Introduction to Information Security

C = EK(M)

Decryption is just the reverse operation; transforming ciphertext C into plaintext M under control of key K

M = DK(C)

It should be (computationally) infeasible for an observer of C to recover either M or K (in a reasonable time) – security requirement

Symmetric-Key Cryptography

• Both sender and receiver keys are the same: KA=KB
• The keys must be kept secret and securely distributed
• Thus, also called “Secret Key Cryptography”
• Data Encryption Standard (DES)

Encryption
Decryption
plaintext
ciphertext
plaintext
Key KA
Key KB=KA
Secure Key Distribution

What is a block cipher?

Block ciphers are the crypto work horse

Canonical examples:

• 3DES: n = 64 bits, k = 168 bits
• AES: n = 128 bits, k = 128, 192, 256 bits

Block of plaintext
n bits
Key
k bits
Block of ciphertext
n bits
E, D

Block Cipher

• Divide input bit stream into n-bit sections, encrypt only that section, no dependency/history between sections

• In a good block cipher, each output bit is a function of all n input bits and all k key bits

Courtesy:
Andreas
Steffen

Block ciphers built by iteration

key expansion
key k1
key k2
key k3
key kn
key k
m
R(k1, ∙)
R(kn, ∙)
R(k3, ∙)
R(k2, ∙)
c
R(k, m) is called a round function
Ex: 3DES (n=48), AES128 (n=10)
m
c
m1
m2
m3

Stream Ciphers

Recall: A stream cipher typically xors plaintext byte-by-byte with PRNG(k)

Example: RC4 (Rivest Cipher 4) is a PRNG based on a key, and is used as a stream cipher in TLS and WPA