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I know there are 4 known weak keys, their value and that double encryption using one of the weak keys should give back the plain text.

I implemented this in OpenSSL. I have a plain text $m$ in a file plain.txt and compute $y = DES_k(m)$ where $k$ is a weak key. Then I compute $y' = DES_k(y)$. Instead of getting $m$ I get only a portion of it and the rest is part of $y$.

Why is this happening?

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If you don't get the expected result, then you are doing it wrong; most probably, use wrong mode of operation and/or wrong padding. You can get the expected result with openssl as

openssl enc -e -des-ecb -in plain.txt -out cipher1.txt -nopad -K 0101010101010101 -iv 0
openssl enc -e -des-ecb -in cipher1.txt -out cipher2.txt -nopad -K 0101010101010101 -iv 0

plain.txt file size should be multiple of 8 (8 bytes is DES block size), and plain.txt and cipher2.txt files should be identical.

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  • $\begingroup$ I'm almost there. I get an error that says "Data not multiple of block length", but when I look at cipher2.txt, more of the plaintext is there, but not all of it. My plaintext file has 19 characters (includes spaces). Is this related to the padding? How do I force the data to be a multiple of the block length? $\endgroup$ – planejane Sep 18 '16 at 16:29
  • $\begingroup$ Eureka. The mode should be ofb. Off to research why. Hints welcomed. ;) Thank you! $\endgroup$ – planejane Sep 18 '16 at 16:51
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    $\begingroup$ @planejane Eureaka is wrong. You should use ECB mode without padding (-nopad option), and plaintext file size should be multiple of 8 (8 bytes is DES block size); OFB mode decryption is the same operation as encryption, so OFB double encryption with the same key is in fact encryption+decryption, and it works with any key, not with a weak key only. $\endgroup$ – kludg Sep 18 '16 at 17:06
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    $\begingroup$ file size should be multiple of 8, so add some characters (dot, spaces, anything you like) to the end of file to make the file size multiple of 8. $\endgroup$ – kludg Sep 18 '16 at 17:15
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    $\begingroup$ @planejane: that handling is padding. You can see it at work: specify -e -des-ecb for encryption and -e -des-ecb -nopad for decryption; then examine the deciphered plaintext with something that can view arbitrary bytes, like hexdump. $\endgroup$ – fgrieu Sep 19 '16 at 3:51

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