Chapter 8: Key Management
Dulal C Kar
Based on Bruce Schneier
Introduction
• Hardest part of cryptography
• Keeping keys secret is hard
• Cryptanalysts often attack key
management protocols and algorithms
Generating Keys
• Security rests on keys
• Key generation algorithm can be under
attack
• Reduced key spaces
– DES has a 56-bit key
– Fixing some of the bits reduces key space
– Example: Key for Norton Discreet for MSDOS actually reduces to a 40-bit key, 10000
times easier to break
Generating Keys (cont’d)
• Poor Key Choices
– Same as poor password choices
– Subject to dictionary attack
– Dictionary attack is much more effective
when used against a key of files and not a
single key
Generating Keys (cont’d)
• Random Keys
– Best keys but how to generate them
– Use reliable random source such as noise
from audio or radioactive decay or
cryptographically secure pseudo random-bit
generator
– Discard weak keys. DES has only 16 weak
keys.
– Generating keys for public-key cryptography
is harder due to math
Generating Keys (cont’d)
• Pass Phrases
– Use easy to remember phrase
– Then use a one-way hash function to obtain a
pseudo-random-bit string from the phrase
– Information theory
• About 1.3 bits of info per character in standard
English
• For 64-bit key, a pass phrase of about 49
characters should be sufficient
Generating Keys (cont’d)
• ANSI X9.17 Key generation
–
–
–
–
“Not east to remember” keys
Suitable for sessions
Uses triple-DES to generate keys
Algorithm
Let EK(X) be triple-DES encryption of X with key K.
K: special key reserved for key generation
V0: secret 64-bit seed
T: timestamp
To generate random key Ri, calculate
Ri = EK(EK(Ti) xor Vi)
To generate Vi+1, calculate
Vi+1 = EK(EK(Ti) xor Ri)
To turn Ri into DES key, simply adjust every eight bit for parity.
Generating Keys (cont’d)
• DoD Key generation
– Recommends using DES in OFB (output feedback)
mode
– Generate a DES key from system interrupt vectors,
system status registers, and system counters
– Generate an initialization vector from the system
clock, system ID, and date and time
– For plaintext, use an externally generated 64-bit
quantity (or type eight characters)
– Use the output as your key
Transferring Keys
• Public key cryptography solves the problem
• Some systems use alternate channels known to
be secure
• Two types of keys by X9.17 standard
– Data keys (distributed more often)
– Key encryption keys (manually distributed, tamper
proof smart card)
• Key distribution
– Split key using secret splitting scheme
– Send each share over a different channel
Transferring Keys (cont’d)
• Key distribution in large networks
– Total number key exchanges for n-users is
n(n-1)/2
– Better to create a central key server (or
servers)
Verifying Keys
• When Bob receives a key, how does he know it
came from Alice and not from some-one
pretending to be Alice?
• Alice can use a digital signature protocol to sign
the key
• Bob has to trust public-key database to verify
Alice’s signature
• KDC can sign Alice’s public key
• Bob has to trust KDC’s public key he has
• In this sense, some people argue that public-key
cryptography is useless
Verifying Keys (cont’d)
• Error detection during key transmission
– Send key as well as a known constant 2 to 4 bytes
encrypted by the key
– At the receiving do the same to verify
• Key-error detection during decryption
– For ASCII plaintext, decrypt and see whether you can
read
– For random plaintext
• Attach a verification block header, a known header encrypted
by the key
• Decrypt at the receiver to verify