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I get this warning while I decrypt a Twofish-encrypted file in GPG.

WARNING: message was not integrity protected.

From what I found online, this warning should only come up for CAST5, not for AES256 or Twofish. Why does this occur? And is there a way to prevent this warning?

I use GPG on Ubuntu 12.04 GNU/Linux.

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    I'd guess you only encrypted the message (with an unauthenticated mode like CFB or CBC) without applying a MAC or digital signature.
    – CodesInChaos
    Commented Mar 23, 2014 at 17:31
  • No, I encrypted the file with gpg this way. > "gpg -s -armor --symmetric TWOFISH filename" From the man page, I found that -s option signs it with my RSA key.
    – user258413
    Commented Mar 25, 2014 at 7:23

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How was the file encrypted? OpenPGP provides an option to integrity protect your ciphertext, called MDC. Please note this is not the same as MAC or a signature.

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  • Force-mdc option in GPG is required only for CAST5, not for 256bit key size like in AES256 or Twofish.
    – user258413
    Commented Mar 25, 2014 at 7:28
  • That's why it throws a warning, not an error I guess?
    – Tim Lamballais
    Commented Mar 25, 2014 at 7:43
  • Yes, it's only a warning - which it should NOT throw for Twofish or AES256 in GPG, because their block sizes are above 64 bits. (I should have said block-size in previous comment instead of key size, sorry!) The default algorithm "CAST5" requires force-mdc due to its lower block size. I found this info here : is.gd/JggekU
    – user258413
    Commented Mar 26, 2014 at 14:52

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