Guessing the IV for CBC If I have the key, the ciphertext under AES-CBC and the plaintext, then how can I guess the initialisation vector?
CBC mode decryption for the first block is defined as:

$$P_0 = IV \oplus D_k( C_0 )$$

where $P_0$ is the first plaintext block, $C_0$ is the first ciphertext block, and $D_k$ is the decryption by the block cipher using the key $k$.

This can be rearranged as:

$$IV = P_0 \oplus D_k( C_0 )$$

which allows you to reconstruct the IV, if you know the key, the plaintext and the ciphertext.