Re: [PATCH v1] man/man3/crypt.3: These functions accept many algorithms, not only DES

[Alejandro Colomar <alx+linux-man@kernel.org>]

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Hi Serge,

> Date: 2026-11-11 09:05:40-0600
> From: "Serge E. Hallyn" <serge@hallyn.com>
>
[...]
> > -     salt is a two‐character string chosen from the set
> > -     [a-zA-Z0-9./].  This string is used to perturb the algo‐
> > -     rithm in one of 4096 different ways.
> > -
> > -     By taking the lowest 7 bits of each of the first eight
> > -     characters of the key, a 56‐bit key is obtained.  This
> > -     56‐bit key is used to encrypt repeatedly a constant string
> > -     (usually a string consisting of all zeros).  The returned
> > -     value points to the hashed password, a series of 13 print‐
> > -     able ASCII characters (the first two characters represent
> > -     the salt itself).  The return value points to static data
> > -     whose content is overwritten by each call.
> > -
> > -     Warning: the key space consists of 2**56 equal 7.2e16 pos‐
> > -     sible values.  Exhaustive searches of this key space are
> > -     possible using massively parallel computers.  Software,
> > -     such as crack(1), is available which will search the por‐
> > -     tion of this key space that is generally used by humans
> > -     for passwords.  Hence, password selection should, at mini‐
> > -     mum, avoid common words and names.  The use of a passwd(1)
> > -     program that checks for crackable passwords during the se‐
> > -     lection process is recommended.
> > -
> > -     The DES algorithm itself has a few quirks which make the
> > -     use of the crypt() interface a very poor choice for any‐
> > -     thing other than password authentication.  If you are
> > -     planning on using the crypt() interface for a cryptography
> > -     project, don’t do it: get a good book on encryption and
> > -     one of the widely available DES libraries.
> > +     salt is a string used for determining the hasing algorithm
> 
> typo: hashing

Thanks!  Fixed.

> > +     (possibly including parameters).  It is also used to per‐
> > +     turb the algorithm.

BTW, I suspect the salt is not used for perturbing the algorithm
anymore.  From musl's source code, at least for SHA-512, I _think_ it's
not reading past the third '$', and thus it's not reading any random-ish
bytes.

Should I drop the second sentence?


Have a lovely night!
Alex

> > +
> > +     The returned value points to the hashed password.  The re‐
> > +     turn value points to static data whose content is over‐
> > +     written by each call.

-- 
<https://www.alejandro-colomar.es>

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