Security Module Introduction

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Security Module Introduction Classical and
Modern Cryptology AMS I-3.1.1 Fall 2005

• Greg Phillips
• greg.phillips_at_rmc.ca
• Royal Military College of Canada
• Electrical and Computer Engineering

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Overview and todays class

• classical and modern cryptology
• cryptology?
• cipher basics
• techniques and attacks
• digital ciphers
• example DES
• public key cryptology and public key
  infrastructure
• computer security (COMPUSEC)
• network security (NETSEC)
• assurance
• computer security demonstration

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What the heck is Cryptology?

• cryptography
• the art of providing secure communication over
  insecure channels
• cryptanalysis
• the art of breaking into such communications
• cryptology
• the combined art of cryptography and cryptanalysis

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Caesar Cipher
ABCDEFGHIJKLMNOPQRSTUVWXYZ
DEFGHIJKLMNOPQRSTUVWXYZABC

• Every letter is substituted with the third letter
  alphabetically following belongs to a class of
  ciphers called substitution ciphers
• The plaintext gregphillips becomes the ciphertext
  juhjskloolsv
• This is called a restricted cryptosystem because
  it relies on keeping the nature of the algorithm
  secret

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Generalized Caesar Cipher
ABCDEFGHIJKLMNOPQRSTUVWXYZ
NOPQRSTUVWXYZABCDEFGHIJKLM

• Every letter is substituted with the nth letter
  alphabetically following, where n is the secret
  key
• Here, n is 13 and gregphillips becomes
  tertcuvyyvcf
• Since there are only 25 interesting keys, a
  cryptanalyst could easily search the entire key
  space using a brute-force search

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More Generalized Caesar Cipher
ABCDEFGHIJKLMNOPQRSTUVWXYZ
SFBHIXZJLTYKGWUMRPVEDONACQ

• Every letter is substituted with another letter,
  randomly chosen. The order of the substituted
  letters becomes the the secret key
• Here, the key is sfbhixzjltykgwumrpvedonacq and
  gregphillips becomes zpizmjlkklmv
• Since there are 26! (41026 or 288) keys,
  brute-force search is impractical without
  automated assistance

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Categories of Attacks

• Ciphertext only
• The cryptanalyst has only a number of intercepted
  ciphertexts.
• Known plaintext.
• The cryptanalyst has a number of ciphertexts with
  corresponding plaintexts.
• Chosen plaintext
• The cryptanalyst gets to choose plaintext
  messages and is given the corresponding
  ciphertext.

The goal of an attack is either to recover the
secret key, or to be able to decipher the next
message without the key.
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Information Theory Attacks

• Rely on the typical frequency distribution of
  letters, digrams, trigrams and words in natural
  languages.
• For example, in English
• letters e (13.05), t (9.02), o (8.21), etc.
• digrams th (3.16), in (1.54), etc.
• trigrams the (4.72), ing (1.42), etc.
• words the (6.42), of (4.02), etc.
• Knowing the original language of the plaintext,
  and with enough plaintext samples, it is
  typically short work to break almost any
  substitution cipher

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The One-time Pad
G R E G P H I L L I P S
10 5 7 22 17 2 2 19 4 12 1
6....
Q W L C G J K E P U Q Y

• A perfectly secure substitution cipher
• Letters are encoded as in the generalized Caesar
  cipher but using a different key for each letter
• This requires a key-string as long as the
  original plaintext
• If the key-string is reused the system becomes
  prone to attack thus one-time pad

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Visual One-time Pad
http//www.cl.cam.ac.uk/Research/DTG/fms27/vck/
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Transposition

• reorder the letters but do not disguise them the
  new ordering is the key
• e.g., with a key of 12 5 4 9 7 8 6 1 11 10 2 3,
  gregphillips would become spglilhgpire
• typically the key is shorter
• than the message, e.g.,
• with a key of 3 1 2 4,
• gregphillips becomes
• egrgiphlplis
• not particularly secure by itself, however it
  obscures digrams, trigrams and words

