[WEB SECURITY] Max size of a password

Erlend Oftedal erlend at oftedal.no
Mon May 23 09:56:24 EDT 2011


I'm wondering if you might want a max size, but just not 14 characters. Maybe 256 or 1024.
Reason: you don't want to hash a 2 GB password 1000 times => DOS.

Erlend

Gautam <itsecanalyst at gmail.com> wrote:

> Claudio, Albino and James, You gave me the exact answer I was looking for,
>i know about hashing and other things, however the entropy stuff probably
>was known to me in full. I read a couple of papers from NIST last night to
>understand this better.
>
>I am also going to add the 2nd factor auth in my recommendation, since most
>of the stuff is what you know, what you have is where I want to take this
>and probably What you are will be next.
>
>Thanks to all.
>
>Gautam
>
>On Sun, May 22, 2011 at 11:24 AM, James Manico <jim at manico.net> wrote:
>
>> Then again, hashing and salting is not enough for secure password storage.
>>
>> 1) Use a strong/modern hash
>> 2) Use a cryptographically strong salt
>> 2a) Isolate that salt from the actual hash
>> 3) Iterate the hashing of the pass and salt 1000 times (as of y2k,
>> doubling that iteration value every three years when you re-hash to
>> slow down re-construction of the custom hash table if the salt is know
>> by the attacker)
>> 4) For %#^* sakes, stop depending on password as the only factor for
>> authentication. Shifting to 2-factor strategies decreases the need for
>> strong password policy or storage mechanisms.
>>
>> Jim Manico
>>
>> On May 22, 2011, at 7:46 PM, Claudio Telmon <claudio at telmon.org> wrote:
>>
>> > 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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