1

I have written a message and encrypted it using cipher block chaining.

What will happen if the receiver loses the Initialization Vector, or doesn't receive at all?

1

When decrypting a message in CBC mode, each ciphertext block ci is decrypted with the chosen key, and then XORed with the previous ciphertext block ci-1.

Since for c1, there is no c0, we use the IV instead. So if the receiver knows the ciphertext and the key used to encrypt it, but not the IV, they can decrypt everything apart from the first block.

3

In a cipher block chain, each block is XORed with the ciphertext of the previous block, not the plaintext. So even if you cannot decipher one block, as long as you have received the complete block intact and correct, you can still use it to decipher the next one.

So, if your receiver doesn't have the Initialization Vector, they will be unable to decipher the first block they receive. But as long as they receive the first block, they will still successfully decipher the second (and each successive) block.

2

The bigger problem is not the garbled first block, as already answered. The real problem is that if you use Authenticated Encryption (or AEAD), as you must, then the message cannot be authenticated without the IV (because the IV must be covered by the MAC), and when the message cannot be authenticated, it must not be decrypted. The job of the MAC is to ensure unauthenticated messages are never passed to AES (or whatever) together with your real key.

Since you are trying to decrypt messages that don't have their IV, we must assume they are also unauthenticated (or you had a MAC that didn't cover the IV, which is broken). Fix that.

You can do CBC + HMAC, encrypt-then-MAC (thus MAC-then-decrypt), with the MAC covering the IV, and it would be secure. But, it would be much better to use AES-GCM or Chacha20-Poly1305. It would be even better to just use libsodium or Google Tink.

  • 1
    This is a great answer, and shows the important difference in the answers you'll get by asking the questions "what will happen" and "what should happen"... – Johnny Apr 14 at 21:58

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