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In cryptography, a one-time pad (OTP) is an encryption technique that cannot be cracked if used correctly. In this technique, a plaintext is paired with a random ...

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- 1. One-time Pad: Encryption e=000 h=001 i=010 k=011 l=100 r=101 s=110 t=111 h e i l h i t l e r 001 000 010 100 001 010 111 100 000 101 111 101 110 101 111 100 000 101 110 000 110 101 100 001 110 110 111 001 110 101 s r l h s s t h s r Encryption: Plaintext Key = Ciphertext Plaintext: Key: Ciphertext:
- 2. One-time Pad: Decryption e=000 h=001 i=010 k=011 l=100 r=101 s=110 t=111 s r l h s s t h s r 110 101 100 001 110 110 111 001 110 101 111 101 110 101 111 100 000 101 110 000 001 000 010 100 001 010 111 100 000 101 h e i l h i t l e r Decryption: Ciphertext Key = Plaintext Ciphertext: Key: Plaintext:
- 3. One-time Pad e=000 h=001 i=010 k=011 l=100 r=101 s=110 t=111 s r l h s s t h s r 110 101 100 001 110 110 111 001 110 101 101 111 000 101 111 100 000 101 110 000 011 010 100 100 001 010 111 100 000 101 k i l l h i t l e r Ciphertext: “key”: “Plaintext”: Double agent claims sender used following “key”
- 4. One-time Pad e=000 h=001 i=010 k=011 l=100 r=101 s=110 t=111 s r l h s s t h s r 110 101 100 001 110 110 111 001 110 101 111 101 000 011 101 110 001 011 101 101 001 000 100 010 011 000 110 010 011 000 h e l i k e s i k e Ciphertext: “Key”: “Plaintext”: Or sender is captured and claims the key is…
- 5. One-time Pad Summary • Provably secure… – Ciphertext provides no info about plaintext – All plaintexts are equally likely • …but, only when be used correctly – Pad must be random, used only once – Pad is known only to sender and receiver • Note: pad (key) is same size as message • So, why not distribute msg instead of pad?
- 6. Codebook Cipher • Literally, a book filled with “codewords” • Zimmerman Telegram encrypted via codebook Februar 13605 fest 13732 finanzielle 13850 folgender 13918 Frieden 17142 Friedenschluss 17149 : : • Modern block ciphers are codebooks! • More about this later…
- 7. Codebook Cipher: Additive • In practice, also used additive • Additive book of “random” numbers – Sender encrypts msg with codebook – Then chooses position in additive book – Adds additive numbers to get ciphertext – Send ciphertext and additive position (MI) – Recipient subtracts additives before decrypting • Why use an additive sequence?
- 8. Zimmerman Telegram • Perhaps most famous codebook ciphertext ever • A major factor in U.S. entry into WWI
- 9. Zimmerman Telegram Decrypted British had recovered partial codebook Then able to fill in missing parts
- 10. Post-WWII History • Claude Shannon father of the science of information theory • Computer revolution lots of data to protect • Data Encryption Standard (DES), 70’s • Public Key cryptography, 70’s • CRYPTO conferences, 80’s • Advanced Encryption Standard (AES), 90’s • The crypto genie is out of the bottle…
- 11. Claude Shannon • The founder of Information Theory • 1949 paper: Comm. Thy. of Secrecy Systems • Fundamental concepts – Confusion obscure relationship between plaintext and ciphertext, substitution ciphers – Diffusion spread plaintext statistics through the ciphertext, transposition ciphers • Proved one-time pad is secure • One-time pad is confusion-only, while transposition is diffusion-only
- 12. Steganography • an alternative to encryption • hides existence of message – using only a subset of letters/words in a longer message marked in some way – using invisible ink – hiding in LSB in graphic image or sound file • has drawbacks – high overhead to hide relatively few info bits
- 13. Modern Block Ciphers • will now look at modern block ciphers • one of the most widely used types of cryptographic algorithms • provide secrecy and/or authentication services • in particular will introduce DES (Data Encryption Standard)
- 14. Stream Cipher There is a plain text stream P = P1P2P3. . . There is a cipher text stream C = C1C2C3. . . There is a key stream K = (k1, k2, k3, . . . )
- 15. Stream Cipher
- 16. Stream cipher Examples Additive cipher K = (k, k, k, . . . ) Monoalphabetic substitution cipher K = mapping of the current PT char to CT char, . . . Vigenere cipher K = (k1, k2, . . . , km, k1, k2, . . .)
- 17. Block cipher
- 18. Block cipher Examples Play fair cipher (block size = 2) DES, AES
- 19. Block Cipher Principles • most symmetric block ciphers are based on a Feistel Cipher Structure • needed since must be able to decrypt ciphertext to recover messages efficiently • block ciphers look like an extremely large substitution • would need table of 264 entries for a 64-bit block • instead create from smaller building blocks • using idea of a product cipher
- 20. Claude Shannon and Substitution- Permutation Ciphers • in 1949 Claude Shannon introduced idea of substitution-permutation (S-P) networks – modern substitution-transposition product cipher • these form the basis of modern block ciphers • S-P networks are based on the two primitive cryptographic operations we have seen before: – substitution (S-box) – permutation (P-box) • provide confusion and diffusion of message
- 21. Feistel Cipher Structure • Horst Feistel devised the feistel cipher – based on concept of invertible product cipher • partitions input block into two halves – process through multiple rounds which – perform a substitution on left data half – based on round function of right half & subkey – then have permutation swapping halves • implements Shannon’s substitution- permutation network concept
- 22. Feistel Cipher Structure
- 23. Feistel Cipher Design Principles • 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 • fast software en/decryption & ease of analysis – are more recent concerns for practical use and testing
- 24. Feistel Cipher: Encryption • Feistel cipher is a type of block cipher design, not a specific cipher • Split plaintext block into left and right halves: P = (L0,R0) • For each round i = 1,2,...,n, compute Li= Ri1 Ri= Li1 F(Ri1,Ki) where F is round function and Ki is subkey • Ciphertext: C = (Ln,Rn)
- 25. Feistel Cipher: Decryption • Start with ciphertext C = (Ln,Rn) • For each round i = n,n1,…,1, compute Ri1 = Li Li1 = Ri F(Ri1,Ki) where F is round function and Ki is subkey • Plaintext: P = (L0,R0) • Formula “works” for any function F – But only secure for certain functions F
- 26. Feistel Cipher Decryption

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