Cryptography

Terms

• Plaintext is the original message
• Ciphertext is the coded message
• Cipher is the algorithm use to transform plaintext to ciphertext
• Key is the info used in cipher known only to sender/receiver
• Cryptanalysis (codebreaking) is the study of principles/methods of deciphering ciphertext without knowing the key
• Cryptology is the field of cryptography and cryptanalysis
• Symmetric Encryption is when both sender and receiver use the same keys
• Asymmetric Encrpytion is when sender/receiver use different keys

Block vs. Stream ciphers

• Block ciphers encrypt block at a time
• Message is broken into blocks and encrypted
• Stream ciphers process a bit or byte at a time during encryption and decryption

Confusion and Diffusion

• Diffusion dissipates statistical structure of plaintext over bul of cipher text
• Confusion makes relationships between ciphertext and key as complex as possible

Fiestel Cipher

• Partitions input block into two halves
  • Employs multiple rounds of processing
  • Performs a substitution on left data half based on a fn of right half & subkey
  • Employs permutation swapping halves

Cipher parameters

• Block Size
  • Increasing size improves security, but slows cipher
• Key Size
  • Increasing size improves security, makes exhaustive key searching harder, but may slow cipher
• Number of rounds
  • Increasing number improves security, but slows cipher
• Subkey generation
  • Greater complexity can make analysis harder but slows cipher
• Round function
  • Greater complexity can make analysis harder, but slows cipher

DES cipher

• Data Encryption Standard
• Encrypts 64-bit data using 56 bit key
• The 56-bit key has 2^56 or 7.2*10^16 values

Source