2015 Summer Paar DES - Decryption Divya Agarwal and Katie Marsh - Rhea

Decryption - Theory

DES as discussed is based on Feistel networks. And in Feistel ciphers only the keyschedule has to be modified for decryption.

Step 1: Generate the same 16 round keys in reverse order for decryption, that is given initial key $ k $ can we get $ k_{16} $ and the rest.

Step 2: As D0=D16 and C0=C16 the first round key can be generated by applying PC-2 right after PC-1 (no rotation here!). Refer Fig 1 for more details.

Fig 1: Reversed key Schedule for DES

Step 3: All other rotations of C and D can be reversed to reproduce the other round keys resulting in:

* No rotation in round 1.
* One bit rotation to the right in rounds 2, 9 and 16.
* Two bit rotations to the right in all other rounds.

Step 4: Note, the decryption and encryption function are the same essentially.

The complete lecture by Prof. Paar can be found here.

References

• C. Paar. Understanding Cryptography. Lecture Notes. Dept. of Electr. Eng. and In­for­ma­ti­on Sci­en­ces, Ruhr University.
• C. Paar and J. Pelzl. Understanding Cryptography. A textbook for Student and Practitioners. Springer 2010.

Questions and comments

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