It is the number of bits (bytes) in the key. It is the number of bits (bytes) in the key

It is the number of bits (bytes) in the key.

• It is the number of bits (bytes) in the key.

• A 2-bit key has four values

• 00, 01, 10, 11 in its key space

• A key of length “n” has a key space of 2^n distinct values.

• E.g. the key is 128 bits

• 101010101010….10010101111111
• There are 2^128 combinations
• 340 282 366 920 938 463 463 374 607 431 768 211 456

Examples: DES, RC4, RC5, IDEA, Skipjack

• Examples: DES, RC4, RC5, IDEA, Skipjack

• Advantages: fast, ciphertext secure

• Disadvantages: must distribute key in advance, key must not be divulged

Symmetric Cipher

• Symmetric Cipher

• EK2(M) = C

• DK2(C) = M

• With the property that:

• DK2(EK2(M)) = M

• And, with different keys (public/private) we have:

• EK1(M) = C

• DK2(C) = M

• DK2(EK1(M)) = M