create rfc 17

rfc/0017_transaction_comment_encryption.md

+# RFC 17: Transaction Comment Encryption
+
+Alice wants to send a transaction to bob with an encrypted comment.
+
+Let `Sa` the private key of Alice and `Pa` its associated public key.
+Let `Sb` the private key of Bob and `Pb` its associated public key.
+
+## generate symetric encryption key
+
+We use nacl function [crypto_box_beforenm]. Then use scrypt with a random nonce.
+
+Let `R = crypto_box_beforenm(Sa, Pb) = crypto_box_beforenm(Sb, Pa)`.
+
+Let `n` be a random nonce of 12 bytes.
+
+The symmetric encryption key `k` is obtained as follows:
+
+`k = scrypt(R, SHA256(n))`
+
+With following parameters for scrypt:
+
+```txt
+N = 4096
+r = 16
+p = 1
+dkLen = message length
+```
+
+## encrypt zith XOR cipher
+
+Encrypt bit per bit with XOR cipher.
+
+## Serialize encrypted message with meta data
+
+| Prefix  | Message type | Message length | Nonce    | Encrypted message | Padding                    |
+|:-------:|:------------:|:--------------:|:--------:|:-----------------:|:--------------------------:|
+| 2 bytes | 1 byte       | 1 byte         | 12 bytes | Any bytes         | `Message length % 4` bytes |
+
+The padding is used so that the quantity of bytes to be serialized is always a multiple of 4. This is necessary for the encoding in base z85.
+
+### Prefix
+
+A constant value that indicates the type of format. For the present format defined in this RFC, the prefix is `0x0100`. A possible future new format should choose a different prefix. This prefix can also be incremented for a future version of the present format.
+
+### Message type
+
+| Code | Significance                     |
+|:----:|:--------------------------------:|
+| 0x00 | Write by a human for a human     |
+| 0x01 | Write by a human for a machine   |
+| 0x10 | Write by a machine for a human   |
+| 0x11 | Write by a machine for a machine |
+
+## Encodinq in transaction document
+
+Encrypted message in encoded in [base z85] in transaction comment directly.
+
+DUBP protocol accept all [base z85] characters except `$`. We replace `$` by `_`.
+
+[base z85]: https://rfc.zeromq.org/spec/32/
+[crypto_box_beforenm]: https://nacl.cr.yp.to/box.html
+[XOR cipher]: https://en.wikipedia.org/wiki/XOR_cipher