[CHECKSUM] modify RFC0016:

  * check that bytes representation of pubkeys are not too long
  * use only one SHA256

rfc/0016_public_key_checksum.md

@@ -15,6 +15,7 @@ License: AGPL-3
 
 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 
+For compatibility, clients MAY accept checksum based on this last standard for user inputs.
 
 ## Public key format
 
@@ -33,14 +34,25 @@ Example of two valid public keys referring to the same  Ed25519 binary public ke
 ```
 
 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.
+  - If the length of the array is inferior to 32, zero-bytes MUST be prepended.
+  - If the length of the array is superior to 32, leading zero-bytes MUST be removed.
+  - If there is one leading non-zero byte and length is superior to 32, then the public key is not valid.
 
 ```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
+
+# remove leading zero-bytes if length is superior to 32
+while len(pubkey_byte) > 32:
+    if pubkey_byte[0] == 0:
+        del pubkey_byte[0]
+    # raise error if leading byte is not null
+    else:
+        raise ValueError("Invalid public key: bytes length is too long")
 ```
 
 ## Checksum display
@@ -51,8 +63,8 @@ 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
+12BjyvjoAf5qik7R8TKDJAHJugsX23YgJGi2LmBUv2nx:8BD
+2BjyvjoAf5qik7R8TKDJAHJugsX23YgJGi2LmBUv2nx:8BD
 ```
 
 This function is used when a public key is typed manually on the keyboard (or issue on QRcode reader)
@@ -76,8 +88,8 @@ This short form consists in:
 
 Example of short-form public keys with checksum:
 ```
-12Bj…v2nx:8pQ
-2Bjy…v2nx:8pQ
+12Bj…v2nx:8BD
+2Bjy…v2nx:8BD
 ```
 
 Short form CAN be used for user input, only in research fields.
@@ -92,17 +104,14 @@ In that case, the same color MUST be used for the separating colon.
 To compute the checksum:
 
 0) use the binary representation of the public key :
+for simplification we use a pubkey that is 32 bits long once decoded.
 `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`
+2) `Base58.encode(sha256(pubkey))`
+`5qCYkJMWgNA54gZz4HMQLbxL5btDmTS3EuHHCTcJaqGi`
 
+3)  We only take the 3 first characters of this value to get the checksum
+`5qC`