Information Assurance Management

Cryptographic Techniques

Week 12-2

Key Length

• Symmetric Key Length

• Security of a Symmetric cryptosystem is a function of the strength of the algorithm and the key length

• Assume perfect algorithm - brute force every possible key

• You need a small amount of ciphertext and the corresponding plaintext

Key Length

• A brute-force attack is a known-plaintext attack.

• Doesn’t need much to launch this attack

• 8 bit key = 256 possible keys (2^8)

• 56 bits = 2^56 or 2285 MIPS-years

• 64 bits = 2^64 or 585,000 MIPS-years

• 128 bits = 10^25 MIPS-years

Key Length

• The Algorithm must be so strong that the only way to break it is with a brute-force attack

• Security should rest with the key, not the details of the algorithm

• 1977 theory about million million processors working parallel

Key Length

• Public-Key Length– Multiply two large prime numbers is the one-

way function– Hard to factor the product to retrieve the two

prime numbers– But getting easier!– General number field sieve

Key Length

• How long should a key be?

• How much is what you are protecting worth?– How long does it need to be protected?– Tactical military secrets = min/hrs=56-64 bits– Short term business deals=days/wks=64 bits– Long term deals = years =64 bits– Trade Secrets = decades = 112 bits

Key Length

– H-bomb secrets = >40 years = 128 bits– Identities of spies=>50 years=128 bits– Personal affairs=>50 years= 128 bits– Diplomatic embarrassments=>65 years=>128

bits– U.S. census data=100 years=>128 bits

• All this is nonsense! Key is longer then you can imagine necessary - no surprises