Encryption

Bitmessage uses the Elliptic Curve Integrated Encryption Scheme (ECIES)[1] to encrypt the payload of the Message and Broadcast objects.

The scheme uses Elliptic Curve Diffie-Hellman (ECDH)[2] to generate a shared secret used to generate the encryption parameters for Advanced Encryption Standard with 256bit key and Cipher-Block Chaining (AES-256-CBC)[3]. The encrypted data will be padded to a 16 byte boundary in accordance to PKCS7[4]. This means that the data is padded with N bytes of value N.

The Key Derivation Function (KDF)[5] used to generate the key material for AES is SHA512[6]. The Message Authentication Code (MAC) scheme used is HMACSHA256[7].

Format

(See also: Protocol specification)

Field Size	Description	Data type	Comments
16	IV	uchar[]	Initialization Vector used for AES-256-CBC
2	uint16_t	Curve type	Elliptic Curve type 0x02CA (714)
2	uint16_t	X length	Length of X component of public key R
X length	uchar[]	X	X component of public key R
2	uint16_t	Y length	Length of Y component of public key R
Y length	uchar[]	Y	Y component of public key R
?	encrypted	uchar[]	Cipher text
32	MAC	uchar[]	HMACSHA256 Message Authentication Code

In order to reconstitute a usable (65 byte) public key (starting with 0x04), the X and Y components need to be expanded by prepending them with 0x00 bytes until the individual component lengths are 32 bytes.

Encryption

1. The destination public key is called K.
2. Generate 16 random bytes using a secure random number generator. Call them IV.
3. Generate a new random EC key pair with private key called r and public key called R.
4. Do an EC point multiply with public key K and private key r. This gives you public key P.
5. Use the X component of public key P and calculate the SHA512 hash H.
6. The first 32 bytes of H are called key_e and the last 32 bytes are called key_m.
7. Pad the input text to a multiple of 16 bytes, in accordance to PKCS7.
8. Encrypt the data with AES-256-CBC, using IV as initialization vector, key_e as encryption key and the padded input text as payload. Call the output cipher text.
9. Calculate a 32 byte MAC with HMACSHA256, using key_m as salt and IV + R + cipher text as data. Call the output MAC.

The resulting data is: IV + R + cipher text + MAC

Decryption

1. The private key used to decrypt is called k.
2. Do an EC point multiply with private key k and public key R. This gives you public key P.
3. Use the X component of public key P and calculate the SHA512 hash H.
4. The first 32 bytes of H are called key_e and the last 32 bytes are called key_m.
5. Calculate MAC' with HMACSHA256, using key_m as salt and IV + R + cipher text as data.
6. Compare MAC with MAC'. If not equal, decryption will fail.
7. Decrypt the cipher text with AES-256-CBC, using IV as initialization vector, key_e as decryption key and the cipher text as payload. The output is the padded input text.

Partial Example

Public key K:

Data	Comments
04 09 d4 e5 c0 ab 3d 25 fe 04 8c 64 c9 da 1a 24 2c 7f 19 41 7e 95 17 cd 26 69 50 d7 2c 75 57 13 58 5c 61 78 e9 7f e0 92 fc 89 7c 9a 1f 17 20 d5 77 0a e8 ea ad 2f a8 fc bd 08 e9 32 4a 5d de 18 57	Public key, 0x04 prefix, then 32 bytes X and 32 bytes Y.

Initialization Vector IV:

Data	Comments
bd db 7c 28 29 b0 80 38 75 30 84 a2 f3 99 16 81	16 bytes generated with a secure random number generator.

Randomly generated key pair with private key r and public key R:

Data	Comments
5b e6 fa cd 94 1b 76 e9 d3 ea d0 30 29 fb db 6b 6e 08 09 29 3f 7f b1 97 d0 c5 1f 84 e9 6b 8b a4	Private key r
04 02 93 21 3d cf 13 88 b6 1c 2a e5 cf 80 fe e6 ff ff c0 49 a2 f9 fe 73 65 fe 38 67 81 3c a8 12 92 df 94 68 6c 6a fb 56 5a c6 14 9b 15 3d 61 b3 b2 87 ee 2c 7f 99 7c 14 23 87 96 c1 2b 43 a3 86 5a	Public key R

Derived public key P (point multiply r with K):

Data	Comments
04 0d b8 e3 ad 8c 0c d7 3f a2 b3 46 71 b7 b2 47 72 9b 10 11 41 57 9d 19 9e 0d c0 bd 02 4e ae fd 89 ca c8 f5 28 dc 90 b6 68 11 ab ac 51 7d 74 97 be 52 92 93 12 29 be 0b 74 3e 05 03 f4 43 c3 d2 96	Public key P
0d b8 e3 ad 8c 0c d7 3f a2 b3 46 71 b7 b2 47 72 9b 10 11 41 57 9d 19 9e 0d c0 bd 02 4e ae fd 89	X component of public key P

SHA512 of public key P X component (H):

Data	Comments
17 05 43 82 82 67 86 71 05 26 3d 48 28 ef ff 82 d9 d5 9c bf 08 74 3b 69 6b cc 5d 69 fa 18 97 b4	First 32 bytes of H called key_e
f8 3f 1e 9c c5 d6 b8 44 8d 39 dc 6a 9d 5f 5b 7f 46 0e 4a 78 e9 28 6e e8 d9 1c e1 66 0a 53 ea cd	Last 32 bytes of H called key_m

Padded input:

Data	Comments
54 68 65 20 71 75 69 63 6b 20 62 72 6f 77 6e 20 66 6f 78 20 6a 75 6d 70 73 20 6f 76 65 72 20 74 68 65 20 6c 61 7a 79 20 64 6f 67 2e 04 04 04 04	The quick brown fox jumps over the lazy dog.0x04,0x04,0x04,0x04

Cipher text:

Data	Comments
64 20 3d 5b 24 68 8e 25 47 bb a3 45 fa 13 9a 5a 1d 96 22 20 d4 d4 8a 0c f3 b1 57 2c 0d 95 b6 16 43 a6 f9 a0 d7 5a f7 ea cc 1b d9 57 14 7b f7 23	3 blocks of 16 bytes of encrypted data.