Based on Bruce Schneier Chapter 7: Key Length Dulal C. Kar

Symmetric Key Length For a block cipher –Cryptanalyst needs a small amount of ciphertext and the corresponding plaintext (known plaintext attack) –Getting this plaintext and ciphertext is easy –Just intercept standard header of an or a standard header of file formatted by a word processor Complexity –For 64 bits, 2 64 keys possible –Assuming a million keys a second, it will take 2285 years –Algorithm must be secure so that there is no alternative to break any other way except using brute- force attack

Time and Cost Estimates for Brute-Force Attack Michael Wiener (1995) –Designed 56-bit DES cracking, specialized parallel processing machine for $1 million, which can crack a key in 3.5 hours. –Moore’s Law –Computing power doubles approximately every 18 months, which means costs go down by a factor of 10 every 5 years –$1 million of machine of 1995 will cost $10000 today –Check table 7.1 for time estimates in 1995

Software Crackers Slower than hardware (1000 times slower) Distributed attack making use of idle time of microcomputers Easy to crack a 40-bit key Using 200 million giant Cray mainframe- like computers, each performing a million encryptions per second –To recover a 128-bit key would take a million times the age of the universe

The Chinese Lottery Imagine, a brute-force, million-test-per second cracking chip was built into every radio and television sold in china See table 7.2 for results

Thermodynamic Limitations Second law of thermodynamics To record a single bit requires amount of energy no less than kT (k = 1.38* erg/deg-Kelvin and T = absolute temp) With T = 3.2 deg-Kelvin, to set/reset a bit, it would consume 4.4* ergs Annual output of our sun is about 1.21*10 41 ergs –enough to power 2.7*10 56 single bit changes –Enough to put a 187-bit counter through all its values –Implies that 256-bit keys will be infeasible for cracking using brute-force attack

Public-Key Key Length Hard to factorize a product to obtain two large prime numbers Idea is used to make a trap-door one-way function Breaking public-key algorithms involve trying to factor the large number (or taking discrete logarithms in a very large finite field) Factoring large numbers is hard but getting easier faster than mathematicians expected If you want your keys to remain secure for 20 years, 1024 bits is likely too short

Recommended Public-Key Key Length Years –Individual: 1280 bits –Corporation: 1536 bits –Government: 2048 bits Year 2015 –Individual: 1536 bits –Corporation: 2048 bits –Government: 2048 bits

Comparing Symmetric and Public-Key Key Length A cryptosystem is likely to be attacked at its weakest point In a system that uses both symmetric and public-key cryptography, key lengths for each should be chosen to make both equally difficult to attack See table 7.9 In general, choose a more secure public key length than your symmetric key length. Public keys stays longer

Birthday Attacks Against One-way Hash Functions Two brute-force attacks 1.Given the hash of message, H(M), create another document M’ such that H(M) = H(M’) 2.Find two random messages, M and M’ such that H(M) = H(M’) Second one is far easier attack Assume the hash function produces an m-bit output 1.Finding a message that hashes to a given hash value would require hashing 2 m random messages 2.Finding two messages that hash to the same value would require 2 m/2 random messages Example, if you want to drop the odds of someone breaking to less than 1 in 2 80, use a 160-bit one-way hash function