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Being Digital

• Most electronic cryptosystems operate at the
  level of bits rather than letters
• The general principles of substitution and
  transposition are still used
• Additional operations
• circular shift
• exclusive or, normally written

1
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Data Encryption Standard (DES)

• Originally proposed by IBM revised by the
  National Security Agency (NSA) and published as
  FIPS 46 by the National Bureau of Standards

plaintext
DES encipher and decipher are the same operation,
which makes hardware implementation of DES simple
and cheap.
DES encipher
56-bit key
ciphertext
DES decipher
plaintext
http//www.nist.gov/itl/div897/pubs/fip46-2.htm
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DES Overview
Input
Initial Permutation
Permuted Input
L0
R0
K0
f
L1 R0
K1
f
...
L2 R1
Pre-output
L16 R15
Inverse Permutation
Output
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Initial and Inverse Permutations(transposition)
Initial Permutation 58 50 42
34 26 18 10 2 60 52 44 36
28 20 12 4 62 54 46 38 30 22
14 6 64 56 48 40 32 24 16
8 57 49 41 33 25 17 9 1 59
51 43 35 27 19 11 3 61 53
45 37 29 21 13 5 63 55 47
39 31 23 15 7
Inverse Permutation 40 8 48
16 56 24 64 32 39 7 47 15
55 23 63 31 38 6 46 14 54 22
62 30 37 5 45 13 53 21 61
29 36 4 44 12 52 20 60 28 35
3 43 11 51 19 59 27 34 2
42 10 50 18 58 26 33 1 41
9 49 17 57 25
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Key Schedule
Permuted Choice 1 57 49 41 33 25
17 9 1 58 50 42 34 26
18 10 2 59 51 43 35 27 19 11
3 60 52 44 36 63 55 47 39
31 23 15 7 62 54 46 38 30
22 14 6 61 53 45 37 29 21 13
5 28 20 12 4
Left Shifts 1 1 2 1 3 2
4 2 5 2 6 2 7 2 8
2 9 1 10 2 11 2 12
2 13 2 14 2 15 2 16 1
Permuted Choice 2 14 17 11 24 1
5 3 28 15 6 21 10 23 19
12 4 26 8 16 7 27 20 13
2 41 52 31 37 47 55 30 40 51
45 33 48 44 49 39 56 34 53 46
42 50 36 29 32
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The Function
f
E bit-selection table 32 1 2 3
4 5 4 5 6 7 8
9 8 9 10 11 12 13 12 13 14
15 16 17 16 17 18 19 20 21 20
21 22 23 24 25 24 25 26 27
28 29 28 29 30 31 32 1
Permutation P 16 7 20 21 29 12 28
17 1 15 23 26 5 18 31 10 2
8 24 14 32 27 3 9 19 13 30
6 22 11 4 25
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How Secure Is DES?

• in an RSA Labs contest, July 1999, a
  special-purpose computer built by the Electronic
  Frontier Foundation cracked the contest message
  (secured by 56-bit single DES) in 22 hours
• used a fast, brute-force attack, searching the
  key space at about 245 billion keys/second
• time to exhaust 56-bit key space 2.4 days
• time to exhaust 40-bit key space 4.5 seconds
• total system cost was 210,000 of which about
  80,000 was RD
• complete plans are freely available on the
  Internet
• as of 2004, estimated cost to build is about
  15,000

http//www.eff.org/Privacy/Crypto/Crypto_misc/DESC
racker/
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EFF DES Cracker
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What Can We Do?

• use longer keys
• use longer keys
• use longer keys
• use other algorithms
• Triple-DES
• CAST
• IDEA
• Advanced Encryption Standard (Rijndael)
• NSA/CSE developed mil-grade crypto
• and longer keys!

http//csrc.nist.gov/CryptoToolkit/aes/ http//en.
wikipedia.org/wiki/Aes
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Next class

• Public Key Cryptology
• and
• Public Key Infrastructure