View source for 2015 Summer Cryptography Prof. Paar Introduction to Cryptography Divya Agarwal and Katie Marsh - Rhea

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[[Category:Paar]]
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DES Key Schedule
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A [http://www.projectrhea.org/learning/slectures.php slecture] on [[Cryptography]] by student Divya Agarwal and Katie Marsh.

Partly based on the [[2015 Summer Cryptography Paar|Cryptography Summer 2015]] lecture material of Prof. Paar.

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Introduction to Key Schedule by Prof. Paar.

=[https://youtu.be/GGtKeWizkaY Link to video on youtube]=
<youtube>https://youtu.be/GGtKeWizkaY</youtube> 
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== Accompanying Lecture Notes ==
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==== DES Key Schedule - Theory ====

The DES ''key schedule'' genrates 16 round keys (or sub-keys) for the 16 encryption rounds. The sub-keys are derived out of the original 64-bit key given as an input. 

Step 1: The original 64-bit key is reduced to 56-bit key using the PC-1 permutation table (Fig 1a). Note: Every 8th bit is ignored in the table(i.e. bits numbered 8, 16, 24, 32, 40, 48, 56, and 64). Nevertheless number the bits from 1 to 64, going left to right(Fig 1b).

[[File:KS2.png|400px|thumb|left|Fig 1a: PC-1 Permutation table]]
[[File:KS1.png|250px|thumb|left|Fig 1b: 64-bit key input bit description]]

=[http://youtu.be/H10mlpjjLAQ Link to video on youtube]=
<youtube>http://youtu.be/H10mlpjjLAQ</youtube> 

Step 2: Split the 56-bit Key in two 28-bit halves: Left - <math>C_0</math> and Right - <math>D_0</math>(Refer Fig 2).

[[File:KS3.png|400px|thumb|left|Fig 2: Key Schedule for DES encryption]]

=[http://youtu.be/sI1FoqiXix0 Link to video on youtube]=
<youtube>http://youtu.be/sI1FoqiXix0</youtube> 

Step 3: With <math>C_0</math> and <math>D_0</math> defined, we now create sixteen blocks <math>C_n</math> and <math>D_n</math>, <math>1≤n≤16</math> using following rules.
* In rounds i = 1,2,9,16 , the two halves are each rotated left by '''one''' bit.
* In all other rounds, where i ≠ 1,2,9,16 , the two halves are each rotated left by '''two''' bits.
* Total number of rotations 4*1 + 12* 2 = 28 which leads to an interesting property : <math>C_0</math>=<math>C_{16}</math> and <math>D_0</math>=<math>D_{16}</math>.

Step 4: We now form the keys <math>K_n</math>, for <math>1≤n≤16</math>, by applying the PC-2 permutation table (Fig 3) to each of the concatenated pairs <math>C_nD_n</math> to generate 48-bit keys out of 56-bit bit <math>C_nD_n</math>.

[[File:KS4.png|250px|thumb|left|Fig 3: PC-2 Permutation table]]

The complete lecture video can be found [https://www.youtube.com/watch?v=l-7YW06BFNs here].

==== DES Key Schedule - Example ====




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== 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.
* J. Orlin Grabbe, The DES Algorithm Illustrated.
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==[[2015_Summer_Cryptography_Prof. Paar_Introduction to Cryptography_Divya Agarwal and Katie Marsh_comments | Questions and comments]]==
If you have any questions, comments, etc. please post them [[2015_Summer_Cryptography_Prof. Paar_Introduction to Cryptography_Divya Agarwal and Katie Marsh_comments|here]].
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