My secret message is encrypted in a byte buffer with AES-128 in CBC mode. The first 16 bytes are the IV and the message follows contiguously. If I want to securely delete this message is it enough to delete (overwrite with random) the IV, the first block, or both?
In an answer to similar post (crypto shredding by erasing the IV instead of the key) it was suggested that if the key and one plain-text block is known then the entire message can be recovered. So it may be better to overwrite the entire secret message...
However, if the secret key has been forgotten decryption is impossible and so the length of the encrypted message is unknown. There may be circumstances where overwriting the entire is undesirable.