How do I convert password hashing from MD5 to SHA?

Question

I've got an old application that has user passwords stored in the database with an MD5 hash. I'd like to replace this with something in the SHA-2 family.

I've thought of two possible ways to accomplish this, but both seem rather clunky.

1) Add a boolean "flag" field. The first time the user authenticates after this, replace the MD5 password hash with the SHA password hash, and set the flag. I can then check the flag to see whether the password hash has been converted.

2) Add a second password field to store the SHA hash. The first time the user authenticates after this, hash the password with SHA and store it in the new field (probably delete their MD5 hash at the same time). Then I can check whether the SHA field has a value; this essentially becomes my flag.

In either case, the MD5 authentication would have to remain in place for some time for any users who log in infrequently. And any users who are no longer active will never be switched to SHA.

Is there a better way to do this?

Solution 1

Essentially the same, but maybe more elegant than adding extra fields: In the default authentication framwork in Django, the password hashes are stored as strings constructed like this:

hashtype$salt$hash

Hashtype is either sha1 or md5, salt is a random string used to salt the raw password and at last comes the hash itself. Example value:

sha1$a1976$a36cc8cbf81742a8fb52e221aaeab48ed7f58ab4

Solution 2

You can convert all your MD5 Strings to SHA1 by rehashing them in your DB if you create your future passwords by first MD5ing them. Checking the passwords requires MD5ing them first also, but i dont think thats a big hit.

php-code (login):

prev: $login = (md5($password) == $storedMd5PasswordHash);

after: $login = (sha1(md5($password)) == $storedSha1PasswordHash);

Works also with salting, got the initial idea from here.

Solution 3

I think you've already got the best possibilities. I like #1 more than #2, since there's no use for the md5 once the sha is set.

There's no way to reverse the MD5, so you have to wait for the user to authenticate again to create a new hash.

Solution 4

No - basically you'll have to keep the MD5 in place until all the users you care about have been converted. That's just the nature of hashing - you don't have enough information to perform the conversion again.

Another option in-keeping with the others would be to make the password field effectively self-describing, e.g.

MD5:(md5 hash)
SHA:(sha hash)

You could then easily detect which algorithm to use for comparison, and avoid having two fields. Again, you'd overwrite the MD5 with SHA as you went along.

You'd want to do an initial update to make all current passwords declare themselves as MD5.

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