You shouldn't store a password in plaintext in your database. So if your database is leaked, they don't have the user's actual passwords. But with the digest scheme, you can't store a hash of the digest, because if all you have is Hash2(Hash1(username:realm:password)) and the nonces, then you can't compute Hash1(Hash1(username:realm:password):server_nonce:client_nonce), assuming that Hash2 is irreversible, which you want it to be.
Hash1 is the hash algorithm for the authorization digest, so MD5 or maybe SHA256 (although only firefox currently supports that). Hash2 is the hash algorithm (including salting) used on the server to hash the hash of the username password before storing in the database.
Hash(username:password) is what you store in the database != plain text.
The password isn't plaintext, but if the Hash(username:password) is compromised that gives an attacker enough information to impersonate that user. The attacker can just use that `Hash(username:password)` to compute the digest, they don't need to know the original password.
Yes, again if the database is compromised, it's compromised... I don't understand why so many people keep mentioning that when HTTPS does not solve that either?!
If a database of passwords is compromised, but the passwords are properly hashed and salted (server-side, not client-side), then you would need a rainbow table for each salt in order to actually get something usable from the compromised data.
Ignoring all the other benefits of HTTPS, it means you can securely transfer the password itself to the server, or a deterministic hash of the password which the server hashes again. Not to mention that you can use a stronger hashing algorithm, like bcrypt. There might be other ways to do this, but https is definitely the most practical.
who cares if you need a rainbow table?
If the passwords are moderately strong and you are using a good hashing algorithm (that is salted), then creating a big enough rainbow table (or brute forcing) is intractable.
Can you exaplin what a "2nd preimage attack" is and how HMAC solves it?
If it's an attack you can do after the database is compromised it's not very interesting, and if it's an attack you can do over the wire then you can just limit the number of login attempts per some duration no?
There are numerous ways to accidentally introduce a weakness to 2nd preimage resistance. Using an HMAC is a way to “reset” the entropy of a hash key and eliminate any unwanted source of information leakage.
Comments
You shouldn't store a password in plaintext in your database. So if your database is leaked, they don't have the user's actual passwords. But with the digest scheme, you can't store a hash of the digest, because if all you have is Hash2(Hash1(username:realm:password)) and the nonces, then you can't compute Hash1(Hash1(username:realm:password):server_nonce:client_nonce), assuming that Hash2 is irreversible, which you want it to be.
client_nonce?
This is how the RFC works: Hash(Hash(username:password):server_nonce)
What is the difference between Hash1 and Hash2?
Hash(username:password) is what you store in the database != plain text.
Also what is realm?
Yes, the client also generates a nonce that is included in the digest. The full digest (according to RFC 2617) is (assuming qop is used):
request-digest = <"> < KD ( H(A1), unq(nonce-value) ":" nc-value ":" unq(cnonce-value) ":" unq(qop-value) ":" H(A2) ) <">
Hash1 is the hash algorithm for the authorization digest, so MD5 or maybe SHA256 (although only firefox currently supports that). Hash2 is the hash algorithm (including salting) used on the server to hash the hash of the username password before storing in the database.
The password isn't plaintext, but if the Hash(username:password) is compromised that gives an attacker enough information to impersonate that user. The attacker can just use that `Hash(username:password)` to compute the digest, they don't need to know the original password.
Yes, again if the database is compromised, it's compromised... I don't understand why so many people keep mentioning that when HTTPS does not solve that either?!
If a database of passwords is compromised, but the passwords are properly hashed and salted (server-side, not client-side), then you would need a rainbow table for each salt in order to actually get something usable from the compromised data.
Ok, how does HTTPS help with that?
And who cares if you need a rainbow table?
Ignoring all the other benefits of HTTPS, it means you can securely transfer the password itself to the server, or a deterministic hash of the password which the server hashes again. Not to mention that you can use a stronger hashing algorithm, like bcrypt. There might be other ways to do this, but https is definitely the most practical.
If the passwords are moderately strong and you are using a good hashing algorithm (that is salted), then creating a big enough rainbow table (or brute forcing) is intractable.
This is vulnerable to 2nd preimage attacks. As a rule of thumb, you should use HMAC whenever you are concatenating values.
Can you exaplin what a "2nd preimage attack" is and how HMAC solves it?
If it's an attack you can do after the database is compromised it's not very interesting, and if it's an attack you can do over the wire then you can just limit the number of login attempts per some duration no?
I can try to explain, but this answer on the theoretical computer science stackexchange does it much better: https://cstheory.stackexchange.com/a/591
There are numerous ways to accidentally introduce a weakness to 2nd preimage resistance. Using an HMAC is a way to “reset” the entropy of a hash key and eliminate any unwanted source of information leakage.
which is itself a problem with using the Digest method from RFC 2617