[Dovecot] CRAM-MD5 Password Generation Algorithm

Douglas Willcocks douglas.willcocks at gmail.com
Sat Apr 12 23:32:04 EEST 2008

On Sat, 12 Apr 2008 14:23:58 -0400, Bill Cole
<dovecot-20061108 at billmail.scconsult.com> wrote:
> At 2:07 PM +0100 4/12/08, Douglas Willcocks  imposed structure on a
> stream of electrons, yielding:
>>On Sat, 12 Apr 2008 07:52:01 +0200, Patrick Ben Koetter
>><p at state-of-mind.de> wrote:
> [...]
>>>  CRAM-MD5 passwords are passwords saved in plaintext format on the
> client
>>>  and
>>>  (!) on the server. Here's why:
>>That's what I first thought.
> It's not technically a plaintext format or an obvious encoding, but
> if I'm reading the Dovecot code correctly, it is some form of the
> intermediate "contexts" derived from the password as part of the
> HMAC-MD5 algorithm.
> It is not clear to me whether one could actually reverse that
> derivation. It seems to me that it should be simple, but no one seems
> to say that explicitly about the practice of storing the
> pre-calculated contexts rather than the actual password, so maybe I'm
> missing something. It may be that actual implementations always use
> the MD5 of the actual password as the key the context calculation,
> rather than using the password itself. That would make the contexts
> 16 bytes each plus padding, which would explain the Dovecot
> 'CRAM-MD5' storage format.
> What IS clear is that with the HMAC-MD5 contexts one can authenticate
> as the user using CRAM-MD5, and that CRAM-MD5 requires the server to
> store either a recoverable plaintext password or the HMAC-MD5
> contexts derived from it.
> [...]

I'm not convinced that the context is reversible.

In the source of version 1.0.13:

The function 'hmac_md5_init' is called (from password-scheme-cram-md5.c:13)
to generate the scheme. Looking at the definition of 'hmac_md5_init' and
the explanation of the algorithm given on
http://www.cryptostuff.com/crypto/index.php?title=hmac, it's quite easy to
see the resemblance (with the exception of the lack of 'message' in

That implies the (multiple) usage of the MD5 hashing algorithm on the
password, making it more or less irreversible.

I may of course have misunderstood the sequence of functions that dovecotpw
actually calls of course...

>>Perhaps this not an irreversible hash, but more something like (althought
>>it's not) base64? The thing is, it _looks_ like a hash.
> I'm pretty sure that it is a pair of 16-byte values derived from the
> password, represented in hexadecimal and concatenated.

That's the representation I'm trying to replicate.

>>For example, using dovecotpw I can generate the hashed (??) version of
>>'password', which is
>>I can then place that in the database and the authentication is
> successful.
>>What's more, according to
>>You only need to store the password in plaintext if you are using _both_
>>To quote:
>>"The problem with non-plaintext auth mechanisms is that the password must
>>be stored either in plaintext, or using a mechanism-specific scheme
> that's
>>incompatible with all other non-plaintext mechanisms. For example if
> you're
>>going to use CRAM-MD5 authentication, the password needs to be stored in
>>either PLAIN or CRAM-MD5 scheme. If you want to allow both CRAM-MD5 and
>>DIGEST-MD5, the password must be stored in plaintext."
> It may be illuminating to look at
> http://tools.ietf.org/html/draft-ietf-sasl-crammd5-09 and pay
> particular attention to Section 5.

I've read through the document, and I now understand where exactly the
precomputed context sits in the whole picture, but I'm still unsure how to
reproduce it without dovecotpw.

>>>>  To generate the passwords to go into the database I can use the
>>>  dovecotpw
>>>>  utility, but I'm wanting to stick some sort of minimal admin
> interface
>>>  on
>>>>  the server to be able to manage the users etc without having to use
> the
>>>>  CLI.
>>>  Use pwgen.
>>The problem (as stated above) is not how to generate passwords, there are
>>thousands of libraries that can do that relatively well.
> What exactly is wrong with using dovecotpw?
> Are you unaware of the existence of system() and backtick operators
> in your preferred languages, or of the ability to write a CGI in
> shell?

I _am_ aware of such functionality in the various languages I previously
mentioned, but I don't want to simply wrap the executable, I would like to
reproduce the algorithm. I have nothing against using dovecotpw, but would
I rather not depend on it for portability reasons.

I may want to run the library on various different machines, architectures
or os' and I don't want to have to compile dovecot for each situation if I
only need a small part of one of the encryption libraries.

> Most generic 'best practice' advice for writing web-based tools tries
> to discourage any mechanism that feeds commands to a shell, but it is
> important to understand why that is and when the generic advice
> should be set aside. No Bourne/POSIX or csh-based shell is
> particularly fast, and system() or `` calls launching a shell to run
> a command line from another language has a fork cost, so using either
> approach for performance-sensitive apps is a bad idea on that basis
> alone. In regards to security, there has been a long ugly history of
> sloppy coders being exposed by their failure to diligently validate
> user input before feeding it to a shell. That problem has been
> addressed to some degree by delusional misfeatures like PHP's 'safe'
> mode, which encourages the coder and the admin to collaborate in
> insecurity under the misimpression of being safe. If you are
> disciplined about how you use what the user tells you, there's
> nothing inherently unsafe in passing it through a shell or even
> writing a simple web app entirely in shell.

I am well aware of the security implications of using the best practices
for web-based system <-> Shell interaction, but that isn't the problem
here. It's not a question of security, it's a question of portability.

> --
> Bill Cole
> bill at scconsult.com


Douglas Willcocks

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