encrypted_payload.rst and encryption.rst docs, crossreferences
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docs/encrypted_payload.rst
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docs/encrypted_payload.rst
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+------------+-------------+-----------+--------------------------------------------+
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| Field Size | Description | Data type | Comments |
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+============+=============+===========+============================================+
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| 16 | IV | uchar[] | Initialization Vector used for AES-256-CBC |
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+------------+-------------+-----------+--------------------------------------------+
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| 2 | Curve type | uint16_t | Elliptic Curve type 0x02CA (714) |
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+------------+-------------+-----------+--------------------------------------------+
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| 2 | X length | uint16_t | Length of X component of public key R |
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+------------+-------------+-----------+--------------------------------------------+
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| X length | X | uchar[] | X component of public key R |
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+------------+-------------+-----------+--------------------------------------------+
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| 2 | Y length | uint16_t | Length of Y component of public key R |
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+------------+-------------+-----------+--------------------------------------------+
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| Y length | Y | uchar[] | Y component of public key R |
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+------------+-------------+-----------+--------------------------------------------+
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| ? | encrypted | uchar[] | Cipher text |
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+------------+-------------+-----------+--------------------------------------------+
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| 32 | MAC | uchar[] | HMACSHA256 Message Authentication Code |
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+------------+-------------+-----------+--------------------------------------------+
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docs/encryption.rst
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docs/encryption.rst
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Encryption
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==========
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Bitmessage uses the Elliptic Curve Integrated Encryption Scheme
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`(ECIES) <http://en.wikipedia.org/wiki/Integrated_Encryption_Scheme>`_
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to encrypt the payload of the Message and Broadcast objects.
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The scheme uses Elliptic Curve Diffie-Hellman
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`(ECDH) <http://en.wikipedia.org/wiki/ECDH>`_ to generate a shared secret used
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to generate the encryption parameters for Advanced Encryption Standard with
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256bit key and Cipher-Block Chaining
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`(AES-256-CBC) <http://en.wikipedia.org/wiki/Advanced_Encryption_Standard>`_.
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The encrypted data will be padded to a 16 byte boundary in accordance to
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`PKCS7 <http://en.wikipedia.org/wiki/Cryptographic_Message_Syntax>`_. This
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means that the data is padded with N bytes of value N.
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The Key Derivation Function
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`(KDF) <http://en.wikipedia.org/wiki/Key_derivation_function>`_ used to
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generate the key material for AES is
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`SHA512 <http://en.wikipedia.org/wiki/Sha512>`_. The Message Authentication
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Code (MAC) scheme used is `HMACSHA256 <http://en.wikipedia.org/wiki/Hmac>`_.
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Format
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------
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(See also: :doc:`protocol`)
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.. include:: encrypted_payload.rst
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In order to reconstitute a usable (65 byte) public key (starting with 0x04),
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the X and Y components need to be expanded by prepending them with 0x00 bytes
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until the individual component lengths are 32 bytes.
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Encryption
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----------
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1. The destination public key is called K.
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2. Generate 16 random bytes using a secure random number generator.
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Call them IV.
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3. Generate a new random EC key pair with private key called r and public key
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called R.
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4. Do an EC point multiply with public key K and private key r. This gives you
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public key P.
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5. Use the X component of public key P and calculate the SHA512 hash H.
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6. The first 32 bytes of H are called key_e and the last 32 bytes are called
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key_m.
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7. Pad the input text to a multiple of 16 bytes, in accordance to PKCS7.
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8. Encrypt the data with AES-256-CBC, using IV as initialization vector,
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key_e as encryption key and the padded input text as payload. Call the
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output cipher text.
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9. Calculate a 32 byte MAC with HMACSHA256, using key_m as salt and
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IV + R + cipher text as data. Call the output MAC.
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The resulting data is: IV + R + cipher text + MAC
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Decryption
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----------
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1. The private key used to decrypt is called k.
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2. Do an EC point multiply with private key k and public key R. This gives you
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public key P.
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3. Use the X component of public key P and calculate the SHA512 hash H.
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4. The first 32 bytes of H are called key_e and the last 32 bytes are called
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key_m.
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5. Calculate MAC' with HMACSHA256, using key_m as salt and
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IV + R + cipher text as data.
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6. Compare MAC with MAC'. If not equal, decryption will fail.
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7. Decrypt the cipher text with AES-256-CBC, using IV as initialization
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vector, key_e as decryption key and the cipher text as payload. The output
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is the padded input text.
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.. highlight:: nasm
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Partial Example
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---------------
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.. list-table:: Public key K:
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:header-rows: 1
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:widths: auto
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* - Data
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- Comments
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* -
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::
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04 09 d4 e5 c0 ab 3d 25
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fe 04 8c 64 c9 da 1a 24
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2c 7f 19 41 7e 95 17 cd
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26 69 50 d7 2c 75 57 13
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58 5c 61 78 e9 7f e0 92
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fc 89 7c 9a 1f 17 20 d5
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77 0a e8 ea ad 2f a8 fc
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bd 08 e9 32 4a 5d de 18
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57
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- Public key, 0x04 prefix, then 32 bytes X and 32 bytes Y.
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.. list-table:: Initialization Vector IV:
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:header-rows: 1
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:widths: auto
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* - Data
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- Comments
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* -
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::
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bd db 7c 28 29 b0 80 38
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75 30 84 a2 f3 99 16 81
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- 16 bytes generated with a secure random number generator.
