md5

deep

Hello,

Im using md5 to encrypt my users passwords. If I understood right there is no way to change the password back??

Everytime a user tried to login, my program will md5(his password) and then match that one with the one in the DB.

BUT if a user lost the password... i want the user to enter his email so that the script can look up the password... but its encrypted?? Is it possible to change it back??

Thanks!

bubblenut

No, that's why it's called a one way encryption algorythm :rolleyes: 😉
What one would normally do in this kind of situation (well what I would do and I'm prett normal ... I think) is generate a new password, encrypt it, set it in the database and then send this to the user, they can then change it to something more convenient if they so wish.

HTH
Bubble

deep

Sounds great, thanks!

onion2k

You could make a loop that MD5s random strings until you find one that matches...

:p 😃

ucbones

not thoroughly convinced that would work, an 8 character string has 36⁸ possible combinations, which is 2821109907456 possible combinations. Assuming each one took 0.01 seconds to process that's some 800 years of process time. Anyone fancy that?

😉

ucbones

onion2k

I wasn't being entirely serious. Thats what the smilies are for.

Although, you're actually wrong. On average you'll only have to search half the keyspace (sometimes you'll find the password really quick, other times it'd take almost all the strings to find..), so its really it'd only be 400 years. 😉

laserlight

MD5 isnt an encryption algorithm.
MD5, or Message Digest 5, is a message digest algorithm.
It takes a message of arbitrary length, and outputs a hash of 128 bits.

It should be computationally infeasible for 2 given messages to produce the same hash.
Also, it should be computationally infeasible to find a message that corresponds to a given hash.

That said, since in theory there are an infinite number of messages, there should also be an infinite number of messages corresponding to a given hash.
So we're only talking about the usage in practice.

You could read [url=ftp://ftp.rfc-editor.org/in-notes/rfc1321.txt]RFC 1321[/url]

Incidentally, PHP now also supports SHA1 hashes, using [man]sha1/man
You can read [url=ftp://ftp.rfc-editor.org/in-notes/rfc3174.txt]RFC 3174[/url] for more info on SHA1

On brute force attacks: the main problem would be that users have a bad habit of using easy to guess passwords, so a dictionary attack might be used, which would considerably reduce the time taken.
This sort of relates to the idea that MD5 is not intended to encrypt passwords.
It is intended to provide a way of checking data integrity, as is SHA1.
It just happens that we can use this to prevent an attacker from obtaining the original password from a hashed password, assuming the password is well chosen.
There is no security other than that.