7 ms·

Is there even any reasonable excuse for limiting the length of passwords to something less than hundreds of characters?

cookiecaper 13y ago
No. A potential common excuse may be, "No one will remember a secure password in excess of 16 characters, we are trying to minimize support volume", but this is not a "reasonable" excuse.
- groby_b 13y ago
  Yes. It's a DoS vector. The question is, what is a reasonable limit - and 16 characters fails horribly in that section.
  - cookiecaper 13y ago
    Right, as the OP said, there is no reasonable excuse to limit character intake to less than multiple hundreds. I agree that some limit is sensible just to ensure someone doesn't insert War and Peace (or base64-encoded binaries or whatever) as their password.
    - Brandon0 13y ago
      I was using War and Peace -- now I have to go change my passwords. Thanks!
      - kibibu 13y ago
        ...a dependence of which we are not conscious.2
Pxtl 13y ago
None that wouldn't be better handled with a warning of "excessive password complexity, we're not sure you'll remember this". Even truncating all password entries (create/change and auth) and simply taking the used subset would be better than blocking me from using my intended password.
- trothoun 13y ago
  Hmmm... if your password was password30jf0sd09jga09ja0i9sdfasi09djf0-sdj9faspiodjf and they only used the first 8 characters, you'd think you had a strong password, but would really have a very weak password. Hashing then truncating as discussed above would be much safer.
  - jlgreco 13y ago
    Really no reason to truncate after hashing. The output size of any cryptographic hash function should be more than small enough to send down the wire and hash again properly. Really though, just reject passwords over a few KB. Nobody will ever notice that limit except for people trying to fuck with you.
Zikes 13y ago
The only reasonable restriction is in overall POST body length. I recall an exploit once on node.js where unreasonably large POST requests could DDOS a server with minimal effort.
- zachlatta 13y ago
  Well, that and the server having to hash a huge password.
quaunaut 13y ago
There's no reason to limit the length of passwords whatsoever, except perhaps to be sure you're not trying to hash 20mb of text.
- just2n 13y ago
  You could really support an arbitrary password size by locally hashing the password to a length at least as long as the one you store in the database to preserve entropy, then send the fixed length hash as the user's "password", and hash it again on the server using something slower, like PBKDF2 with a few thousand rounds.
  - Hello71 13y ago
    How do you think hashing functions work?
    - TillE 13y ago
      At least on the web, hashing isn't usually done locally (ie, in Javascript).
      - alexjeffrey 13y ago
        this doesn't add extra security - the first hash becomes a substitute password. A cracker could modify their steam account to take said hash as input and sent it directly over the wire.
        just2n 13y ago
        It doesn't remove any security, either. If you perform a hash on the server that sends an arbitrary input size to a fixed number of bits, the attacker needs only to try O(2^n) different inputs where n is the number of bits of that hash. By using a hash with at least n bits as a substitute password, there's no less security. It does mitigate the DoS vector, however.
misterdai 13y ago
Only the support issue, but then I'd have thought that people using really long passwords as normally the ones who'll make an effort to remember / store them. They mention "Encrypted passwords", which would vary the amount of storage required in their database and also make the passwords retrievable if the attacker had gained access to the key / algorithm etc... I'd had preferred if they used a one way hash instead, obviously with a secure hash algorithm, uniquely salted and rehashed multiple times. All passwords would then be the same length when stored and users wouldn't have to have such a low maximum limit. There are worse offenders for low character limits, like Adobe with a 12 char maximum. Used to be a site that listed some, just a shame it's no longer available http://web.archive.org/web/20100526105638/http://www.weakpasswords.org/list http://web.archive.org/web/20100526105638/http://www.weakpas...

No. A potential common excuse may be, "No one will remember a secure password in excess of 16 characters, we are trying to minimize support volume", but this is not a "reasonable" excuse.

groby_b 13y ago
Yes. It's a DoS vector. The question is, what is a reasonable limit - and 16 characters fails horribly in that section.
- cookiecaper 13y ago
  Right, as the OP said, there is no reasonable excuse to limit character intake to less than multiple hundreds. I agree that some limit is sensible just to ensure someone doesn't insert War and Peace (or base64-encoded binaries or whatever) as their password.
  - Brandon0 13y ago
    I was using War and Peace -- now I have to go change my passwords. Thanks!
    - kibibu 13y ago
      ...a dependence of which we are not conscious.2

Pxtl 13y ago

None that wouldn't be better handled with a warning of "excessive password complexity, we're not sure you'll remember this". Even truncating all password entries (create/change and auth) and simply taking the used subset would be better than blocking me from using my intended password.

trothoun 13y ago
Hmmm... if your password was password30jf0sd09jga09ja0i9sdfasi09djf0-sdj9faspiodjf and they only used the first 8 characters, you'd think you had a strong password, but would really have a very weak password. Hashing then truncating as discussed above would be much safer.
- jlgreco 13y ago
  Really no reason to truncate after hashing. The output size of any cryptographic hash function should be more than small enough to send down the wire and hash again properly. Really though, just reject passwords over a few KB. Nobody will ever notice that limit except for people trying to fuck with you.

Zikes 13y ago

The only reasonable restriction is in overall POST body length. I recall an exploit once on node.js where unreasonably large POST requests could DDOS a server with minimal effort.

zachlatta 13y ago
Well, that and the server having to hash a huge password.

quaunaut 13y ago

There's no reason to limit the length of passwords whatsoever, except perhaps to be sure you're not trying to hash 20mb of text.

just2n 13y ago
You could really support an arbitrary password size by locally hashing the password to a length at least as long as the one you store in the database to preserve entropy, then send the fixed length hash as the user's "password", and hash it again on the server using something slower, like PBKDF2 with a few thousand rounds.
- Hello71 13y ago
  How do you think hashing functions work?
  - TillE 13y ago
    At least on the web, hashing isn't usually done locally (ie, in Javascript).
    - alexjeffrey 13y ago
      this doesn't add extra security - the first hash becomes a substitute password. A cracker could modify their steam account to take said hash as input and sent it directly over the wire.
      - just2n 13y ago
        It doesn't remove any security, either. If you perform a hash on the server that sends an arbitrary input size to a fixed number of bits, the attacker needs only to try O(2^n) different inputs where n is the number of bits of that hash. By using a hash with at least n bits as a substitute password, there's no less security. It does mitigate the DoS vector, however.

misterdai 13y ago

Only the support issue, but then I'd have thought that people using really long passwords as normally the ones who'll make an effort to remember / store them. They mention "Encrypted passwords", which would vary the amount of storage required in their database and also make the passwords retrievable if the attacker had gained access to the key / algorithm etc... I'd had preferred if they used a one way hash instead, obviously with a secure hash algorithm, uniquely salted and rehashed multiple times. All passwords would then be the same length when stored and users wouldn't have to have such a low maximum limit. There are worse offenders for low character limits, like Adobe with a 12 char maximum. Used to be a site that listed some, just a shame it's no longer available http://web.archive.org/web/20100526105638/http://www.weakpasswords.org/list http://web.archive.org/web/20100526105638/http://www.weakpas...