1
Network Security
Chapter 2
–
Introduction to
Cryptography
George Hamer

CSc 492/592

Fall 2008
2
1.
Defintion
process data into unintelligible form,
reversibly, without data loss
typically
digitally
usually one

to

one in size
compression
analog cryptography: voice changers, shredder
other services:
integrity checking: no tampering
authentication: not an impostor
George Hamer

CSc 492/592

Fall 2008
This is IMPORTANT!!!!
Fundamental Tenet of Cryptography
If lots of smart people have failed to
solve a problem, then it probably will not
be solved (soon).
George Hamer

CSc 492/592

Fall
2008
3
Cryptography Caveats
Cannot
prove that code is secure
assume until otherwise
but: can prove (some)
systems/protocols secure (assuming
secure code)
Difficult to explain algorithm securely
Cryptographic system =
algorithm(published or secret) + secret
value (
key)
Assume Trudy has algorithm
George Hamer

CSc 492/592

Fall
2008
4
Computational Difficulty
algorithm needs to be efficient
may
use inefficient for short key
brute

force cryptanalysis: try all keys
until “looks like” plaintext
any scheme can be broken
depends on $ = f(time)
Longer key
more secure
Encryption = O(n+1)
Decryption = O(2
n+1
) twice as hard
George Hamer

CSc 492/592

Fall
2008
5
Computational Difficulty cont.
cryptanalysis tools:
–
special

purpose hardware
–
parallel machines
–
Internet coarse

grain
parallelism
–
. . .
George Hamer

CSc 492/592

Fall
2008
6
Secret Key vs. Secret Algorithm
Secret algorithm
another hurdle
hard to keep secret if widely used:
reverse engineering, social
engineering
commercial: published
wide
review, trust
military: avoid giving enemy good
ideas (not just messages)
George Hamer

CSc 492/592

Fall
2008
7
Trivial Codes
Caesar cipher: substitution cipher: A

> B, B

> C, etc
Captain Midnight secret Decoder ring:
shift by variable n, IBM

> HAL
Only 26 possibilities
monoalphabetic
cipher:
generalization, arbitrary mapping
letter to letter, only 2
26
possibilities
Can be broken with statistical analysis
George Hamer

CSc 492/592

Fall
2008
8
Cryptanalysis
Cipher text only: requires exhaustive
search until it “look like” recognizable
text. Requires much cipher text
Known plain text: requires
<plaintext, ciphertext> pairs. May
not remain secret forever!
Chosen plain text: requires that I can
get text encrypted
George Hamer

CSc 492/592

Fall
2008
9
Some large numbers
Time to next ice age
14,000 yrs
DES 56 bit keys
7*10
16
keys
Probability of MD5 collision 1/3*10
38
Age of planet
10
9
years
Time until sun goes nova
10
14
years
Age of universe
10
10
years
Number of atoms in universe 10
77
George Hamer

CSc 492/592

Fall
2008
10
Brute Force Attacks
Number of encryptions/sec: 1 million to 1 billion bits/sec
1999: 56

bit key broken in 22.5 h with 1,800 chips
($250,000) (245 * 10
9
keys/sec see eff.org)
1995: 56

bit key broken in 1 week with 120,000
processors ($6.7M)
56

bit key broken in 1 month with 28,000 processors
($1.6M)
64

bit key broken in 1 week with 3.1*10
7
processors
($1.7 billion)
128

bit key broken in 1 week with 5.6*10
26
processors
Chinese Lottery: With machines that test at the rate of a
million keys every second, take 64 seconds to break DES
with a billion such machines running in parallel.
George Hamer

CSc 492/592

Fall
2008
11
Types of Cryptography
Hash function: no key needed
Secret key: one key used
Public key: two keys used, one public
and one private
George Hamer

CSc 492/592

Fall
2008
12
Secret Key Cryptography
George Hamer

CSc 492/592

Fall
2008
13
Secret Key Cryptography cont.
Cipher text ≈ same length as plain
text
symmetric cryptography
substitution codes, DES, IDEA
Message transmission: agree on key
(how?), communicate over insecure
channel
George Hamer

CSc 492/592

Fall
2008
14
Strong Authentication
George Hamer

CSc 492/592

Fall
2008
15
Strong Authentication cont.
= prove knowledge of key without
revealing it
Notice that Fred can obtain <plain,
cipher> text pairs
Not completely secure!
Integrity check = fixed

length checksum
for message CRC not sufficient
easy
to pick
newmessage
with same CRC
encrypt MIC (
message integrity check)
George Hamer

CSc 492/592

Fall
2008
16
Public Key Cryptography
asymmetric cryptography
publicly invented in 1975
two keys: private (d) and public (e)
much slower than secret key
cryptography
George Hamer

CSc 492/592

Fall
2008
17
Public Key Cryptography cont.
George Hamer

CSc 492/592

Fall
2008
18
Public Key Cryptography cont.
George Hamer

CSc 492/592

Fall
2008
19
Digital Signatures
Encrypt
hash(m)
with private key
Doesn’t reveal text
Authorship
Integrity
non

repudiation: can’t do with secret

key cryptography
George Hamer

CSc 492/592

Fall
2008
20
Hash Algorithm
= message digest, one

way
transformation h(m)
Length(h(m)) << Length(message)
usually fixed lengths: 48
–
128 bits
Easy to compute
h(m)
Given
h(m
), but not m, there is no
way to compute m
Computationally infeasible to find m
1
and
m
2
where h(m
1
)= h(m
2
)
George Hamer

CSc 492/592

Fall
2008
21
22
George Hamer

CSc 492/592

Fall
2008
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