Does bcrypt on top of md5 reduce its security?

Question

A GitHub project I recently checked has a flawed security as they're using an unsalted md5() to hash the passwords. I opened a bug and they acknowledged it. However, the transition from md5() to bcrypt would mean that every user would need to log in so the password can be migrated.

So my question is, does using bcrypt over md5() reduce it's security?

// For already logged in users:
if (password_verify(md5($_POST['pass']), $DBHash)) {

  }

In that way, the whole database could be migrated easily, instead of migrating each user individually.

Should there be any other problem besides the fact that PHP might timeout if not handled correctly? (around 1 second for each hash).

Answer 1

Absolutely not. In fact, this is almost the standard procedure for migration from a weak hashing scheme to bcrypt in every project I worked with.

What usually happens is that a flag is set for each user indicating that their passwords use the old scheme, and whenever they login the application uses the backward-compatible login logic. Once the user logs in, you have their plaintext password, and now you can simply take the new password and store it using bcrypt only, the unset the flag. All new users use the new bcrypt-only scheme.

Alternatively, you can go by CodesInChaos's suggestion and stick to the new bcrypt(md5()). While he's indeed correct that there's no security gain, I personally like to keep it bcrypt() only.