I want to know is there any way to derive the initialization vector, if I know all the other details.
AES SHA 256bits CBC pkcs5padding
Input: Hello
Key: 123
Output: xORGCDL88sa8cUrK8JNSCQ==
Now, how to find IV?
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$\begingroup$ Does the padding oracle attack is possible? If not, no! CBC mode is CPA secure. $\endgroup$ – kelalaka Oct 21 '20 at 9:27
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1$\begingroup$ @kelalaka: CBC mode is not CPA secure if the adversary knows the key $\endgroup$ – poncho Oct 21 '20 at 13:07
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$\begingroup$ @poncho uh, The key is known. Forget security. The question is much easier than. use the libraries.. $\endgroup$ – kelalaka Oct 21 '20 at 13:09
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I want to know is there any way to derive the initialization vector, if I know all the other details.
Well, the first block (16 bytes for AES) of CBC mode encryption is defined as:
$$C_0 = E_k( IV \oplus P_0 )$$
where $P_0$ is the first block of the padded plaintext, and $C_0$ is the first block of the ciphertext.
If you know $C_0, P_0, k$, then you can rearrange this into:
$$IV = P_0 \oplus D_k( C_0 )$$
where $D_k$ is the AES transform in decryption mode, based on key $k$.
This allows you to rederive the IV directly.
