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Date: Tue, 15 Sep 2015 10:26:36 +0000
From: Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Specification of a modular crypt format

I copied Thomas' draft on the following google doc, where anyone can
comment, but not edit. Edit rights are granted to Thomas and to the PHC
panel:
https://docs.google.com/document/d/1QJva4xsY3eHNm2YiT0yPS9spRqd-N8NamYRpF5lkIzM/edit?usp=sharing

On Mon, Sep 14, 2015 at 10:43 PM Thomas Pornin <pornin@...et.org> wrote:

> On Mon, Sep 14, 2015 at 05:05:11PM +0100, Hugo Landau wrote:
> > The need for key-value extensibility for password hashing functions
> > is, presumably, nil
>
> Actually I am not doing it for extensibility, but for readability.
> If you have three parameters (say, the p, m and t or Argon2) then
> you can have something like:
>
>     $argon2i$42$5$5000$...
>
> but then you have to remember, as a human reader, in which order the
> p, m and t appear in the string. In my proposal you get:
>
>     $argon2i$m=42,p=5,t=5000$...
>
> which is clearer (for humans). Since the order is fixed, this should not
> be more complex to parse (instead of checking "does it start with '$'",
> code checks for "does it start with ',p='", which is still a single
> strncmp() call for a C-based parser).
>
> Accepting named parameters in arbitrary order, and/or ignoring
> unrecognized parameters, would indeed make parsing more complex, but I
> personally don't want that. I want the whole encoding to be fully
> deterministic so that test vectors work well. This also implies that
> the parsing can remain relatively easy.
>
> I'll try to write the parsing code in a few days, so that we may have a
> baseline for the complexity of implementations.
>
>
> > I also think parameter values can be limited to non-negative integers at
> > this time, unless you are aware of any password hashing schemes which
> > would be incompatible with this.
>
> In Makwa, at least, it is preferable to have a binary identifier for the
> used modulus (in the current spec, it is a hash of the modulus,
> truncated to 8 bytes). This parameter does not map to a non-negative
> integer (well, you _could_ make it an integer, since every sequence of
> bits can be interpreted as an integer, but you see what I mean).
>
> Another case would be PBKDF2, which is configurable with an iteration
> count, an output length, and an underlying PRF (usually HMAC with a
> given hash function). The hash function name could be made part of the
> function identifier (i.e. you have 'pbkdf2-sha1', not 'pbkdf2' with a
> 'sha1' parameter), but, conceptually, you could make the hash function
> one of the parameters (as a symbolic string).
>
>
> > I'm not convinced by this rationale. Firstly, I'm not aware of any
> > base64 functionality provided by programming language standard libraries
> > which does line wrapping, etc.
>
> .NET's System.Convert.ToBase64String() includes newlines by default
> (which might be OS-dependent, even). However, there are four versions
> of that call, two of which accepting an extra parameter of value
> Base64FormattingOptions.None to avoid inserting line breaks every 76
> characters (yes, 76, not 64).
>
> "openssl base64" (command-line tool, for shell scripts) also includes
> line breaks (and requires them upon decoding...).
>
> None of this is an irremediable point. "True" Base64 is not harder to
> implement than the "crypt" variant. I'll give it a go (I already did in
> both C and Java, neither of which having innate abilities at Base64).
>
>
> (I note that the B64 implementations already in place for bcrypt and the
> SHA-2-based crypts works only in the encoding direction, which is the
> easier of the two. For salt _decoding_, we need a decoder, which is
> relatively harder to implement.)
>
>
> > Developers in this circumstance (or, preferably, people developing
> > password hashing libraries)
>
> Yes, much preferably, please.
>
>
>         --Thomas Pornin
>

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