If random data is prepended to a file encrypted with GNUPG's symmetric encryption, how may an attacker find out if the input file is deliberately corrupted or the passphrase is wrong?
If it is not easily ascertainable from which offset the actual encrypted data starts, is there any added security in always prepending a few kilobytes to an encrypted file in order to slow down an attacker?
In order to brute force the file, the attacker must start at offset 0 and continue incrementally with all likely passphrases until he gets a match.
Here I am assuming perhaps incorrectly that there is no magic number easily identifying the file as encrypted with GNUPG or there is no validation of the contents without trying the passphrase.
But if this assumption does not hold for GNUPG, what about Truecrypt's file based containers?
So far I know Truecrypt file based containers are indistinguishable from random noise, but the size gives away that the file may be a Truecrypt container.
If my TC container's actual size is 5 GB, and I prepend it with 5 GB random data, will brute forcing by an attacker who doesn't know the real offset be too costly?
Is there any design reason why crypto software do not let the user select the start offset for decryption in addition to the passphrase?