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Comment by kkl | Hacker News Reader

Why would you prefer CTR over CBC (assuming properly authenticated)? The only benefit I could think of is performance.

CTR saves you the trouble of padding your plaintext before encrypting, thus eliminating an entire class of cryptography attacks (i.e. padding oracles). The security margins of CBC and CTR are otherwise similar.

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.)

Be very careful with random-access CTR.

Can you elaborate? (Either by direct explanation or by reference to something explaining the hazard there?)

Could you please point to an explanation why random access CTR decryption is dangerous? Or why it's more or less secure than random access CBC decryption?

Or is random access CTR encryption dangerous? I don't see how, unless you reuse the keystream / make it a two-time pad.

https://www.imperialviolet.org/2014/06/27/streamingencryptio...

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.)

But it is still safe to do random access AES-CTR if you've already checked the MAC. Random access is not a problem, decrypting before authenticating is a problem.

I think that's why the statement was "be careful with" rather than "don't"

I feel like "if an AEAD mode isn't available, use CTR with HMAC with the following caveats..." is a bit like saying "if you drive without a seatbelt, make sure to drive extra carefully".

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.

> 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.

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?

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