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I'm trying to complete an encryption based CTF and am testing for the use of DES encryption with weak or semi-weak key pairs by taking the keys listed on wikipedia (https://en.wikipedia.org/wiki/Weak_key#Weak_keys_in_DES) and encrypting the given cipher text with said keys to see if it yields any plaintext. The listed semi-weak key pairs on the page though are 16 bytes long, when surely a DES key is 8 bytes? How can such keys be used in DES encryption operations?

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A weak key pair is just that, a pair of keys. So a pair of 8 byte keys is 16 bytes.

For their example weak keys:

0x011F011F010E010E and 0x1F011F010E010E01

This can be broken down into:

01 1F 01 1F 01 0E 01 0E and 1F 01 1F 01 0E 01 0E 01

The first byte of the first key is 01, the second byte of the first key is 1F, etc.

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  • On Wikipedia is has two 16 byte keys as a single pair though.
    – muke
    Commented Apr 7, 2019 at 3:12
  • @muke No? I see a pair of 8 byte keys. Remember that each hex character is half a byte, so the key 011F011F010E010E is 01:1F:01:1F:01:0E:01:0E, aka 8 bytes.
    – forest
    Commented Apr 7, 2019 at 3:13
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    Oh, I wasn't aware of this, that makes sense then. Thanks.
    – muke
    Commented Apr 7, 2019 at 3:13

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