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* In rounds i = 1,2,9,16 , the two halves are each rotated left by '''one''' bit. | * 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. | * In all other rounds, where i ≠ 1,2,9,16 , the two halves are each rotated left by '''two''' bits. | ||
| − | * Total | + | * 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: | + | 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>. |
Revision as of 07:49, 17 June 2015
Introduction to Cryptography
A slecture on Cryptography by student Divya Agarwal and Katie Marsh.
Partly based on the Cryptography Summer 2015 lecture material of Prof. Paar.
Contents
Link to video on youtube
Accompanying Lecture Notes
DES- Key Schedule
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 1). 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.
Step 2: Split the 56-bit Key in two 28-bit halves: Left - $ C_0 $ and Right - $ D_0 $(Refer Fig 2).
Step 3: With $ C_0 $ and $ D_0 $ defined, we now create sixteen blocks $ C_n $ and $ D_n $, $ 1≤n≤16 $ 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 : $ C_0 $=$ C_{16} $ and $ D_0 $=$ D_{16} $.
Step 4: We now form the keys $ K_n $, for $ 1≤n≤16 $, by applying the PC-2 permutation table (Fig 3) to each of the concatenated pairs $ C_nD_n $ to generate 48-bit keys out of 56-bit bit $ C_nD_n $.
Questions and comments
If you have any questions, comments, etc. please post them here.
Back to 2015 Summer Cryptography Prof. Paar
