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encrypted_payload.rst and encryption.rst docs, crossreferences

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

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Encryption
==========
Bitmessage uses the Elliptic Curve Integrated Encryption Scheme
`(ECIES) <http://en.wikipedia.org/wiki/Integrated_Encryption_Scheme>`_
to encrypt the payload of the Message and Broadcast objects.
The scheme uses Elliptic Curve Diffie-Hellman
`(ECDH) <http://en.wikipedia.org/wiki/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) <http://en.wikipedia.org/wiki/Advanced_Encryption_Standard>`_.
The encrypted data will be padded to a 16 byte boundary in accordance to
`PKCS7 <http://en.wikipedia.org/wiki/Cryptographic_Message_Syntax>`_. This
means that the data is padded with N bytes of value N.
The Key Derivation Function
`(KDF) <http://en.wikipedia.org/wiki/Key_derivation_function>`_ used to
generate the key material for AES is
`SHA512 <http://en.wikipedia.org/wiki/Sha512>`_. The Message Authentication
Code (MAC) scheme used is `HMACSHA256 <http://en.wikipedia.org/wiki/Hmac>`_.
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.
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.
.. 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.

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@ -199,27 +199,9 @@ Inventory vectors consist of the following data format:
Encrypted payload
^^^^^^^^^^^^^^^^^
Bitmessage uses `ECIES <https://en.wikipedia.org/wiki/Integrated_Encryption_Scheme>`_ to encrypt its messages. For more information see Encryption
Bitmessage uses `ECIES <https://en.wikipedia.org/wiki/Integrated_Encryption_Scheme>`_ to encrypt its messages. For more information see :doc:`encryption`
+------------+-------------+-----------+--------------------------------------------+
| Field Size | Description | Data type | Comments |
+============+=============+===========+============================================+
| 16 | IV | uchar[] | Initialization Vector used for AES-256-CBC |
+------------+-------------+-----------+--------------------------------------------+
| 2 | Curve type | uint16_t | Elliptic Curve type 0x02CA (714) |
+------------+-------------+-----------+--------------------------------------------+
| 2 | X length | uint16_t | Length of X component of public key R |
+------------+-------------+-----------+--------------------------------------------+
| X length | X | uchar[] | X component of public key R |
+------------+-------------+-----------+--------------------------------------------+
| 2 | Y length | uint16_t | Length of Y component of public key R |
+------------+-------------+-----------+--------------------------------------------+
| Y length | Y | uchar[] | Y component of public key R |
+------------+-------------+-----------+--------------------------------------------+
| ? | encrypted | uchar[] | Cipher text |
+------------+-------------+-----------+--------------------------------------------+
| 32 | MAC | uchar[] | HMACSHA256 Message Authentication Code |
+------------+-------------+-----------+--------------------------------------------+
.. include:: encrypted_payload.rst
Unencrypted Message Data
^^^^^^^^^^^^^^^^^^^^^^^^