Cryptography.ppt
- 1. Introduction to Symmetric Block Cipher Jing Deng Based on Prof. Rick Han’s Lecture Slides Dr. Andreas Steffen’s Security Tutorial
- 2. Cryptography • Encryption algorithm also called a cipher • Cryptography has evolved so that modern encryption and decryption use secret keys • Cryptographic algorithms can be openly published • Only have to protect the keys Encryption Decryption plaintext ciphertext plaintext Encryption Decryption plaintext ciphertext plaintext Key KA Key KB
- 3. 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
- 4. Cryptanalysis • Brute force: try every key • Ciphertext-only attack: • Attacker knows ciphertext of several messages encrypted with same key (but doesn’t know plaintext). • Possible to recover plaintext (also possible to deduce key) by looking at frequency of ciphertext letters • Known-plaintext attack: • Attackers observes pairs of plaintext/ciphertext encrypted with same key. • Possible to deduce key and/or devise algorithm to decrypt ciphertext.
- 5. Cryptanalysis (2) • Chosen-plaintext attack: • Attacker can choose the plaintext and look at the paired ciphertext • Attacker has more control than known-plaintext attack and may be able to gain more info about key • Adaptive Chosen-Plaintext attack: • Attacker chooses a series of plaintexts, basing the next plaintext on the result of previous encryption • Examples • Differential cryptanalysis – DES is resistant it • Linear cryptanalysis • Cryptanalysis attacks often exploit the redundancy of natural language • Lossless compression before encryption removes redundancy
- 6. Examples • Simple and non-secure ciphers – Shift Cipher – Caesar Cipher – Affine Cipher – Vigenere Cipher – Hill Cipher • Information-secure cipher – One-Time Pad
- 7. Confusion and Diffusion • Terms courtesy of Claude Shannon, father of Information Theory • “Confusion” = Substitution • a -> b • Caesar cipher • “Diffusion” = Transposition or Permutation • abcd -> dacb • DES Encryption Decryption plaintext ciphertext plaintext Key KA Key KB
- 8. Confusion and Diffusion (2) • Modern substitution ciphers take in N bits and substitute N bits using lookup table: called S- Boxes • “Confusion” : a classical Substitution Cipher Courtesy: Andreas Steffen
- 9. Confusion and Diffusion (3) • “Diffusion” : a classical Transposition cipher • modern Transposition ciphers take in N bits and permute using lookup table : called P-Boxes Courtesy: Andreas Steffen
- 10. 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
- 11. Example: DES • Data Encryption Standard (DES) • Encodes plaintext in 64-bit chunks using a 64-bit key (56 bits + 8 bits parity) • Uses a combination of diffusion and confusion to achieve security • Was cracked in 1997 • Parallel attack – exhaustively search key space • Decryption in DES – it’s symmetric! Use KA again as input and then the same keys except in reverse order
- 12. Example: DES (2) • DES • 64-bit input is permuted • 16 stages of identical operation • differ in the 48-bit key extracted from 56-bit key - complex • R2= R1 is encrypted with K1 and XOR’d with L1 • L2=R1, … • Final inverse permutation stage
- 13. Example: DES (3)
- 14. Beyond DES • Triple-DES: put the output of DES back as input into DES again with a different key, loop again: 3*56 = 168 bit key • Advanced Encryption Standard (AES) – Requirements: • shall be designed so that the key length may be increased as needed. • block size n = 128 bits, key size k = 128, 192, 256 bits – Candidates: MARS, twofish, RC6, Serpent, Rijndael – successor (Rijndael)
- 15. Encryption Mode (ECB) • Electronic Code Book (ECB) mode for block ciphers of a long digital sequence • Vulnerable to replay attacks: if an attacker thinks block C2 corresponds to $ amount, then substitute another Ck • Attacker can also build a codebook of <Ck, guessed Pk> pairs
- 16. Encryption Mode (CBC) • Cipher Block Chaining (CBC) mode for block ciphers • Inhibits replay attacks and codebook building: identical input plaintext Pi =Pk won’t result in same output code due to memory-based chaining • IV = Initialization Vector – use only once
- 17. Stream Cipher • Stream ciphers • Rather than divide bit stream into discrete blocks, as block ciphers do, XOR each bit of your plaintext continuous stream with a bit from a pseudo-random sequence • At receiver, use same symmetric key, XOR again to extract plaintext