Cryptography Basics. IT443 Network Security Administration Slides courtesy of Bo Sheng

Transcription

1 Cryptography Basics IT443 Network Security Administration Slides courtesy of Bo Sheng 1

2 Outline Basic concepts in cryptography systems Secret key cryptography Public key cryptography Hash functions 2

3 Encryption/Decryption plaintext encryption ciphertext decryption plaintext key key Plaintext: a message in its original form Ciphertext: a message in the transformed, hidden form Encryption: the process that transforms a plaintext into a ciphertext Decryption: the process that transforms a ciphertext to the corresponding plaintext Key: the value used to control encryption/decryption. 3

4 Cryptanalysis code breaking, attacking the cipher Difficulty depends on sophistication of the cipher amount of information available to the code breaker Any cipher can be broken by exhaustive trials, but rarely practical 4

5 Caesar Cipher Replace each letter with the one 3 letters later in the alphabet ex.: plaintext CAT ciphertext FDW plaintext alphabet A B C D E F G H I J K ciphertext alphabet A B C D E F G H I J K Trivial to break 5

6 Mono-Alphabetic Ciphers Generalized substitution cipher: an arbitrary (but fixed) mapping of one letter to another 26! ( 4.0* ) possibilities plaintext alphabet A B C D E F G H I J K ciphertext alphabet A B C D E F G H I J K 6

7 Attacking Mono-Alphabetic Ciphers Broken by statistical analysis of letter, word, and phrase frequencies of the language Frequency of single letters in English language, taken from a large corpus of text: 7

8 Ciphertext Only Attacks Ex.: attacker can intercept encrypted communications, nothing else Breaking the cipher: analyze patterns in the ciphertext provides clues about the encryption method/key 8

9 Known Plaintext Attacks Ex.: attacker intercepts encrypted text, but also has access to some of the corresponding plaintext (definite advantage) Makes some codes (e.g., mono-alphabetic ciphers) very easy to break 9

10 Chosen Plaintext Attacks Ex.: attacker can choose any plaintext desired, and intercept the corresponding ciphertext Allows targeted code breaking (choose exactly the messages that will reveal the most about the cipher) 10

11 Secret Keys vs 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 examine the algorithms 11

12 Outline Basic concepts in cryptography system Secret key cryptography Public key cryptography Hash functions 12

13 Secret Key Cryptography plaintext encryption ciphertext decryption plaintext key Same key key Same key is used for encryption and decryption Also known as Symmetric cryptography Conventional cryptography 13

14 Secret Key Cryptography Stream cipher Block cipher Converts one input plaintext block of fixed size k bits to an output ciphertext block of k bits DES, IDEA, AES, AES Selected from an open competition, organized by NSA Joan Daemen and Vincent Rijmen (Belgium) Block size=128 bits, Key Size= 128/192/256 bits 14

15 Key Size Keys should be selected from a large potential set, to prevent brute force attacks Secret key sizes 40 bits were considered adequate in 70 s 56 bits used by DES were adequate in the 80 s 128 bits are adequate for now If computers increase in power by 40% per year, need roughly 5 more key bits per decade to stay sufficiently hard to break 15

16 Public Key Cryptography plaintext encryption ciphertext decryption plaintext Public key Private key 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 Another mode: digital signature 16

17 Public Key Cryptography plaintext Sign ciphertext Verify plaintext Private key Public key 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. 17

18 Public Key Cryptography It must be computationally easy to generate a public / private key pair hard to determine the private key, given the public key It must be computationally easy to encrypt using the public key easy to decrypt using the private key hard to recover the plaintext message from just the ciphertext and the public key 18

19 Symmetric vs Asymmetric Symmetric algorithms are much faster In the order of a 1000 times faster Symmetric algorithms require a shared secret Impractical if the communicating entities don t have another secure channel Both algorithms are combined to provide practical and efficient secure communication E.g., establish a secret session key using asymmetric crypto and use symmetric crypto for encrypting the traffic 19

20 Outline Basic concepts in cryptography system Secret key cryptography Public key cryptography Hash functions 20

21 Cryptographic Hash Functions Message of arbitrary length Hash A fixed-length short message Also known as Message digest One-way transformation One-way function Hash Length of H(m) much shorter than length of m Usually fixed lengths: 128 or 160 bits 21

22 Properties of Crypto Hashes Consider a hash function H Performance: Easy to compute H(m) One-way property: Given H(m) but not m, it s computationally infeasible to find m Weak collision resistance (free): Given H(m), it s computationally infeasible to find m such that H(m ) = H(m). Strong collision resistance (free): Computationally infeasible to find m 1, m 2 such that H(m 1 ) = H(m 2 ) 22

23 Crypto Hash Applications File / Message integrity Check if a downloaded file is corrupted Detect if a file has been changed by someone after it was stored Compute a hash H(F) of file F openssl dgst -md5 filename 23

24 Crypto Hash Applications Password verification Password cannot be stored in plaintext In a hashed format Linux: /etc/passwd, /etc/shadow cat /etc/shadow 24

25 Crypto Hash Applications User authentication Alice wants to authenticate herself to Bob Assuming they already share a secret key K Alice Bob computes Y=H(R K) time verifies that Y=H(R K) 25

26 MD5 (128 bits) Modern Hash Functions Previous versions (i.e., MD2, MD4) have weaknesses. Broken; collisions published in August 2004 Too weak to be used for serious applications SHA (Secure Hash Algorithm) Weaknesses were found SHA-1 (160 bits) Broken, but not yet cracked Collisions in 2 69 hash operations, much less than the brute-force attack of 2 80 operations Results were circulated in February 2005, and published in CRYPTO 05 in August 2005 SHA-256, SHA-384, SHA

27 Electronic Code Book (ECB) Plaintext M 1 M 2 M 3 M padding Key E E E E Ciphertext C 1 C 2 C 3 C 4 27

28 Cipher Block Chaining (CBC) M 1 M 2 M 3 M 4 Initialization Vector Key padding E E E E C 1 C 2 C 3 C 4 28