Cryptography and Network Security chapter 4.ppt

Cryptography and
Network Security
Chapter 4
Fifth Edition
by William Stallings
Lecture slides by Lawrie Brown

Chapter 4 – Basic Concepts
in Number Theory and Finite
Fields
The next morning at daybreak, Star flew indoors, seemingly keen for
a lesson. I said, "Tap eight." She did a brilliant exhibition, first
tapping it in 4, 4, then giving me a hasty glance and doing it in 2, 2,
2, 2, before coming for her nut. It is astonishing that Star learned to
count up to 8 with no difficulty, and of her own accord discovered
that each number could be given with various different divisions, this
leaving no doubt that she was consciously thinking each number. In
fact, she did mental arithmetic, although unable, like humans, to
name the numbers. But she learned to recognize their spoken
names almost immediately and was able to remember the sounds of
the names. Star is unique as a wild bird, who of her own free will
pursued the science of numbers with keen interest and astonishing
intelligence.
— Living with Birds, Len Howard

Introduction
 will now introduce finite fields
 of increasing importance in cryptography
 AES, Elliptic Curve, IDEA, Public Key
 also important in many other areas of
computer engineering
 error detection, error correction, matching, ...
 concern operations on “numbers”
 where what constitutes a “number” and the
type of operations varies considerably
 start with basic number theory concepts

Divisors
 say a non-zero number b divides a if for
some m have a=mb (a,b,m all integers)
 that is b divides into a with no remainder
 denote this as b|a (“b divides a”)
 and say that b is a divisor of a
 eg. all of 1,2,3,4,6,8,12,24 divide 24
 eg. 13 | 182; –5 | 30; 17 | 289; –3 | 33; 17 | 0

Division Algorithm
 if we divide a by n we get integer quotient
q and integer remainder r such that:
 a = qn + r where 0 <= r < n; q = floor(a/n)
 remainder r often referred to as a residue

Greatest Common Divisor (GCD)
 a common problem in number theory
 GCD (a,b) of a and b is the largest integer
that divides evenly into both a and b
 e.g. GCD(60,24) = 12
 define gcd(0, 0) = 0
 often want no common factors (except 1)
define such numbers as relatively prime
 e.g. GCD(8,15) = 1
 hence 8 & 15 are relatively prime

Example GCD(1970,1066)
1970 = 1 x 1066 + 904 gcd(1066, 904)
1066 = 1 x 904 + 162 gcd(904, 162)
904 = 5 x 162 + 94 gcd(162, 94)
162 = 1 x 94 + 68 gcd(94, 68)
94 = 1 x 68 + 26 gcd(68, 26)
68 = 2 x 26 + 16 gcd(26, 16)
26 = 1 x 16 + 10 gcd(16, 10)
16 = 1 x 10 + 6 gcd(10, 6)
10 = 1 x 6 + 4 gcd(6, 4)
6 = 1 x 4 + 2 gcd(4, 2)
4 = 2 x 2 + 0 gcd(2, 0)

GCD(1160718174, 316258250)
Dividend Divisor Quotient Remainder
a = 1160718174 b = 316258250 q1 = 3 r1 = 211943424
b = 316258250 r1 = 211943424 q2 = 1 r2 = 104314826
r1 = 211943424 r2 = 104314826 q3 = 2 r3 = 3313772
r2 = 104314826 r3 = 3313772 q4 = 31 r4 = 1587894
r3 = 3313772 r4 = 1587894 q5 = 2 r5 = 137984
r4 = 1587894 r5 = 137984 q6 = 11 r6 = 70070
r5 = 137984 r6 = 70070 q7 = 1 r7 = 67914
r6 = 70070 r7 = 67914 q8 = 1 r8 = 2156
r7 = 67914 r8 = 2156 q9 = 31 r9 = 1078
r8 = 2156 r9 = 1078 q10 = 2 r10 = 0

Modular Arithmetic
 define modulo operator “a mod n” to be
remainder when a is divided by n
 where integer n is called the modulus
 b is called a residue of a mod n
 since with integers can always write: a = qn + b
 usually chose smallest positive remainder as residue
• ie. 0 <= b <= n-1
 process is known as modulo reduction
• eg. -12 mod 7 = -5 mod 7 = 2 mod 7 = 9 mod 7
 a & b are congruent if: a mod n = b mod n
 when divided by n, a & b have same remainder
 eg. 100 mod 11 = 34 mod 11
so 100 is congruent to 34 mod 11

Modular Arithmetic Operations
 can perform arithmetic with residues
 uses a finite number of values, and loops
back from either end
Zn = {0, 1, . . . , (n – 1)}
 modular arithmetic is when do addition &
multiplication and modulo reduce answer
 can do reduction at any point, i.e.
a+b mod n = [a mod n + b mod n] mod n

Modular Arithmetic Operations
1. [(a mod n) + (b mod n)] mod n
= (a + b) mod n
2. [(a mod n) – (b mod n)] mod n
= (a – b) mod n
3. [(a mod n) x (b mod n)] mod n
= (a x b) mod n
e.g.
[(11 mod 8) + (15 mod 8)] mod 8 = 10 mod 8 = 2 (11 + 15) mod 8 = 26 mod 8 = 2
[(11 mod 8) – (15 mod 8)] mod 8 = –4 mod 8 = 4 (11 – 15) mod 8 = –4 mod 8 = 4
[(11 mod 8) x (15 mod 8)] mod 8 = 21 mod 8 = 5 (11 x 15) mod 8 = 165 mod 8 =
5

