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68249Re: Encryption: Vim should use authenticated encryption mode

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  • Ulrik
    Feb 14 4:07 PM
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      On 2013-02-14 22:08, Bram Moolenaar wrote:
      >
      > Ulrik Sverdrup wrote:
      >
      >> The blowfish encryption mode is vulnerable (not to revelation of the
      >> plaintext), but the encryption is not checked for integrity or
      >> authenticity. This means that someone might corrupt the encrypted file
      >> (hexedit or similar), and vim will decrypt it without notice of error or
      >> warning.
      >>
      >> This attack allows someone to modfiy encrypted files so that the owner
      >> doesn't notice. With sufficient tries or skill it might be possible to
      >> change a file's values in a predictable way at a certain offset.
      >>
      >> The solution is an authenticated encryption mode. The common way to do
      >> it is 'Encrypt-then-MAC' where a message authentication code is formed
      >> from the ciphertext and the key. This code when matching will prove that
      >> the document is unchanged and was produced by someone with access to the
      >> key. This code will detect the previous attack case, and additionally it
      >> allows vim to detect that the wrong password was entered. Security
      >> practise says that Vim must fail with an error if the MAC does not match.
      >
      > I think that a verification key will actually make it easier to crack
      > the password. Currently, when an attacker tries all kinds of passwords,
      > he also needs a way to verify the decrypted text is actually readable.
      > That is not so easy to do. With a verification key the verify part
      > becomes really easy and fast.
      >
      > It is extremely difficult to change the file in a way that after
      > decryption it is readable text. Probably just as difficult as cracking
      > the password. When knowing that a file is only plain text, checking for
      > invalid Unicode characters is probably sufficient to notice that the
      > decryption failed.
      >

      Using Vim 7.3 patches 1-547, this is not true, and it is trivially
      testable (otherwise I would not have claimed it).

      Using :set cm=blowfish :X goodenough
      I produced file A that ends with "I owe you 200 USD"

      using hex editor I flipped 1 single bit to produce file B, that ends
      with "I owe you 300 USD". You can diff the two binary files by using:

      diff <(xxd A) <(xxd B)

      a one-bit difference in the ciphertext leads to a one-bit difference in
      the plain text, and we have a false document and undedetected corruption.

      To reproduce, here are files A and B:

      xxd -r >A <<EOF
      0000000: 5669 6d43 7279 7074 7e30 3221 4638 a780 VimCrypt~02!F8..
      0000010: 332a 14a3 e680 d2dd 2003 d079 9b8a 6ca7 3*...... ..y..l.
      0000020: 0e43 da8b b1bb 6aad 0f1a c38c f4ba 24ba .C....j.......$.
      0000030: 181b c7d6 9b8a 6ca7 0e43 da8b b1bb 6aad ......l..C....j.
      0000040: 0f1a c38c f4ba 24ba 181b c7d6 9b8a 6ca7 ......$.......l.
      0000050: 0e43 da8b b1bb 6aad 0f1a c38c ec09 c98f .C....j.........
      0000060: 2322 0fd6 1aff 59b1 47cc a61f 5a62 c89c #"....Y.G...Zb..
      0000070: eba3 d824 ec09 c98f 2322 0fd6 1aff 59b1 ...$....#"....Y.
      0000080: 47cc a61f 5a62 c89c eba3 d824 ec09 c98f G...Zb.....$....
      0000090: 2322 0fd6 1aa1 78f8 5b9b aa4c dbfb 6d56 #"....x.[..L..mV
      00000a0: 32e5 962e b15c 000a f6 2....\...
      EOF

      xxd -r >B <<EOF
      0000000: 5669 6d43 7279 7074 7e30 3221 4638 a780 VimCrypt~02!F8..
      0000010: 332a 14a3 e680 d2dd 2003 d079 9b8a 6ca7 3*...... ..y..l.
      0000020: 0e43 da8b b1bb 6aad 0f1a c38c f4ba 24ba .C....j.......$.
      0000030: 181b c7d6 9b8a 6ca7 0e43 da8b b1bb 6aad ......l..C....j.
      0000040: 0f1a c38c f4ba 24ba 181b c7d6 9b8a 6ca7 ......$.......l.
      0000050: 0e43 da8b b1bb 6aad 0f1a c38c ec09 c98f .C....j.........
      0000060: 2322 0fd6 1aff 59b1 47cc a61f 5a62 c89c #"....Y.G...Zb..
      0000070: eba3 d824 ec09 c98f 2322 0fd6 1aff 59b1 ...$....#"....Y.
      0000080: 47cc a61f 5a62 c89c eba3 d824 ec09 c98f G...Zb.....$....
      0000090: 2322 0fd6 1aa1 78f8 5b9b aa4c dbfb 6d56 #"....x.[..L..mV
      00000a0: 33e5 962e b15c 000a f6 3....\...
      EOF


      Note: I didn't search or brute force this, I only counted the right byte
      offset in the file and flipped a bit. I really hope I am somehow
      mistaken, but I don't think I am.

      Regarding quickening brute force by using a MAC, this is a false, the
      MAC can have equivalent security factor to the block cipher, it should
      really not be a concern.

      HTH,
      ulrik

      PS. the password is 'goodenough' literally.

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