[WEB SECURITY] Max size of a password

Claudio Telmon claudio at telmon.org
Sun May 22 03:59:46 EDT 2011


On 05/22/2011 12:33 AM, Gautam wrote:
> Now my delima is
> 
> SHA256(*AAA*)  = some 256 bit hash
> 
> now
> 
> SHA256(*AAABB*) = is also 256 bit hash
> 
> so with this reasoning will it make sense if i say no limit or just a
> reasonable limit of 14 character since the result is always going to be
> 128bit text be it 8 characters or 14 characters.

A good hash function can provide you an entropy which is limited by the
length of the output, but the entropy available in the input domain is
also a limit. 14 characters as input from a user are not random bits and
won't provide 128 bit of entropy for the result. If users are permitted
(and suggested) to choose long passphrases, they may end up feeding the
function with enough entropy. If they are limited to 14 characters, they
will select passwords that will be shorter that 14 characters and are
not random, so they will always feed less that 128 bit of entropy to the
function, and the output will also have less that 128 bits of entropy,
even if it is 128 bit long.

ciao

- Claudio

-- 

Claudio Telmon
claudio at telmon.org
http://www.telmon.org





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