Download presentation

Presentation is loading. Please wait.

Published byErin Cotten Modified over 2 years ago

1
CS 6262 Spring 02 - Lecture #7 (Tuesday, 1/29/2002) Introduction to Cryptography

2
Definitions n Process data into unintelligible form, reversible, without data loss n Usually one-to-one (not compression) n Analog cryptography example: voice changers n Other services: u Integrity checking: no tampering u Authentication: not an imposter n Plaintext encryption ciphertext decryption plaintext

3
Computational Difficulty n Algorithm needs to be efficient. u Otherwise only short keys can be used. n Most schemes can be broken: depends on $$$. u E.G. Try all possible keys. n Longer key is often more secure: u Encryption O(N+1). u Brute-force cryptanalysis: O( 2N+1 ), twice as hard with each additional bit. n Cryptanalysis tools: u Special-purpose hardware. u Parallel machines. u Internet coarse-grain parallelism.

4
Secret Key Vs. Secret Algorithm n Secret algorithm: additional hurdle n Hard to keep secret if used widely: u Reverse engineering, social engineering n Commercial: published u Wide review, trust n Military: avoid giving enemy good ideas

5
Some Trivial Schemes n Caesar cipher: substitution cipher: u A D, B E n Captain Midnight Secret Decoder rings: u shift variable by n: IBM HAL, or : F (letter + offset) mod 26 u only 26 possible ways of secret coding. n Monoalphabetic cipher: u generalization, arbitrary mapping of one letter to another u 26!, approximately 4 10 26 u statistical analysis of letter frequencies n One-time pad u A random sequence of 0’s and 1’s XORed to plaintext

6
Cryptanalysis: Breaking an Encryption Scheme n Ciphertext only: u Exhaustive search until “recognizable plaintext” u Need enough ciphertext n Known plaintext: u Secret may be revealed (by spy, time), thus pair is obtained u Great for monoalphabetic ciphers n Chosen plaintext: u Choose text, get encrypted u Useful if limited set of messages

7
Models for Evaluating Security n Unconditional security (perfect secrecy) n Complexity-theoretic security n Provable security n Computational security n Ad hoc security

8
Brute Force Attacks n Number of encryption/sec: 1 million to 1 billion/sec n 56-bit key broken in 1 week with 120,000 processors ($6.7m) n 56-bit key broken in 1 month with 28,000 processors ($1.6m) n 64-bit key broken in 1 week with 3.1 10 7 processors ($1.7b) n 128-bit key broken in 1 week with 5.6 10 26 processors

9
Types of Cryptography n Hash functions: no key n Secret key cryptography: one key n Public key cryptography: two keys - public, private

10
Secret Key Cryptography n Same key is used for encryption and decryption u Symmetric cryptography n Ciphertext approximately the same length as plaintext n Substitution codes, DES, IDEA n Message transmission: u Agree on key (but how?) u Communicate over insecure channel n Secure storage: crypt

11
Secret Key Cryptography (Cont’d) n Strong authentication: prove knowledge of key without revealing it: u Send challenge r, verify the returned encrypted {r} u Fred can obtain chosen plaintext, cihpertext pairs F Challenge should chosen from a large pool n Integrity check: fixed-length checksum for message u Send MIC along with the message

12
Public Key Cryptography n Asymmetric cryptography n Invented/published in 1975 n Two keys: private (d), public (e) u Encryption: public key; Decryption: private key u Signing: private key; Verification: public key n Much slower than secret key cryptography

13
Public Key Cryptography (Cont’d) n Data transmission: u Alice encrypts m a using e B, Bob decrypts to m a using d b. n Storage: u Can create a safety copy: using public key of trusted person. n Authentication: u No need to store secrets, only need public keys. u Secret key cryptography: need to share secret key for every person to communicate with.

14
Public Key Cryptography (Cont’d) n Digital signatures u Encrypt hash h(m) with private key F Authorship F Integrity F Non-repudiation: can’t do with secret key cryptography

15
Hash Algorithms n Message digests, one-way transformations n Length of h(m) much shorter then length of m n Usually fixed lengths: 48-128 bits n Easy to compute h(m) n Given h(m), no easy way to find m n Computationally infeasible to find m 1, m 2 s.t. h(m 1 ) = h(m 2 ) n Example: (m+c) 2, take middle n digits

16
Hash Algorithms (Cont’d) n Password hashing u Doesn’t need to know password to verify it u Store h(p+s), s (salt), and compare it with the user-entered p u Salt makes dictionary attack less convenient n Message integrity u Agree on a password u Compute h(m|p) and send with m u Doesn’t require encryption algorithm, so the technology is exportable

Similar presentations

Presentation is loading. Please wait....

OK

Lecture 5: Cryptographic Hashes

Lecture 5: Cryptographic Hashes

© 2018 SlidePlayer.com Inc.

All rights reserved.

Ads by Google

Ppt on finite difference method Ppt on supply chain management of nokia windows Ppt on viruses and bacteria worksheets Ppt on tcp/ip protocol stack Ppt on fdi in retail sector Ppt on power transmission system in automobile Ppt on energy conservation Ppt on how industries are polluting our water resources Ppt on waxes structure Download ppt on tsunami