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. -- -- You received this message from the "vim_dev" maillist. Do not top-post! Type your reply below the text you are replying to. For more information, visit http://www.vim.org/maillist.php --- You received this message because you are subscribed to the Google Groups "vim_dev" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/groups/opt_out.
