Safely erasing encrypted data 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.
If I want to securely delete this message is it enough to delete (overwrite with random) the IV, the first block, or both?


No, it's not enough.  Not even if you erase both.

(Not unless your message is at most two blocks long, that is.)

Erasing the IV makes the first block of the message undecryptable.  Erasing the IV and the first $n$ blocks of ciphertext makes the first $n+1$ blocks undecryptable.  In either case, the rest of the message can still be decrypted normally.

For modes other than CBC, the details may vary, but for all commonly used modes the conclusion remains the same: to ensure that no part of the message can be decrypted even if the key is compromised, you will need to erase the entire ciphertext (or the IV and almost all of the ciphertext, at least).