The initialization vector (IV) is exclusive or\'d against the plain text before

Question

The initialization vector (IV) is exclusive or'd against the plain text before encryption for the first block sent in order to prevent an attacker from learning that duplicate message blocks are being sent. This technique is often used with streaming modes like CBC.

I have researched some of the questions that were asked about whether it was safe to pass the IV in the clear. The general consensus seems to be that the IV can be safely passed in the clear, but the IV should be random to prevent certain kinds of attacks. Using a counter for the IV is also thought to be vulnerable as well and only randomly generated IVs should be used. I don't really understand this since the reason stated for passing a random IV is that it can not be easily guessed.

In any case, my question is as follows:

It should be relatively easy to send the first block of a message encrypted, but without any IV processing. This first block could contain the IV which would then be used for all remaining blocks of the message. Since the IV in the encrypted message is random, there should never be any duplicate first blocks. How come this is not being done and isn't it at least a little safer than sending the IV in the clear?

Explanation / Answer

Depending on the mode of operation, transmitting the IV encrypted (with the same key as used for the rest of the process) can actually weaken security a lot.

For example, in the CFB and OFB modes, the IV is encrypted and the result XORed with the first block of the plaintext to produce the first block of ciphertext. Thus, an adversary who knows the encrypted IV can trivially undo the XOR to recover the first block of plaintext! The same is true for the CTR mode, if the IV/nonce is used directly as the initial counter value.

The CBC mode works differently, and does not break as catastrophically if you encrypt the IV before transmitting it. However, as Henrick Hellstr

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