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Date: Sun, 6 Sep 2015 17:20:10 +0100
From: Hugo Landau <>
Subject: Interest in specification of modular crypt format


Now that a winner has been announced, I wondered if the PHC has any
interest in specifying a modular crypt format to supplement the final
specification for Argon2?

The modular crypt format is used by UNIX-like systems to encode
passwords in system databases (sha256-crypt is "$5$", sha512-crypt is
"$6$" etc.), but is also more widely used as a password storage format.
Specifying the modular crypt format early as part of the specification
itself would avoid any risk of balkanization of a supposed standard and
encourage adoption.

The most exhaustive documentation of the modular crypt format that I am
aware of is that maintained by the Python passlib project:

Take note of PBKDF2, which, while a standard, appears to have numerous
different incompatible formats: Passlib's PBKDF2, cta_pbkdf2_sha1,
dlitz_pbkdf2_sha1, etc.
Specifying a format is trivial but may be highly advantageous.

Hugo Landau

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