The document provides an overview of cipher techniques including: - Classical techniques like transposition ciphers, substitution ciphers including the Caesar and Playfair ciphers, and polyalphabetic ciphers like the Vigenere cipher. - Modern techniques like stream ciphers which encrypt bits one at a time using a pseudorandom keystream, and block ciphers which encrypt blocks of text. - It also discusses cryptanalysis techniques for analyzing ciphers and discusses how to build more secure systems using techniques like the one-time pad or combining multiple ciphers.

1. Cipher Techniques
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2. Road Map
 Basic Terminology
 Cryptosystem
 Classical Cryptography
 Algorithm Types and Modes
 Data Encryption Standard
 Other Stream & Block Ciphers
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3. Basic Terminology
 plaintext - the original message
 ciphertext - the coded message
 cipher - algorithm for transforming plaintext to ciphertext
 key - info used in cipher known only to sender/receiver
 encipher (encrypt) - converting plaintext to ciphertext
 decipher (decrypt) - recovering ciphertext from plaintext
 cryptography - study of encryption principles/methods
 cryptanalysis (codebreaking) - the study of principles/ methods
of deciphering ciphertext without knowing key
 cryptology - the field of both cryptography and cryptanalysis
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4. Cryptosystem
A cryptosystem is a five-tuple (P,C,K,E,D),
where the following are satisfied:
1. P is a finite set of possible plaintexts.
2. C is a finite set of possible ciphertexts.
3. K, the key space, is a finite set of possible
keys
4. ∀K∈K, ∃EK∈E (encryption rule), ∃DK∈D
(decryption rule).
Each EK: P→C and DK: C→P are functions
such that ∀x∈P, DK(EK(x)) = x.
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5. Cryptography
 Cryptography
 Symmetric / private key / single key
 Asymmetric / public-key / two - key
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6. Symmetric Cryptography
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7. Asymmetric Cryptography
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8. Requirements
 Two requirements for secure use of
symmetric encryption:
 a strong encryption algorithm
 a secret key known only to sender / receiver
Y = EK(X)
X = DK(Y)
 assume encryption algorithm is known
 implies a secure channel to distribute key
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9. Symmetric cryptography
 Transposition Techniques
 Substitution techniques
 Caesar Cipher
 Monoalphabetic Cipher
 Polyalphabethic Cipher
 Playfair Cipher
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10. Types of Cryptanalytic Attacks
adversary needs
strongest attack  ciphertext only
 only know algorithm / ciphertext, statistical, can
identify plaintext, or worse: the key
 known plaintext
 know/suspect plaintext & ciphertext to attack
cipher
 chosen plaintext
 select plaintext and obtain ciphertext to attack
cipher
 chosen ciphertext
 select ciphertext and obtain plaintext to attack
adversary’s attacks cipher
can be weaker  chosen text
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 select either plaintext or ciphertext to en/decrypt 10
to

11. Brute Force Search
 always possible to simply try every key
 most basic attack, proportional to size of key
space
 assume either know / recognise plaintext
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12. Transposition Ciphers
 Consider classical transposition or
permutation ciphers
 these hide the message by rearranging the
letter order
 without altering the actual letters used
 can recognise these since have the same
frequency distribution as the original text
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13. Rail Fence cipher
 writemessage letters out diagonally over a
number of rows
 then read off cipher row by row
 eg. write message out as:
m e m a t r h t g p r y
e t e f e t e o a a t
 giving ciphertext
MEMATRHTGPRYETEFETEOAAT
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14. Row Transposition Ciphers
a more complex scheme
 write letters of message out in rows over a
specified number of columns
 then reorder the columns according to some
key before reading off the rows
Key: 4 3 1 2 5 6 7
Plaintext: a t t a c k p
o s t p o n e
d u n t i l t
w o a m x y z
Ciphertext: TTNAAPTMTSUOAODWCOIXKNLYPETZ
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15. Classical Substitution Ciphers
 where letters of plaintext are replaced by
other letters or by numbers or symbols
 or if plaintext is viewed as a sequence of bits,