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.. list-table:: Randomly generated key pair with private key r and public key R:
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:header-rows: 1
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:widths: auto
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* - Data
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- Comments
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* -
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::
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5b e6 fa cd 94 1b 76 e9
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d3 ea d0 30 29 fb db 6b
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6e 08 09 29 3f 7f b1 97
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d0 c5 1f 84 e9 6b 8b a4
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- Private key r
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* -
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::
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04 02 93 21 3d cf 13 88
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b6 1c 2a e5 cf 80 fe e6
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ff ff c0 49 a2 f9 fe 73
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65 fe 38 67 81 3c a8 12
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92 df 94 68 6c 6a fb 56
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5a c6 14 9b 15 3d 61 b3
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b2 87 ee 2c 7f 99 7c 14
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23 87 96 c1 2b 43 a3 86
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5a
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- Public key R
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.. list-table:: Derived public key P (point multiply r with K):
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:header-rows: 1
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:widths: auto
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* - Data
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- Comments
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* -
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::
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04 0d b8 e3 ad 8c 0c d7
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3f a2 b3 46 71 b7 b2 47
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72 9b 10 11 41 57 9d 19
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9e 0d c0 bd 02 4e ae fd
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89 ca c8 f5 28 dc 90 b6
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68 11 ab ac 51 7d 74 97
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be 52 92 93 12 29 be 0b
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74 3e 05 03 f4 43 c3 d2
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96
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- Public key P
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* -
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::
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0d b8 e3 ad 8c 0c d7 3f
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a2 b3 46 71 b7 b2 47 72
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9b 10 11 41 57 9d 19 9e
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0d c0 bd 02 4e ae fd 89
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- X component of public key P
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.. list-table:: SHA512 of public key P X component (H):
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:header-rows: 1
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:widths: auto
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* - Data
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- Comments
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* -
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::
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17 05 43 82 82 67 86 71
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05 26 3d 48 28 ef ff 82
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d9 d5 9c bf 08 74 3b 69
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6b cc 5d 69 fa 18 97 b4
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- First 32 bytes of H called key_e
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* -
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::
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f8 3f 1e 9c c5 d6 b8 44
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8d 39 dc 6a 9d 5f 5b 7f
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46 0e 4a 78 e9 28 6e e8
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d9 1c e1 66 0a 53 ea cd
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- Last 32 bytes of H called key_m
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.. list-table:: Padded input:
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:header-rows: 1
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:widths: auto
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* - Data
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- Comments
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* -
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::
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54 68 65 20 71 75 69 63
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6b 20 62 72 6f 77 6e 20
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66 6f 78 20 6a 75 6d 70
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73 20 6f 76 65 72 20 74
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68 65 20 6c 61 7a 79 20
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64 6f 67 2e 04 04 04 04
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- The quick brown fox jumps over the lazy dog.0x04,0x04,0x04,0x04
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.. list-table:: Cipher text:
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:header-rows: 1
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:widths: auto
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* - Data
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- Comments
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* -
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::
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64 20 3d 5b 24 68 8e 25
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47 bb a3 45 fa 13 9a 5a
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1d 96 22 20 d4 d4 8a 0c
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f3 b1 57 2c 0d 95 b6 16
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43 a6 f9 a0 d7 5a f7 ea
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cc 1b d9 57 14 7b f7 23
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- 3 blocks of 16 bytes of encrypted data.
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@ -199,27 +199,9 @@ Inventory vectors consist of the following data format:
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Encrypted payload
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Encrypted payload
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^^^^^^^^^^^^^^^^^
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^^^^^^^^^^^^^^^^^
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Bitmessage uses `ECIES <https://en.wikipedia.org/wiki/Integrated_Encryption_Scheme>`_ to encrypt its messages. For more information see Encryption
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Bitmessage uses `ECIES <https://en.wikipedia.org/wiki/Integrated_Encryption_Scheme>`_ to encrypt its messages. For more information see :doc:`encryption`
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+------------+-------------+-----------+--------------------------------------------+
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.. include:: encrypted_payload.rst
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| Field Size | Description | Data type | Comments |
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+============+=============+===========+============================================+
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| 16 | IV | uchar[] | Initialization Vector used for AES-256-CBC |
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+------------+-------------+-----------+--------------------------------------------+
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| 2 | Curve type | uint16_t | Elliptic Curve type 0x02CA (714) |
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+------------+-------------+-----------+--------------------------------------------+
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| 2 | X length | uint16_t | Length of X component of public key R |
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+------------+-------------+-----------+--------------------------------------------+
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| X length | X | uchar[] | X component of public key R |
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+------------+-------------+-----------+--------------------------------------------+
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| 2 | Y length | uint16_t | Length of Y component of public key R |
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+------------+-------------+-----------+--------------------------------------------+
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| Y length | Y | uchar[] | Y component of public key R |
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+------------+-------------+-----------+--------------------------------------------+
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| ? | encrypted | uchar[] | Cipher text |
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+------------+-------------+-----------+--------------------------------------------+
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| 32 | MAC | uchar[] | HMACSHA256 Message Authentication Code |
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+------------+-------------+-----------+--------------------------------------------+
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Unencrypted Message Data
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Unencrypted Message Data
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^^^^^^^^^^^^^^^^^^^^^^^^
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^^^^^^^^^^^^^^^^^^^^^^^^
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