# List out and explain the components of block ciphers in symmetric key encryption?

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) 