then substitution involves replacing plaintext
bit patterns with ciphertext bit patterns
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16. Caesar Cipher
 earliest known substitution cipher
 by Julius Caesar
 first attested use in military affairs
 replaces each letter by 3rd letter after it
 example:
meet me after the toga party
PHHW PH DIWHU WKH WRJD SDUWB
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17. Caesar Cipher
 can define transformation as:
a b c d e f g h i j k l m n o p q r s t u v w x y z
D E F G H I J K L M N O P Q R S T U V W X Y Z A B C
 mathematically give each letter a number
a b c d e f g h i j k l m
0 1 2 3 4 5 6 7 8 9 10 11 12
n o p q r s t u v w x y Z
13 14 15 16 17 18 19 20 21 22 23 24 25
 then have Caesar cipher as:
C = E(p) = (p + k) mod (26)
p = D(C) = (C – k) mod (26)
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18. Monoalphabetic Cipher
 rather than just shifting the alphabet
 could shuffle (jumble) the letters arbitrarily
 each plaintext letter maps to a different random
ciphertext letter
 hence key is 26 letters long
Plain: abcdefghijklmnopqrstuvwxyz
Cipher: DKVQFIBJWPESCXHTMYAUOLRGZN
Plaintext: ifwewishtoreplaceletters
Ciphertext: WIRFRWAJUHYFTSDVFSFUUFYA
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19. Playfair Cipher
 not even the large number of keys in a
monoalphabetic cipher provides security
 one approach to improving security was to
encrypt multiple letters
 the Playfair Cipher is an example
 invented by Charles Wheatstone in 1854, but
named after his friend Baron Playfair
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20. Playfair Key Matrix
 a 5X5 matrix of letters based on a keyword
 (I and J aren’t distinguished)
 fill in letters of keyword (sans duplicates)
 fill rest of matrix with other letters
 eg. using the keyword MONARCHY
MONAR
CHYBD
EFGIK
LPQST
UVWXZ
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21. Encrypting and Decrypting
 plaintext encrypted two letters at a time:
1. each letter is replaced by the one in its row in the column
of the other letter of the pair, eg. “hs" encrypts to "BP",
and “ea" to "IM" or "JM" (as desired). Except when that
doesn’t work!
2. if a pair is a repeated letter, insert a filler like 'X', eg.
"balloon" transformed to "ba lx lo on"
3. if both letters fall in the same row, replace each with
letter to right (wrapping back to start from end), eg.
“ar" encrypts as "RM"
4. if both letters fall in the same column, replace each with
the letter below it (again wrapping to top from bottom),
eg. “mu" encrypts to "CM"
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22. Polyalphabetic Ciphers
 another approach to improving security is to use
multiple cipher alphabets
 called polyalphabetic substitution ciphers
 makes cryptanalysis harder with more alphabets to
guess and flatter frequency distribution
 use a key to select which alphabet is used for each
letter of the message
 use each alphabet in turn
 repeat from start after end of key is reached
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23. Vigenère Cipher
 simplest polyalphabetic substitution cipher is
the Vigenère Cipher
 effectively multiple caesar ciphers
 key is multiple letters long K = k1 k2 ... kd
 ith letter specifies ith alphabet to use
 use each alphabet in turn
 repeat from start after d letters in message
 decryption simply works in reverse
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24. Example
 write the plaintext out
 write the keyword repeated above it
 use each key letter as a caesar cipher key
 encrypt the corresponding plaintext letter
 eg using keyword deceptive
key: deceptivedeceptivedeceptive
plaintext: wearediscoveredsaveyourself
ciphertext:ZICVTWQNGRZGVTWAVZHCQYGLMGJ
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25. Autokey Cipher
 ideally want a key as long as the message
 Vigenère proposed the autokey cipher
 with keyword is prefixed to message as key
 knowing keyword can recover the first few letters
 use these in turn on the rest of the message
 but still have frequency characteristics to attack
 eg. given key deceptive
key: deceptivewearediscoveredsav
plaintext: wearediscoveredsaveyourself
ciphertext:ZICVTWQNGKZEIIGASXSTSLVVWLA
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26. One-Time Pad
 ifa truly random key as long as the message
is used, the cipher will be secure
 called a One-Time pad
 is unbreakable since ciphertext bears no
statistical relationship to the plaintext
 since for any plaintext & any ciphertext
there exists a key mapping one to other
 unconditional security! why look any
further??
