phc-discussions - Re: [PHC] Specification of a modular crypt format

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Date: Sat, 19 Sep 2015 19:02:51 +0200
From: Thomas Pornin <pornin@...et.org>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Specification of a modular crypt format

Hello,

here is some example C code that implements encoding and decoding of
hash strings for Argon2i.

According to the remarks made here, I followed rules that differ from my
earlier draft in the following respects:

 -- The decoder does not validate input strings; it may accept strings
 that do not comply with the specification (e.g. "p=01" instead of
 "p=1").

 -- I am using "normal" Base64 (not the crypt variant), although I do
 not include the '=' padding signs.

 -- I added two optional parameters "keyid" and "data", that correspond
 to an identifier for the key (if any), and associated data. Both are
 binary (encoded in Base64); the key ID is up to 8 bytes, while the
 associated data is up to 32 bytes. When they are empty, they are omitted
 (i.e. there is no difference in encoding between "no associated data"
 and "associated data has length 0").

 -- I set the parameter order to "m, t, p, keyid, data".

The code includes some generic Base64 encoder/decoder (about 150 lines),
then code specific to Argon2i (about 200 lines), and some test vectors.
The Base64 code is constant-time with regards to the data contents (no
data-dependent table lookups or branches); this is probably useless in
most situations, but easy enough to achieve.

This sample code is meant to give some discussion basis about
implementation complexity. Feel free to reuse under "public domain"
rules (i.e. do everything you want except blame me).

	--Thomas Pornin

View attachment "phc-mcf-parse.c" of type "text/x-csrc" (13298 bytes)