Encryption ========== Bitmessage uses the Elliptic Curve Integrated Encryption Scheme `(ECIES) `_ to encrypt the payload of the Message and Broadcast objects. The scheme uses Elliptic Curve Diffie-Hellman `(ECDH) `_ 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) `_. The encrypted data will be padded to a 16 byte boundary in accordance to `PKCS7 `_. This means that the data is padded with N bytes of value N. The Key Derivation Function `(KDF) `_ used to generate the key material for AES is `SHA512 `_. The Message Authentication Code (MAC) scheme used is `HMACSHA256 `_. Format ------ (See also: :doc:`protocol`) .. include:: encrypted_payload.rst 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. [#f1]_ 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 [#f2]_ + 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. .. highlight:: nasm Partial Example --------------- .. list-table:: Public key K: :header-rows: 1 :widths: auto * - 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. .. list-table:: Initialization Vector IV: :header-rows: 1 :widths: auto * - 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. .. list-table:: Randomly generated key pair with private key r and public key R: :header-rows: 1 :widths: auto * - 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 .. list-table:: Derived public key P (point multiply r with K): :header-rows: 1 :widths: auto * - 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 .. list-table:: SHA512 of public key P X component (H): :header-rows: 1 :widths: auto * - 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 .. list-table:: Padded input: :header-rows: 1 :widths: auto * - 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 .. list-table:: Cipher text: :header-rows: 1 :widths: auto * - 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. .. list-table:: MAC: :header-rows: 1 :widths: auto * - Data - Comments * - :: f2 52 6d 61 b4 85 1f b2 34 09 86 38 26 fd 20 61 65 ed c0 21 36 8c 79 46 57 1c ea d6 90 46 e6 19 - 32 bytes hash .. rubric:: Footnotes .. [#f1] The pyelliptic implementation used in PyBitmessage takes unpadded data, see :obj:`.pyelliptic.Cipher.ciphering`. .. [#f2] The pyelliptic encodes the pubkey with curve and length, see :obj:`.pyelliptic.ECC.get_pubkey`