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27. Product Ciphers
 ciphers using substitutions or transpositions are not
secure because of language characteristics
 hence consider using several ciphers in succession
to make harder (Shannon)
 two substitutions make a more complex substitution
 two transpositions make more complex transposition
 but a substitution followed by a transposition makes a new
much harder cipher
 this is bridge from classical to modern ciphers
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28. Rotor Machines
 before modern ciphers, rotor machines were most
common product cipher
 were widely used in WW2
 German Enigma, Allied Hagelin, Japanese Purple
 implemented a very complex, varying substitution
cipher
 used a series of cylinders, each giving one
substitution, which rotated and changed after each
letter was encrypted
 with 3 cylinders have 263=17576 alphabets
 3! rearrangements of cylinders in Enigma
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29. Steganography
 an alternative to encryption
 hides existence of message
 using only a subset of letters/words in a longer
message marked in some way
 using invisible ink
 hiding in LSB in graphic image or sound file
 has drawbacks
 high overhead to hide relatively few info bits
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30. Algorithm Types and Modes
 An Algorithm type defines what size of plain
text should be encrypted in each step of
algorithm
 An Algorithm mode defines the details of the
cryptographic algorithm, once the type is
decided.
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31. Algorithm Types
 Stream Ciphers
 Block Ciphers
Algorithm Modes
 ElectronicCode Book Work On Block Cipher
 Cipher Block Chaining
 Cipher FeedBack
Work On Block Ciphers acting as
 Output FeedBack Stream Cipher
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32. Stream, Block Ciphers
 E encipherment function
 Ek(b) encipherment of message b with key k
 In what follows, m = b1b2 …, each bi of fixed length
 Block cipher
 Ek(m) = Ek(b1)Ek(b2) …
 Stream cipher
 k = k1k2 …
 Ek(m) = Ek1(b1)Ek2(b2) …
 If k1k2 … repeats itself, cipher is periodic and the kength of
its period is one cycle of k1k2 …
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33. Stream Ciphers
 Often (try to) implement one-time pad by
xor’ing each bit of key with one bit of
message
 Example:
m = 00101
k = 10010
c = 10111
 But how to generate a good key?
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34. Synchronous Stream Ciphers
 n-stage Linear Feedback Shift Register:
consists of
 n bit register r = r0…rn–1
 n bit tap sequence t = t0…tn–1
 Use:
 Use rn–1 as key bit
 Compute x = r0t0 ⊕ … ⊕ rn–1tn–1
 Shift r one bit to right, dropping rn–1, x becomes r0
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35. Operation
…
r0 … rn–1 ⊕ bi
…
ci
r0´ … rn–1´ ri´ = ri–1,
0<i≤n
r0t0 + … + rn–1tn–1
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36. Example
 4-stage LFSR; t = 1001
r ki new bit computation new r
0010 0 01⊕00⊕10⊕01 = 0 0001
0001 1 01⊕00⊕00⊕11 = 1 1000
1000 0 11⊕00⊕00⊕01 = 1 1100
1100 0 11⊕10⊕00⊕01 = 1 1110
1110 0 11⊕10⊕10⊕01 = 1 1111
1111 1 11⊕10⊕10⊕11 = 0 0111
0111 1 11⊕10⊕10⊕11 = 1 1011
 Key sequence has period of 15 (010001111010110)
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37. NLFSR
 n-stage Non-Linear Feedback Shift Register:
consists of
 n bit register r = r0…rn–1
 Use:
 Use rn–1 as key bit
 Compute x = f(r0, …, rn–1); f is any function
 Shift r one bit to right, dropping rn–1, x becomes r0
Note same operation as LFSR but more general
bit replacement function
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38. Example
 4-stage NLFSR; f(r0, r1, r2, r3) = (r0 & r2) | r3
r ki new bit computation new r
1100 0 (1 & 0) | 0 = 0
0110
0110 0 (0 & 1) | 0 = 0
0011
0011 1 (0 & 1) | 1 = 1
1001
1001 1 (1 & 0) | 1 = 1
1100
1100 0 (1 & 0) | 0 = 0
0110
0110
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0 (0 & 1) | 0 = 0
38
0011

39. Self-Synchronous Stream
Cipher
 Takekey from message itself (autokey)
 Example: Vigenère, key drawn from plaintext
 key XTHEBOYHASTHEBA
 plaintext THEBOYHASTHEBAG
 ciphertext QALFPNFHSLALFCT
 Problem:
 Statistical regularities in plaintext show in key
 Once you get any part of the message, you can
decipher more
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40. Another Example
 Take key from ciphertext (autokey)
 Example: Vigenère, key drawn from
ciphertext
 key XQXBCQOVVNGNRTT
 plaintext THEBOYHASTHEBAG
 ciphertext QXBCQOVVNGNRTTM
 Problem:
 Attacker gets key along with ciphertext, so
deciphering is trivial
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41. Block Cipher
 Block Cipher – treat a
block of plaintext as a whole
 Feistel Cipher
 DES/3DES/AES
 Stream coding – encrypt one
bit or byte at a time
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42. Block Ciphers
 Encipher, decipher multiple bits at once
 Each block enciphered independently
 Problem: identical plaintext blocks produce
identical ciphertext blocks
 Example: two database records
 MEMBER: HOLLY INCOME $100,000
 MEMBER: HEIDI INCOME $100,000
 Encipherment:
 ABCQZRME GHQMRSIB CTXUVYSS RMGRPFQN
 ABCQZRME ORMPABRZ CTXUVYSS RMGRPFQN
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43. Solutions
 Insert information about block’s position into
the plaintext block, then encipher
 Cipher block chaining:
 Exclusive-or current plaintext block with previous
ciphertext block:
 c0 = Ek(m0 ⊕ I)
 ci = Ek(mi ⊕ ci–1) for i > 0
where I is the initialization vector
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44. Algorithm Modes
 ElectronicCode Book Work On Block Cipher
 Cipher Block Chaining
 Cipher FeedBack
Work On Block Ciphers acting as
 Output FeedBack Stream Cipher
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45. ECB (Electronic CodeBook) Mode
 Encryption: for 1≤j≤t, cj <= EK(xj).
