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Date: Sat, 26 Sep 2015 21:11:01 +0300
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Specification of a modular crypt format

On Sun, Sep 13, 2015 at 08:26:50PM +0200, Thomas Pornin wrote:
> The hash output size for password verification should be fixed
> (otherwise, if people have to choose that length, it is unavoidable that
> some will do something stupid).

Unfortunately yes, but OTOH for attack (e.g. in JtR) or non-initial uses
(where someone else has already made the determination on output length
anyway), it is desirable to have a canonical encoding to which such
weird hashes could be converted.  e.g. JtR will have to encode those
hashes in john.pot somehow, and preferably in a deterministic way.

So maybe the output length limits should be relaxed to apply to new
hashes only.

A concern, though, is that outlen is yet another risky parameter in
terms of DoS attacks e.g. on httpd via .htpasswd files, if it is
encoded, along with m_cost and t_cost (and whatever hashing scheme
specific parameters those correspond to).  That's not just "new hashes",
but also processing of existing untrusted hashes.  (Similar attacks on
JtR are also possible.  This issue was brought up a few times between
JtR jumbo developers, but we made no decision on how to deal with it yet.)

Since it's "yet another" risky parameter rather than the only one, I
think the concern above might not be a sufficient reason not to have
(encode/decode) that parameter.  We need a workaround or a solution for
all of them anyway (e.g., some API to determine would-be memory and time
costs of computing a hash, in units that e.g. httpd or JtR could check
against configured limits).

Static buffers e.g. for crypt(3) return value may limit what outlen
values a given implementation would support, and it may be fine for many
to support only 256-bit.

Encoding outlen if it's a non-optional parameter is easy - just encode
another number.  The smallest supported value can be way below 256-bit
then (with unreasonably low values being to have a canonical encoding
for attacks on existing weird hashes).

However, encoding outlen if it's optional, yet having the encoding
deterministic by definition, is not so easy.  With my proposal for
omitting default values, we can only omit a default if it's the smallest
valid value.  Omitting a default in the middle of valid range is
error-prone.  So if we make 256-bit the default (I agree that we
should), then we only have a straightforward way to encode values of
256-bit or higher, and we have no canonical encodings for shorter
hashes for attack use.  This is a dilemma I don't yet have a good
proposal for.  Any ideas?

> While collisions are not a concern, it
> would probably be a good marketing move to use an output size which is
> "naturally" immune to collisions, i.e. 256 bits. This is what SHA-256
> outputs, and is larger than the 192 bits of bcrypt. A 32-byte output is
> not too huge with regards to the rest of the string, and also with
> regards to existing hash strings that rely on SHA-256 crypt or, even
> more so, SHA-512 crypt. While a 128-bit output would save some resources
> and make more sense (cryptographically speaking), a 256-bit output
> should ensure wider acceptance by the user base.

I agree with this reasoning.

> I limit the salt length for Argon2 to a maximum of 48 bytes (64
> characters after B64 encoding) so as to help decoders (stack buffers
> again...). Argon2 can process much longer salts, but that does not make
> a lot of sense for password verification, where a 16-byte salt (say, an
> encoded UUID) is already fine.

We may stipulate some limits on salt length for defensive
implementations, but we should also define canonical encodings for
arbitrary salt lengths for attack use.  Luckily, this is trivial.

Alexander

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