IT443 – Network Security Administration Instructor: Bo Sheng Cryptography Basics IT443 – Network Security Administration Instructor: Bo Sheng

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

Encryption/Decryption Plaintext: a message in its original form Ciphertext: a message in the transformed, unrecognized 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. plaintext ciphertext plaintext encryption decryption key key

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

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

Mono-Alphabetic Ciphers Generalized substitution cipher: an arbitrary (but fixed) mapping of one letter to another 26! ( 4.0*1026  288) possibilities plaintext alphabet ciphertext alphabet A B C D E F G H I J K … A B C D E F G H I J K …

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:

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

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

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)

The “Weakest Link” in Security Cryptography is rarely the weakest link Weaker links Implementation of cipher Distribution or protection of keys … …

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 are examine the algorithms

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

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

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

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

Public Key Cryptography 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 plaintext ciphertext plaintext encryption decryption Public key Private key

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

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 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

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

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

Hash Function 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 Message of arbitrary length A fixed-length short message Hash

Properties of Hash 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 m1, m2 such that H(m1) = H(m2)

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

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

Hash Applications User authentication Alice Bob Alice wants to authenticate herself to Bob Assuming they already share a secret key K Alice Bob time  “I’m Alice” R computes Y=H(R|K) Y verifies that Y=H(R|K)

Modern Hash Functions MD5 (128 bits) SHA (Secure Hash Algorithm) 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 269 hash operations, much less than the brute-force attack of 280 operations Results were circulated in February 2005, and published in CRYPTO ’05 in August 2005 SHA-256, SHA-384, …

Birthday Attack What is the smallest group size k such that The probability that at least two people in the group have the same birthday is greater than 0.5? 23 Implication for hash function H of length m With probability at least 0.5 If we hash about 2m/2 random inputs, Two messages will have the same hash image m=64, 1ns per hash Brute force (264): 1013 seconds over 300 thousand years Birthday attack (232): 4 seconds

Lab 1 Sample codes eecs.mit.edu’s IP is 18.62.1.6 Assume their subnetwork use 28-bit prefix 18. 62. 1. 00000110 Scan 18.62.1.0 ~ 18.62.1.15 dig -x 18.62.1.0 +short /home/shengbo/it443/scanip.sh /home/shengbo/it443/scanip.pl

Electronic Code Book (ECB) 128 M1 M2 M3 M4 46 + padding Plaintext  Key E C1 C2 C3 C4 128 Ciphertext 

Cipher Block Chaining (CBC) M1 M2 M3 M4 46 + padding 128 128 128 Initialization Vector C1 C2 C3 C4 128 E Key