 Decryption: for 1≤j≤t, xj <= DK(cj).
 Identical plaintext (under the same key) result in
identical ciphertext
 blocks are enciphered independently of other
blocks
 bit errors in a single ciphertext affect decipherment
of that block only
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46. ECB Mode (Cont’d)
xj
n
key E E-1 key
n
x’j = xj
cj
encipherment decipherment
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47. CBC (Cipher-Block Chaining)
Mode
C0=IV Cj
C j-1
n key
xj ⊕ E-1
⊕
C j-1
key E
Cj
<Encipherment> n X’j = xj
<Decipherment>
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48. CBC Mode (Cont’d)
 Encryption: c0 ← IV, cj ← EK(cj−1⊕ xj)
 Decryption: c0 ← IV, xj ← cj−1 ⊕ E−1K(cj)
 chaining causes ciphertext cj to depend on all preceding
plaintext
 a single bit error in cj affects decipherment of blocks cj and
cj+1
 self-synchronizing: error cj (not cj+1, cj+2) is correctly
decrypted to xj+2.
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49. CFB-r(Cipher FeedBack) Mode
r-bit Shift r-bit Shift
I1=IV
key E key E
leftmost r bits Oj leftmost r bits Oj
xj ci ci xj
Encipherment Decipherment
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50. OFB(Output FeedBack) Mode
with full(or r-bit) feedback
Ij r-bit Shift Ij r-bit Shift
I1=IV
key E key E
Leftmost r-bits Oj Leftmost r-bits Oj
xj cj cj xj
Encipherment Deciphering
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51. Data Encryption
Standard
The Data Encryption Standard (DES)
specifies a FIPS approved
cryptographic algorithm as required
by FIPS 140-1.(Federal Information
Processing Standards 140-1)
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52. April 9, 2013
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53. Enciphering
 The 64 bits of the input block to be
enciphered are first subjected to the following
initial permutation IP:
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54.  IP
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
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55.  The permuted input block is then the input
to a complex key-dependent computation.
 The output of that computation (preoutput)
is then subjected to the next permutation
which is the inverse of the initial
permutation.
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56.  IP-1 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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57.  Let K be a block of 48 bits chosen from the
64-bit (how? explained next). Then the
output L'R' of an iteration with input LR is
defined by:
L' = R
R' = L (+) f (R,K)
 L'R' is the output of the 16th iteration then
R'L' is the preoutput block.
