CSC 474/574 Information Systems Security
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1 CSC 474/574 Information Systems Security Topic 2.1 Introduction to Cryptography CSC 474/574 By Dr. Peng Ning 1 Cryptography Cryptography Original meaning: The art of secret writing Becoming a science that relies on mathematics (number theory, algebra) Process data into intelligible form, reversible, without data loss Usually one-to-one (not compression) CSC 474/574 By Dr. Peng Ning 2
2 Encryption/Decryption encryption ciphertext decryption key key Plaintext: a message in its original form Ciphertext: a message in the transformed, unrecognized form Encryption: the process that transforms a into a ciphertext Decryption: the process that transforms a ciphertext to the corresponding Key: the value used to control encryption/decryption. CSC 474/574 By Dr. Peng Ning 3 Cryptanalysis Ciphertext only: Analyze only with the ciphertext Example: Exhaustive search until recognizable Smarter ways available Known : Secret may be revealed (by spy, time), thus <ciphertext, > pair is obtained Great for mono-alphabetic ciphers CSC 474/574 By Dr. Peng Ning 4
3 Cryptanalysis (Cont d) Chosen : Choose text, get encrypted Useful if limited set of messages Chosen ciphertext: Choose ciphertext Get feedback from decryption, etc. CSC 474/574 By Dr. Peng Ning 5 Security of An Encryption Algorithm Unconditionally secure It is impossible to decrypt the ciphertext One-time pad (the key is as long as the ) Computationally secure The cost of breaking the cipher exceeds the value of the encrypted information The time required to break the cipher exceeds the useful lifetime of the information CSC 474/574 By Dr. Peng Ning 6
4 Secret Keys v.s. Secret Algorithms Security by obscurity We can achieve better security if we keep the algorithms secret Hard to keep secret if used widely Reverse engineering, social engineering Publish the algorithms Security of the algorithms depends on the secrecy of the keys Less unknown vulnerability if all the smart (good) people in the world are examine the algorithms CSC 474/574 By Dr. Peng Ning 7 Secret Keys v.s. Secret Algorithms (cont d) Commercial world Published Wide review, trust Military Keep algorithms secret Avoid giving enemy good ideas Military has access to the public domain knowledge anyway. CSC 474/574 By Dr. Peng Ning 8
5 Some Trivial Codes Caesar cipher: substitution cipher: Replace each letter with the one 3 letters later A D, B E Captain Midnight Secret Decoder rings: shift variable by n: IBM HAL only 26 possibilities CSC 474/574 By Dr. Peng Ning 9 Some Trivial Codes (Cont d) Mono-alphabetic cipher: generalization, arbitrary mapping of one letter to another 26!, approximately statistical analysis of letter frequencies CSC 474/574 By Dr. Peng Ning 10
6 Some Trivial Codes (Cont d) Hill Cipher Encryption: C = KP or Decryption: P = K -1 C Problem: Known attack CSC 474/574 By Dr. Peng Ning 11 Some Trivial Codes (cont d) Poly-alphabetic Ciphers A set of related mono-alphabetic substitution rules is used A key determines which particular rule is chosen for a given transformation CSC 474/574 By Dr. Peng Ning 12
7 Some Trivial Codes (Cont d) All the previous codes are based on substitution Transposition (permutation) Key: Plaintext: A T T A C K P O S T P O N E D U N T I L T W O A M X Y Z Ciphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ CSC 474/574 By Dr. Peng Ning 13 Types of Cryptography Number of keys Hash functions: no key Secret key cryptography: one key Public key cryptography: two keys - public, private The way in which the is processed Block cipher: divides input elements into blocks Stream cipher: process one element (e.g., bit) a time CSC 474/574 By Dr. Peng Ning 14
8 Secret Key Cryptography encryption ciphertext decryption key Same key key Same key is used for encryption and decryption Also known as Symmetric cryptography Conventional cryptography CSC 474/574 By Dr. Peng Ning 15 Secret Key Cryptography (cont d) Basic technique Product cipher: Multiple applications of interleaved substitutions and permutations S P S P S ciphertext key CSC 474/574 By Dr. Peng Ning 16
9 Secret Key Cryptography (cont d) Ciphertext approximately the same length as Examples Stream Cipher: RC4 Block Cipher: DES, IDEA, AES CSC 474/574 By Dr. Peng Ning 17 Applications of Secret Key Cryptography Transmitting over an insecure channel Challenge: How to share the key? Secure Storage on insecure media Authentication Challenge-response To prove the other party knows the secret key Must be secure against chosen attack Integrity check Message integrity code (MIC) CSC 474/574 By Dr. Peng Ning 18
10 Public Key Cryptography encryption ciphertext decryption Public key Private key Invented/published in 1975 A public/private key pair is used Public key can be publicly known Private key is kept secret by the owner of the key Much slower than secret key cryptography Also known as Asymmetric cryptography CSC 474/574 By Dr. Peng Ning 19 Public Key Cryptography (Cont d) message Sign Digital signature Verify Yes/No Private key Public key Another mode: digital signature Only the party with the private key can create a digital signature. The digital signature is verifiable by anyone who knows the public key. The signer cannot deny that he/she has done so. CSC 474/574 By Dr. Peng Ning 20
11 Applications of Public Key Cryptography Data transmission: Alice encrypts m a using Bob s public key e B, Bob decrypts m a using his private key d B. Storage: Can create a safety copy: using public key of trusted person. Authentication: No need to store secrets, only need public keys. Secret key cryptography: need to share secret key for every person to communicate with. CSC 474/574 By Dr. Peng Ning 21 Applications of Public Key Cryptography (Cont d) Digital signatures Sign hash H(m) with the private key Authorship Integrity Non-repudiation: can t do with secret key cryptography Key exchange Establish a common session key between two parties CSC 474/574 By Dr. Peng Ning 22
12 Hash Algorithms Message of arbitrary length Hash H A fixed-length short message Also known as Message digests One-way transformations One-way functions Hash functions Length of H(m) much shorter then length of m Usually fixed lengths: 128 or 160 bits CSC 474/574 By Dr. Peng Ning 23 Hash Algorithms (Cont d) Desirable properties of hash functions Performance: Easy to compute H(m) One-way property: Given H(m) but not m, it s difficult to find m Weak collision free: Given H(m), it s difficult to find m such that H(m ) = H(m). Strong collision free: Computationally infeasible to find m 1, m 2 such that H(m 1 ) = H(m 2 ) CSC 474/574 By Dr. Peng Ning 24
13 Applications of Hash Functions Primary application Generate/verify digital signature Message m H H(m) Sign Signature Sig(H(m)) Private key Message m H H(m) Verify Yes/No Signature Sig(H(m)) Public key CSC 474/574 By Dr. Peng Ning 25 Applications of Hash Functions (Cont d) Password hashing Doesn t need to know password to verify it Store H(password+salt) and salt, and compare it with the user-entered password Salt makes dictionary attack more difficult Message integrity Agree on a secrete key k Compute H(m k) and send with m Doesn t require encryption algorithm, so the technology is exportable CSC 474/574 By Dr. Peng Ning 26
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