[doc] add RFC0016

README.md

@@ -28,6 +28,7 @@ All documents should be implementation agnostic, as specific documentations shou
 * RFC_0008: [Duniter Messages Encryption and Signature](https://git.duniter.org/documents/rfcs/blob/messages_encryption_and_signature/rfc/0008%20Messages%20Encryption%20and%20Signature.md)
 * RFC_0013: [Duniter Encrypted Wallet Import Format](https://git.duniter.org/documents/rfcs/blob/dewif/rfc/0013_Duniter_Encrypted_Wallet_Import_Format.md)
 * RFC_0014: [DUBP Mnemonic](https://git.duniter.org/documents/rfcs/blob/dubp-mnemonic/rfc/0014_Dubp_Mnemonic.md)
+* RFC_0016: [Checksum for Duniter public keys](rfc/0016_public_key_checksum.md)
 
 ### Drafts RFCs
 

rfc/0016_public_key_checksum.md

+# 0016 Checksum for Duniter public keys
+
+```txt
+RFC: 0016
+Title: Checksum for Duniter public keys
+Type: Convention
+Status: Proposed Standard
+Author: matograine <matograine@zaclys.net>
+Created: 2021-03-07
+Last edited: 2021-03-07
+License: AGPL-3
+```
+
+## Sources
+
+This document proposes an enhancement on Tortue's standard for public key checksum: 
+https://github.com/Tortue95/Duniter_Paper_Wallet/blob/master/Duniter_Auth_Protocol.md 
+
+## Public key format
+
+A public key is to be understood as an Ed25519 public key.
+It is represented as a Base58 string which length ranges {43,44} characters.
+
+Example of a public key:
+```
+J4c8CARmP9vAFNGtHRuzx14zvxojyRWHW2darguVqjtX
+```
+
+Example of two valid public keys referring to the same  Ed25519 binary public key:
+```
+12BjyvjoAf5qik7R8TKDJAHJugsX23YgJGi2LmBUv2nx
+2BjyvjoAf5qik7R8TKDJAHJugsX23YgJGi2LmBUv2nx
+```
+
+When converting a public key to its binary representation, the length of the bytes array MUST be verified.
+If the length of the array is inferior to 32, zero-bytes MUST be prepended.
+
+```python
+# convert public key string to bytes
+pubkey_byte = bytearray(base58.b58decode(pubkey))
+# prepend zero-bytes until the public key is 32 bytes long
+while len(pubkey_byte) < 32:
+    pubkey_byte = bytearray(b"\x00") + pubkey_byte
+```
+
+## Checksum display
+
+The checksum aims at verifying the consistency of the public key.
+It consists in three (3) base58 characters.
+It is displayed after the public key, separated by a colon `:`.
+
+Example of two valid representations of a public key with their checksum:
+```
+12BjyvjoAf5qik7R8TKDJAHJugsX23YgJGi2LmBUv2nx:8pQ
+2BjyvjoAf5qik7R8TKDJAHJugsX23YgJGi2LmBUv2nx:8pQ
+```
+
+This function is used when a public key is typed manually on the keyboard (or issue on QRcode reader)
+to avoid sending coins to the wrong public key. Because no control exist on the Duniter protocol.
+
+This technique is better than adding only some checksum bytes at the end of the key, before the base58.encode because:
+- The public key visible for the user is the same.
+- If the user miss some characters, the software client can detect it.
+- The checksum is not mandatory
+
+### Short form
+
+When displaying a public key on little space, a short form can be used for public keys.
+This short form consists in:
+
+* the 4 first characters of the public key
+* the ellipsis character `…`
+* the 4 last characters of the public key
+* a colon `:`
+* the checksum
+
+Example of short-form public keys with checksum:
+```
+12Bj…v2nx:8pQ
+2Bjy…v2nx:8pQ
+```
+
+Short form CAN be used for user input, only in research fields.
+In that case, users will probably use three points `...` instead of an ellipsis `…`.
+Short form MUST NOT be used as a direct input of a public key.
+
+For accessibility, a color CAN be used to display the checksum.
+In that case, the same color MUST be used for the separating colon.
+
+## Compute the checksum
+
+To compute the checksum:
+
+0) use the binary representation of the public key :  
+`pubkey = bytearray(base58.b58decode("J4c8CARmP9vAFNGtHRuzx14zvxojyRWHW2darguVqjtX"))`
+
+1) `sha256(pubkey)`  
+`0x47c7aee49dfb9bea99949d04623281d8ad6188be8f6a698b0eb5994fa44d0a67`
+
+2) `sha256(sha256(pubkey))`  
+`0x04a7ad22fbe357fbf5f58b2996fe5840fa2f977b9f6aa4c62575e68f75882672`
+
+3) `Base58.encode(sha256(sha256(pubkey))`  
+`KAvdGW7rpV68WDQPVN2TCrCoyvAHqMK1nSTS8y68bmB`
+
+4)  We only take the 3 first characters of this value to get the checksum  
+`KAv`
+