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58. One round of DES
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59. April 9, 2013
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60.  PC-1 (Key Permutation)
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
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61.  Iteration corresponds to left shifts:
1 2 3 4 5 6 7 8
1 1 2 2 2 2 2 2
9 10 11 12 13 14 15 16
1 2 2 2 2 2 2 1
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62.  PC-2 (Compression Permutation)
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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63. One round of DES
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64.  The Cipher Function f : A sketch of
the calculation of f (R, K) is given by
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65. Expansion Permutation
1 2 3 4 5 6 7 8 9 10 11 12
1 2 3 4 5 6 7 8 9 10 111213 14 15 16
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66. 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
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67. One round of DES
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68. S
1
14 4 13 1 2 15 11 8 3 10 6 12 5 9 0 7
O 15 7 4 14 2 13 1 10 6 12 11 9 5 3 8
4 1 14 8 13 6 2 11 15 12 9 7 3 10 5 0
15 12 8 2 4 9 1 7 5 11 3 14 10 O 6 13
S
2
15 1 8 14 6 11 3 4 9 7 2 13 12 O 5 10
3 13 4 7 15 2 8 14 12 0 1 10 6 9 11 5
0 14 7 11 10 4 13 1 5 8 12 6 9 3 2 15
13 8 10 1 3 15 4 2 11 6 7 12 0 5 14 9
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69. S
3
10 0 9 14 6 3 15 5 1 13 12 7 11 4 2 8
13 7 O 9 3 4 6 10 2 8 5 14 12 11 15 1
13 6 4 9 8 15 3 0 11 1 2 12 5 10 14 7
1 10 13 0 6 9 8 7 4 15 14 3 11 5 2 12
S
4
7 13 14 3 0 6 9 10 1 2 8 5 11 12 4 15
13 8 11 5 6 15 O 3 4 7 2 12 1 10 14 9
10 6 9 0 12 11 7 13 15 1 3 14 5 2 8 4
3 15 O 6 10 1 13 8 9 4 5 11 12 7 2 14
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70. S
5
2 12 4 1 7 10 11 6 8 5 3 15 13 O 14 9
14 11 2 12 4 7 13 1 5 0 15 10 3 9 8 6
4 2 1 11 10 13 7 8 15 9 12 5 6 3 O 14
11 8 12 7 1 14 2 13 6 15 O 9 10 4 5 3
S
6
12 1 10 15 9 2 6 8 O 13 3 4 14 7 5 11
10 15 4 2 7 12 9 5 6 1 13 14 O 11 3 8
9 14 15 5 2 8 12 3 7 0 4 10 1 13 11 6
4 3 2 12 9 5 15 10 11 14 1 7 6 0 8 13
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71. S
7
4 11 2 14 15 0 8 13 3 12 9 7 5 10 6 1
13 0 11 7 4 9 1 10 14 3 5 12 2 15 8 6
1 4 11 13 12 3 7 14 10 15 6 8 0 5 9 2
6 11 13 8 1 4 10 7 9 5 0 15 14 2 3 12
S
8
13 2 8 4 6 15 11 1 10 9 3 14 5 0 12 7
1 15 13 8 10 3 7 4 12 5 6 11 0 14 9 2
7 11 4 1 9 12 14 2 0 6 10 13 15 3 5 8
2 1 14 7 4 10 8 13 15 12 9 0 3 5 6 11
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72.  S1
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0 14 4 13 1 2 15 11 8 3 10 6 12 5 9 0 7
1 0 15 7 4 14 2 13 1 10 6 12 11 9 5 3 8
2 4 1 14 8 13 6 2 11 15 12 9 7 3 10 5 0
3 15 12 8 2 4 9 1 7 5 11 3 14 10 0 6 13
 For example, for input 011011 the row is 01,
that is row 1, and the column is determined
by 1101, that is column 13. In row 1 column
13 appears 5 so that the output is 0101.
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73. One round of DES
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74.  The permutation function P yields a 32-
bit output from a 32-bit input by
permuting the bits of the input block
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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75. Primitive functions for the data
encryption algorithm
 The choice of the primitive functions KS,
S1, ..., S8 and P is critical to the strength of an
encipherment resulting from the algorithm
 The recommended set of functions are
described as S1, ..., S8 and P in the
algorithm.
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76. Deciphering
 The permutation IP-1 applied to the
preoutput block is the inverse of the
initial permutation IP applied to the
input.
 R = L'
L = R' (+) f (L', K)
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77. Other Stream Ciphers
 RC4
 Variable key size stream cipher
 Proprietary for 7 years (1987 - 1994)
 In 1994 source code was posted to mailing list
 Works in OFB
 Encryption is 10 times faster than DES
 SEAL (Software-optimized Encryption ALgorithm)
 length-increasing pseudorandom function which maps a 32-bit sequence
number n to an L-bit keystream under control of a 160-bit secret key a
 In the preprocessing stage, the key is stretched into larger tables using the
table-generation function Ga (based on SHA-1)
 Subsequent to this preprocessing, keystream generation requires about 5
machine instructions per byte
 order of magnitude faster than DES
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78. Other Block Ciphers
 FEAL
 Fast N-round block cipher
 Suffers a lot of attacks, and hence introduce new attacks
on block ciphers
 Japan standard
 IDEA
 64-64-128-8
 James Massey
 Using algebraic functions (mult mod 2n+1, add mod 2n)
 SAFER, RC-5, AES
April 9, 2013
78

79. Thank You
reachable at
naasir_k@donboscoit.ac.in
April 9, 2013
79