NIP44 encrypted messages for nostr. Spec copied from github.com/nostr-protocol/nips. Audited on 2023.12 by Cure53, Dr M. Heiderich, Dr. Mazaheri, Dr. Bleichenbacher.
Language | License | Copied from | Note |
---|---|---|---|
F# | GPL 2 | https://github.com/lontivero/Nostra | |
Go | MIT | https://git.ekzyis.com/ekzyis/nip44 | Unsafe |
Kotlin | MIT | https://github.com/vitorpamplona/amethyst | |
Rust | MIT | https://github.com/mikedilger/nip44 | |
Swift | MIT | https://github.com/nostr-sdk/nostr-sdk-ios | |
TypeScript / JS | Public domain | https://github.com/nostr-protocol/nips |
optional
The NIP introduces a new data format for keypair-based encryption. This NIP is versioned to allow multiple algorithm choices to exist simultaneously. This format may be used for many things, but MUST be used in the context of a signed event as described in NIP 01.
Note: this format DOES NOT define any kind
s related to a new direct messaging standard,
only the encryption required to define one. It SHOULD NOT be used as a drop-in replacement
for NIP 04 payloads.
Currently defined encryption algorithms:
0x00
- Reserved0x01
- Deprecated and undefined0x02
- secp256k1 ECDH, HKDF, padding, ChaCha20, HMAC-SHA256, base64
Every nostr user has their own public key, which solves key distribution problems present in other solutions. However, nostr's relay-based architecture makes it difficult to implement more robust private messaging protocols with things like metadata hiding, forward secrecy, and post compromise secrecy.
The goal of this NIP is to have a simple way to encrypt payloads used in the context of a signed event. When applying this NIP to any use case, it's important to keep in mind your users' threat model and this NIP's limitations. For high-risk situations, users should chat in specialized E2EE messaging software and limit use of nostr to exchanging contacts.
On its own, messages sent using this scheme have a number of important shortcomings:
- No deniability: it is possible to prove an event was signed by a particular key
- No forward secrecy: when a key is compromised, it is possible to decrypt all previous conversations
- No post-compromise security: when a 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:
created_at
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 may be partially mitigated by only sending messages to trusted relays, and asking relays to delete stored messages after a certain duration has elapsed.
NIP-44 version 2 has the following design characteristics:
- Payloads are authenticated using a MAC before signing rather than afterwards because events are assumed to be signed as specified in NIP-01. The outer signature serves to authenticate the full payload, and MUST be validated before decrypting.
- ChaCha is used instead of AES because it's faster and has better security against multi-key attacks.
- ChaCha is used instead of XChaCha because XChaCha has not been standardized. Also, xChaCha's improved collision resistance of nonces isn't necessary since every message has a new (key, nonce) pair.
- HMAC-SHA256 is used instead of Poly1305 because polynomial MACs are much easier to forge.
- SHA256 is used instead of SHA3 or BLAKE because it is already used in nostr. Also BLAKE's speed advantage is smaller in non-parallel environments.
- A custom padding scheme is used instead of padmé because it provides better leakage reduction for small messages.
- Base64 encoding is used instead of another compression algorithm because it is widely available, and is already used in nostr.
- Calculate a 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
andsalt=utf8_encode('nip44-v2')
- HKDF output will be a
conversation_key
between two users. - It is always the same, when key roles are swapped:
conv(a, B) == conv(b, A)
- Execute ECDH (scalar multiplication) of public key B by private key A
Output
- Generate a random 32-byte nonce
- Always use CSPRNG
- Don't generate a nonce from message content
- Don't re-use the same nonce between messages: doing so would make them decryptable, but won't leak the long-term key
- Calculate message keys
- The keys are generated from
conversation_key
andnonce
. Validate that both are 32 bytes long - Use HKDF-expand, with sha256,
OKM=conversation_key
,info=nonce
andL=76
- Slice 76-byte HKDF output into:
chacha_key
(bytes 0..32),chacha_nonce
(bytes 32..44),hmac_key
(bytes 44..76)
- The keys are generated from
- 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
- Encrypt padded content
- Use ChaCha20, with key and nonce from step 3
- Calculate MAC (message authentication code)
- AAD (additional authenticated data) is used - instead of calculating MAC on ciphertext,
it's calculated over a concatenation of
nonce
andciphertext
- Validate that AAD (nonce) is 32 bytes
- AAD (additional authenticated data) is used - instead of calculating MAC on ciphertext,
it's calculated over a concatenation of
- Base64-encode (with padding) params using
concat(version, nonce, ciphertext, mac)
Encrypted payloads MUST be included in an event's payload, hashed, and signed as defined in NIP 01, using schnorr signature scheme over secp256k1.
Before decryption, the event's pubkey and signature MUST be validated as defined in NIP 01. The public key MUST be a valid non-zero secp256k1 curve point, and the signature must be valid secp256k1 schnorr signature. For exact validation rules, refer to BIP-340.
- 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 throwingbase64 is invalid
, implementations MUST indicate that the encryption version is not yet supported
- Decode base64
- Base64 is decoded into
version, nonce, ciphertext, mac
- If the version is unknown, implementations must indicate that the encryption version is not 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
- Base64 is decoded into
- Calculate conversation key
- See step 1 of (encryption)[#Encryption]
- Calculate message keys
- See step 3 of (encryption)[#Encryption]
- 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
- Decrypt ciphertext
- Use ChaCha20 with key and nonce from step 3
- 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 step 3 of the (encryption)[#Encryption] process matches the actual padding
- 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 methodshkdf_extract(IKM, salt)
andhkdf_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 a shared point, and we encode the shared point's 32-byte x coordinate, using methodbytes(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]
, wherex
is a byte array andi, j <= 0
returns a(j - i)
-byte array with a copy of thei
-th byte (inclusive) to thej
-th byte (exclusive) ofx
.
- 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)
andbase64_decode(bytes)
are Base64 (RFC 4648, with padding)concat
refers to byte array concatenationis_equal_ct(a, b)
is constant-time equality check of 2 byte arraysutf8_encode(string)
andutf8_decode(bytes)
transform string to byte array and backwrite_u8(number)
restricts number to values 0..255 and encodes into Big-Endian uint8 byte arraywrite_u16_be(number)
restricts number to values 0..65535 and encodes into Big-Endian uint16 byte arrayzeros(length)
creates byte array of lengthlength >= 0
, filled with zerosfloor(number)
andlog2(number)
are well-known mathematical methods
The following is a collection of python-like pseudocode functions which implement the above primitives, intended to guide impelmenters. A collection of implementations in different languages is available at https://github.com/paulmillr/nip44.
# 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)
The v2 of the standard was audited by Cure53 in December 2023. Check out audit-2023.12.pdf and auditor's website.
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 a 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 pub2valid.get_message_keys
: calculate chacha_key, chacha_nocne, hmac_key from conversation_key and noncevalid.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 errorinvalid.decrypt
: decrypting message content must throw an error