Effect of "decrypting" clear-text then "re-encrypting" it

Question

I have a (unique) situation where I receive data that is a combination of encrypted (with AES-CBC) and clear-text blocks and I cannot tell which blocks are encrypted. I plan to decrypt all of the data, including the clear-text, I receive and store it locally. Before retransmitting the data, I will encrypt it again. I need the blocks that were originally in clear-text to be in their original clear-text form for the retransmission.

If I decrypt the entire original message, even the blocks that were in clear-text, and then re-encrypt it with AES-CBC and the same key used for decryption, will the originally clear-text blocks be the same after re-encryption as they were when they were received.

Answer 1

Always encrypting the same text to the same value leads to data leakage, violating semantic security. To prevent this, CBC mode uses an initialization vector (IV). The IV is just a random string of bits equal to the block size of the cipher. This is XORed with the first block of clear-text to be encrypted. This makes the first encrypted block random. Each successive clear-text block is XORed with the cipher text of the previous block so that each block after the first is also random. From the Wikipedia:

The IV is then stored beside the encrypted text to allow decryption when the process is reversed:

This use of the IV ensures that encrypting the same message twice will not produce the same encrypted text. CBC mode even protects against repetition in a single cipher string. So repeating bytes in the clear-text message will not encrypt to the same byte sequence.

Note that a poorly implemented CBC encryption might reuse IVs and therefore produce the same encrypted text for the same message. See this answer for more information.