Using Encryption and Authentication Correctly
paragonie.com
paragonie.com
GCM is far more preferred to either CBC or CTR because it's less for the implementer to screw up.
NaCl's ChaCha20-Poly1305 is even better, because it's fast and constant-time.
'cperciva made the CTR+HMAC recommendation here: http://www.daemonology.net/blog/2009-06-11-cryptographic-rig...
Properly authenticated encryption that uses CBC+HMAC-SHA2 with PKCS7 padding is probably okay, but new developments should prefer AEAD modes above all else, and CTR+HMAC-SHA2 if no AEAD modes are available.
(The kind folks in ##crypto on freenode have pointed out to me that CTR also allows random-access decryption, where CBC mode does not. We haven't ever implemented this feature and cannot comment on it.)
Really, if your car doesn't have seatbelts, find an aftermket kit or buy a new car. Likewise, there are BSD-licensed libraries that provide GCM and ChaCha20-Poly1305. These are your aftermarket seatbelt kits for your deathtrap-car of a crypto library.
That being said....
If I were to drive without a seatbelt... CTS instead of CBC mode prevents padding oracle attacks. If you have a flaw that causes IV reuse, CTS and CBC leak less information than CTR mode.
For those driving without seatbelts... CBC and CTR modes do most definitely allow random-access decryption. Seek to block N and decrypt it, then seek to block N-1, read and XOR with the plaintext from your previous decryption. (It's a bit more complicated for the last two blocks of CTS mode, but only a bit.) CBC and CTS do not, however, allow random-access modification, since a single changed bit will on average flip half of the bits in every following block, to the end of the message.
On mobile devices, this can mean not being able to leverage significantly optimized vendor-provided crypto libraries.
I'd be interested in benchmarking the difference between a decent BSD-license GCM implementation and our use of AES-CBC+HMAC ETM on iOS hardware.
Is there a particular AES-GCM implementation you'd recommend starting with?
Or is random access CTR encryption dangerous? I don't see how, unless you reuse the keystream / make it a two-time pad.
Thomas's answer probably has to do with the risks of decrypting a stream and being unable to authenticate it first. (See also: the Cryptographic Doom Principle.)
Interesting to note that different security protocols on the Internet prefer different schemes:
1. SSH does Encrypt and MAC
2. SSL uses MtE
3. IPSec prefers EtM
Just writing this to be clear: it's not a debate. Ignore Schneier on this. In his defense: the most notable things he wrote about MtE were written before this was well-understood.
The "MAC" here is just a block of zeros, and if decryption successfully reveals such a block of zeros, then it's deemed authentic.
This is a pretty specific quality to their construction though, and it doesn't really work elsewhere.
Generally, though, and without intending snark:
If you're discussing MTE v ETM, and Bruce Schneier comes up, the answer is ETM.
https://paragonie.com/blog/2015/04/secure-authentication-php...
Our strategy (designed for a "remember me" checkbox) is actually a little more cautious than a simple nonce, in that we actually generate two tokens:
One is a selector (used to retrieve a record from the database, which is an operation that cannot be performed in constant time), while the other is a validator.
We store an SHA256 hash of the validator in the database. When the auto-login is invoked, we pull the hash and destination user ID from the database (based on the selector), the compare
if (hash_equals(hash('sha256', $verifier), $storedHash)) {
$_SESSION['userid'] = $storedUserId;
$this->generateAndStorePersistentToken(storedUserId);
}