Hi
On 9/25/23 21:43, Levi Morrison via internals wrote:
I did a tiny bit of my own research, and could not find any
recommendations more specific than "10 or more" as the cost factor.
Typically, the advice is "use a more modern system like argon2id".
Please see this email of mine regarding Argon2:
https://news-web.php.net/php.internals/120996
Other than that, the recommendation for BCrypt's cost factor (and
basically also Argon's tunables) is "as high as feasible for your use case".
See also this post on Fediverse (it's also referenced in the initial
email of the voting thread):
https://phpc.social/@tychotithonus@infosec.exchange/111025157601179075
However, I did notice some sites mention that systems ought to check
for a maximum length of 72 bytes when using bcrypt:
https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html#input-limits
As far as I can tell, PHP does not do this check. I am not sure if the
implementation(s) used suffer(s) from the limitation that is the
source of this recommendation. Perhaps someone has time to investigate
this? Anyway, it's "future work."
BCrypt-as-specified has a limit of at most 72 bytes, none of which may
be NUL. Additional characters will simply be ignored. Think of it like
the password would be passed through `substr($password, 0, 72)`. The
implementation used is crypt_blowfish by Openwall [1]. The limit in the
source code is given by this loop:
https://github.com/php/php-src/blob/2e8cdd8eecac5d34619bbd03916d0b7bcc2cc023/ext/standard/crypt_blowfish.c#L580-L582
(BF_N (16) + 2) * 4 is 72
The behavior for longer passwords is well-defined, so I'd say that PHP
doesn't need any additional check. The only thing that could be done
differently is throwing an exception and this is likely not a good idea
for such a generic function as 'password_hash'.
I have voted for 11, but will not be hurt in any way if 12 wins.
Best regards
Tim Düsterhus
[1] https://github.com/openwall/crypt_blowfish
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