Commit 3a35fd82 authored by Éloïs's avatar Éloïs
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create rfc 17

parent 1a1adf15
# 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 with 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 | `4 - (Message length % 4)` bytes |
The maximum length of a message is 186 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
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