# Tag Info

12

It actually leaks information. You are sending: Encrypted IV: $AES(k,IV)$ First ciphertext block of CBC: $AES(k, M_1 \oplus IV)$ Eavesdropper can observe whether the two blocks are equal, which happens iff $M_1$ is all zeroes.

3

It seems you want to make the IV secret for security purposes, in direct opposition to common knowledge and NIST recommendation that non secret keying material (such as a non-secret initialization vector) be... non secret. So that goes against some of the wisdom espoused a few years ago by Bart Preneel in this video, which says that IVs should be kept ...

2

Is the above example correct? If not, how can the IV to be used for decryption be determined? Yes, it is correct. The vector/mask in CBC mode is generally the previous ciphertext block. The algorithms in 2a and 2b simply extends this notion to the IV so that the CBC mode encryption doesn't have to be re-initialized. The outcome of the IV ...

2

The output of the block cipher is used as the new key, and also passed to the "output block" function, which is referenced in the NIST document as $B^m_R$. The purpose of the IV $R$ and the function $B^m_R$ is to reduce the output to a smaller size in a manner that hides the true output of $f$. Too large an output allows key recovery. The output of this ...

1

If the IV is all zeroes, then you basically have ECB for the first block. Basically you're proposing to use this first block's encryption as the IV for the second block. You're implying that the first block will always be unique, but low entropy, which sounds like a counter or time stamp. There are attacks when the IV is predictable, and while this is a ...

1

No. ​ An adversary who knows a ciphertext and one of its plaintext blocks p can trivially find the corresponding xored-plaintext x. ​ Since block ciphers with fixed keys are bijections, if there is a previous plaintext block then x has at least as much entropy as that previous plaintext block. (In particular, x can very easily be different from your other ...

1

No, not as described in the question. Putting aside the block size confusion that Richie Frame mentions in a comment (AES block size is always 128), there is no advantage to encrypting a second half of an IV in GCM mode in particular, and rarely in other modes. In GCM mode the actual IV is used to derive a nonce for CTR mode encryption. By adding a block ...

1

So if you consider ASCII encoding you have two plain text blocks: "SEND ME THE DATA" and " ENCRYPTED" + padding (which we will ignore). Note the space before "ENCRYPTED". Now if you change the IV you will indeed only change the first block. What you should however do is to change the first block of the ciphertext. That is used as vector (not the ...

Only top voted, non community-wiki answers of a minimum length are eligible