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What bad thing happens if a counter is reused with the same key used to encrypt two messages in CTR mode?
CTR must never, ever reuse an nonce+Key pair. It can completely destroy the encryption. This is a major reason to avoid CTR unless you know what you’re doing. It is difficult to use correctly and has horrible failure modes.
Why the IV for CTR can be a nonce?
The “nonce” is better known as the Initialization Vector — with “IV” being the universal short name for that concept. CTR mode works by encrypting the successive values of a counter (CTR stands for “CounTeR”), so the IV in CTR mode is merely the value at which the counter starts.
How does AES CTR mode work?
The Counter (CTR) mode is a typical block cipher mode of operation using block cipher algorithm. In this version, we provide Advanced Encryption Standard (AES) processing ability, the cipherkey length for AES should be 128/192/256 bits.
Why are block cipher modes convenient?
It is easier because of direct encryption of each block of input plaintext and output is in form of blocks of encrypted ciphertext. Generally, if a message is larger than b bits in size, it can be broken down into bunch of blocks and the procedure is repeated.
Can You reuse an IV in CTR security?
CTR security requires that you never reuse an IV for two message encryptions with the same key.
Why are IV / key pairs not reused in CTR mode?
The problem with this is if you encrypt two different plaintexts with the same K e y, I V values, then the attacker gets two pairs: Where he can see the values C 1, C 2. With those, he can then compute: and thus deriving the value of the two plaintexts exclusive-or’ed together.
Can a random AES key be an IV?
Another note is to never use a IV which is all 0 just for convenience. IV’s are equal in size with of your block. In case of AES-128, the blocksize is 128, the keysize is 128 and hence the IV is 128 bits. The best way to do this is by creating a random AES key and using it as IV.
Which is the safest IV for AES-128?
CBC is as far as I know the safest (together with PCBC). If you don’t make the IV random (i.e., you use some repeating group of numbers), it will be easier to figure out the key if the cookie always start with the same clear text. The IV size for AES-128 is 128 bits.