Slideshare uses cookies to improve functionality and performance, and to provide you with relevant advertising. If you continue browsing the site, you agree to the use of cookies on this website. See our User Agreement and Privacy Policy.

Slideshare uses cookies to improve functionality and performance, and to provide you with relevant advertising. If you continue browsing the site, you agree to the use of cookies on this website. See our Privacy Policy and User Agreement for details.

Successfully reported this slideshow.

Like this presentation? Why not share!

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 ...

No Downloads

Total views

5,487

On SlideShare

0

From Embeds

0

Number of Embeds

9

Shares

0

Downloads

191

Comments

6

Likes

1

No notes for slide

- 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

No public clipboards found for this slide

Login to see the comments