  chacha_nonce = keys[32:44]
  hmac_key = keys[44:76]
  return (chacha_key, chacha_nonce, hmac_key)

def encrypt(plaintext, conversation_key, nonce):
  (chacha_key, chacha_nonce, hmac_key) = get_message_keys(conversation_key, nonce)
  padded = pad(plaintext)
  ciphertext = chacha20(key=chacha_key, nonce=chacha_nonce, data=padded)
  mac = hmac_aad(key=hmac_key, message=ciphertext, aad=nonce)
  return base64_encode(concat(write_u8(2), nonce, ciphertext, mac))

def decrypt(payload, conversation_key):
  (nonce, ciphertext, mac) = decode_payload(payload)
  (chacha_key, chacha_nonce, hmac_key) = get_message_keys(conversation_key, nonce)
  calculated_mac = hmac_aad(key=hmac_key, message=ciphertext, aad=nonce)
  if not is_equal_ct(calculated_mac, mac): raise Exception('invalid MAC')
  padded_plaintext = chacha20(key=chacha_key, nonce=chacha_nonce, data=ciphertext)
  return unpad(padded_plaintext)

# Usage:
#   conversation_key = get_conversation_key(sender_privkey, recipient_pubkey)
#   nonce = secure_random_bytes(32)
#   payload = encrypt('hello world', conversation_key, nonce)
#   'hello world' == decrypt(payload, conversation_key)

Encryption

1. Calculate conversation key
   • Execute ECDH (scalar multiplication) of public key B by private key A. Output shared_x must be unhashed, 32-byte encoded x coordinate of the shared point.
   • Use HKDF-extract with sha256, IKM=shared_x and salt=utf8_encode('nip44-v2')
   • HKDF output will be conversation_key between two users
   • It is always the same, when key roles are swapped: conv(a, B) == conv(b, A)
2. Generate random 32-byte nonce
   • Always use CSPRNG
   • Don't generate nonce from message content
   • Don't re-use the same nonce between messages: doing so would make them decryptable, but won't leak long-term key
3. Calculate message keys
   • The keys are generated from conversation_key and nonce. Validate that both are 32 bytes
   • Use HKDF-expand, with sha256, OKM=conversation_key, info=nonce and L=76
   • Slice 76-byte HKDF output into: chacha_key (bytes 0..32), chacha_nonce (bytes 32..44), hmac_key (bytes 44..76)
4. Add padding
   • Content must be encoded from UTF-8 into byte array
   • Validate plaintext length. Minimum is 1 byte, maximum is 65535 bytes
   • Padding format is: [plaintext_length: u16][plaintext][zero_bytes]
   • Padding algorithm is related to powers-of-two, with min padded msg size of 32
   • Plaintext length is encoded in big-endian as first 2 bytes of the padded blob
5. Encrypt padded content
   • Use ChaCha20, with key and nonce from step 3
6. Calculate MAC (message authentication code) with AAD
   • AAD is used: instead of calculating MAC on ciphertext, it's calculated over a concatenation of nonce and ciphertext
   • Validate that AAD (nonce) is 32 bytes
7. Base64-encode (with padding) params: concat(version, nonce, ciphertext, mac)

After encryption, it's necessary to sign it. Use NIP-01 to serialize the event, with result base64 assigned to event's content. Then, use NIP-01 to sign the event using schnorr signature scheme over secp256k1.

Decryption

Before decryption, it's necessary to validate the message's pubkey and signature. The public key must be a valid non-zero secp256k1 curve point, and signature must be valid secp256k1 schnorr signature. For exact validation rules, refer to BIP-340.

1. Check if first payload's character is #
   • # is an optional future-proof flag that means non-base64 encoding is used
   • The # is not present in base64 alphabet, but, instead of throwing base64 is invalid, an app must say the encryption version is not yet supported
2. Decode base64
   • Base64 is decoded into version, nonce, ciphertext, mac
   • If the version is unknown, the app, an app must say the encryption version is not yet supported
   • Validate length of base64 message to prevent DoS on base64 decoder: it can be in range from 132 to 87472 chars
   • Validate length of decoded message to verify output of the decoder: it can be in range from 99 to 65603 bytes
3. Calculate conversation key
   • See step 1 of Encryption
4. Calculate message keys
   • See step 3 of Encryption
5. Calculate MAC (message authentication code) with AAD and compare
   • Stop and throw an error if MAC doesn't match the decoded one from step 2
   • Use constant-time comparison algorithm
6. Decrypt ciphertext
   • Use ChaCha20 with key and nonce from step 3
7. Remove padding
   • Read the first two BE bytes of plaintext that correspond to plaintext length
   • Verify that the length of sliced plaintext matches the value of the two BE bytes
   • Verify that calculated padding from encryption's step 3 matches the actual padding

Audit

The v2 of the standard has been subject to an audit by Cure53 in December 2023. Check out audit-2023.12.pdf and auditor's website.

Tests and code

A collection of implementations in different languages is available at https://github.com/paulmillr/nip44.

We publish extensive test vectors. Instead of having it in the document directly, a sha256 checksum of vectors is provided:

269ed0f69e4c192512cc779e78c555090cebc7c785b609e338a62afc3ce25040  nip44.vectors.json

Example of test vector from the file:

{
  "sec1": "0000000000000000000000000000000000000000000000000000000000000001",
  "sec2": "0000000000000000000000000000000000000000000000000000000000000002",
  "conversation_key": "c41c775356fd92eadc63ff5a0dc1da211b268cbea22316767095b2871ea1412d",
  "nonce": "0000000000000000000000000000000000000000000000000000000000000001",
  "plaintext": "a",
  "payload": "AgAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABee0G5VSK0/9YypIObAtDKfYEAjD35uVkHyB0F4DwrcNaCXlCWZKaArsGrY6M9wnuTMxWfp1RTN9Xga8no+kF5Vsb"
}

The file also contains intermediate values. A quick guidance with regards to its usage:

• valid.get_conversation_key: calculate conversation_key from secret key sec1 and public key pub2
• valid.get_message_keys: calculate chacha_key, chacha_nocne, hmac_key from conversation_key and nonce
• valid.calc_padded_len: take unpadded length (first value), calculate padded length (second value)
• valid.encrypt_decrypt: emulate real conversation. Calculate pub2 from sec2, verify conversation_key from (sec1, pub2), encrypt, verify payload, then calculate pub1 from sec1, verify conversation_key from (sec2, pub1), decrypt, verify plaintext.
• valid.encrypt_decrypt_long_msg: same as previous step, but instead of a full plaintext and payload, their checksum is provided.
• invalid.encrypt_msg_lengths
• invalid.get_conversation_key: calculating converastion_key must throw an error
• invalid.decrypt: decrypting message content must throw an error