Difference between XORing the IV and just pre-pending it to the plaintext before encryption with AES-CBC [duplicate]

Question

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• Using CBC with a fixed IV and a random first plaintext block 5 answers

The normal way of using AES-CBC is to XOR the IV with the first plaintext block

This way you have to pass the IV along the ciphertext.

What if instead of XORing we just put a fixed size randomIV in front of the plaintext:

Encrypt(randomIV + plaintext) = C1

And we don't have to send/tansmit separatly the IV since it is inside C1. Decryption works similarly:

Decrypt(C1) = randomIV + plaintext

And we just remove the randomData (fixed size) to end up with the plaintext.

In essence, the first block of random data, when encrypted will work EXACTLY as a randomIV since it will be XORed with the second block

Except from performance loss (having to encrypt an additional block instead of XORing), can you think why this might not be secure.

Answer 1

If I got you right, you propose to encrypt IV and then run the same CBC-mode encryption using the encrypted IV; it just gives an adversary one more ciphertext block.