libcrux: Potential Panic on Overlong Ciphertext Buffer

An application that passes in a ciphertext buffer of length greater than ptxt.len() + TAG_LEN to libcrux_chacha20poly1305::encrypt or libcrux_chacha20poly1305::xchacha20_poly1305::encrypt would experience a panic.

Impact

An application where the length of the ciphertext buffer is under attacker control could be made to crash.

Mitigation

The fix makes it so that libcrux_chacha20poly1305::encrypt and libcrux_chacha20poly1305::xchacha20_poly1305::encrypt no longer panic in this case, but instead write out the ciphertext and tag into the first ptxt.len() + TAG_LEN bytes of the provided buffer.