Block Cipher
- A block cipher is one in which a block of plaintext is treated as a whole and used to produce a ciphertext block.
- Typically, a block size of 64 or 128 bits is used.
- the two users share a symmetric encryption key
- The vast majority of network-based symmetric cryptographic applications make use of block ciphers.
- Example of Block ciphers: Playfair cipher, Hill cipher
P-box (Permutation box):
- P-box is equivalent to a transposition cipher; it transposes bits.
- Three types of P-boxes are shown: straight P-box, expansion P-box and compression P-box
P-box can be implemented in hardware (pre-wired) or in software. In software, it is represented as a Lookup table ..
strait p box
straight P-box
compression P-box and Expansion P box
- Example of Permutation operation as a look up table for straight P-box
Above Table (table 5.1 in Forouzan) shows 64 entries corresponding to 64 input bits–
- Entries in the table are inputs locations, and
- position(index) of the entry in the table corresponds to output location.
- Ie ., bit in the 58th location of input will go to 1st location of the output, etc
Substitution Box
- S-box (substitution Box) – can be considered as a substitution cipher. It can have different number of inputs and outputs. S-box can be keyed or keyless
- In the example, no of inputs is equal to number of outputs.
Corresponding input/output relation
- Following table defines i/o relation for a 3 x 2 S-box. It produces a 2 bit output for an input of 3 bits
- The leftmost bit of input defines the row, rightmost 2 input bits defines the column
- Values on the cross section of selected row & column is the output
- eg: for input 001, output is 10
(for input 100 also, output is 10)