Modulo 8 Addition Example
+ 0 1 2 3 4 5 6 7
0 0 1 2 3 4 5 6 7
1 1 2 3 4 5 6 7 0
2 2 3 4 5 6 7 0 1
3 3 4 5 6 7 0 1 2
4 4 5 6 7 0 1 2 3
5 5 6 7 0 1 2 3 4
6 6 7 0 1 2 3 4 5
7 7 0 1 2 3 4 5 6

Modulo 8 Multiplication
+ 0 1 2 3 4 5 6 7
0 0 0 0 0 0 0 0 0
1 0 1 2 3 4 5 6 7
2 0 2 4 6 0 2 4 6
3 0 3 6 1 4 7 2 5
4 0 4 0 4 0 4 0 4
5 0 5 2 7 4 1 6 3
6 0 6 4 2 0 6 4 2
7 0 7 6 5 4 3 2 1

Euclidean Algorithm
 an efficient way to find the GCD(a,b)
 uses theorem that:
 GCD(a,b) = GCD(b, a mod b)
 Euclidean Algorithm to compute GCD(a,b) is:
Euclid(a,b)
if (b=0) then return a;
else return Euclid(b, a mod b);

Extended Euclidean Algorithm
 calculates not only GCD but x & y:
ax + by = d = gcd(a, b)
 useful for later crypto computations
 follow sequence of divisions for GCD but
assume at each step i, can find x &y:
r = ax + by
 at end find GCD value and also x & y
 if GCD(a,b)=1 these values are inverses

Finding Inverses
EXTENDED EUCLID(m, b)
1. (A1, A2, A3)=(1, 0, m);
(B1, B2, B3)=(0, 1, b)
2. if B3 = 0
return A3 = gcd(m, b); no inverse
3. if B3 = 1
return B3 = gcd(m, b); B2 = b–1 mod m
4. Q = A3 div B3
5. (T1, T2, T3)=(A1 – Q B1, A2 – Q B2, A3 – Q B3)
6. (A1, A2, A3)=(B1, B2, B3)
7. (B1, B2, B3)=(T1, T2, T3)
8. goto 2

Inverse of 550 in GF(1759)
Q A1 A2 A3 B1 B2 B3
— 1 0 1759 0 1 550
3 0 1 550 1 –3 109
5 1 –3 109 –5 16 5
21 –5 16 5 106 –339 4
1 106 –339 4 –111 355 1
-111(1759) + 355(550) = 1

Group
 a set S of elements or “numbers”
 may be finite or infinite
 with some operation ‘.’ so G=(S,.)
 Obeys CAIN:
 Closure: a,b in S, then a.b in S
 Associative law: (a.b).c = a.(b.c)
 has Identity e: e.a = a.e = a
 has iNverses a-1: a.a-1 = e
 if commutative a.b = b.a
 then forms an Abelian group

Cyclic Group
 define exponentiation as repeated
application of operator
 example: a3 = a.a.a
 and let identity be: e=a0
 a group is cyclic if every element is a
power of some fixed element a
 i.e., b = ak for some a and every b in group
 a is said to be a generator of the group

Ring
 a set of “numbers”
 with two operations (addition and multiplication)
which form:
 an Abelian group with addition operation
 and multiplication:
 has closure
 is associative
 distributive over addition: a(b+c) = ab + ac
 if multiplication operation is commutative, it
forms a commutative ring
 if multiplication operation has an identity and no
zero divisors, it forms an integral domain

Field
 a set of numbers
 with two operations which form:
 Abelian group for addition
 Abelian group for multiplication (ignoring 0)
 ring
 have hierarchy with more axioms/laws
 group -> ring -> field

Finite (Galois) Fields
 finite fields play a key role in cryptography
 can show number of elements in a finite
field must be a power of a prime pn
 known as Galois fields
 denoted GF(pn)
 in particular often use the fields:
 GF(p)
 GF(2n)

Galois Fields GF(p)
 GF(p) is the set of integers {0,1, … , p-1}
with arithmetic operations modulo prime p
 these form a finite field
 since have multiplicative inverses
 find inverse with Extended Euclidean algorithm
 hence arithmetic is “well-behaved” and can
do addition, subtraction, multiplication, and
division without leaving the field GF(p)

GF(7) Multiplication Example
 0 1 2 3 4 5 6
0 0 0 0 0 0 0 0
1 0 1 2 3 4 5 6
2 0 2 4 6 1 3 5
3 0 3 6 2 5 1 4
4 0 4 1 5 2 6 3
5 0 5 3 1 6 4 2
6 0 6 5 4 3 2 1

Polynomial Arithmetic
 can compute using polynomials
f(x) = anxn + an-1xn-1 + … + a1x + a0 = ∑ aixi
• n.b. not interested in any specific value of x
• which is known as the indeterminate
 several alternatives available
 ordinary polynomial arithmetic
 poly arithmetic with coefs mod p
 poly arithmetic with coefs mod p and
polynomials mod m(x)

Ordinary Polynomial Arithmetic
 add or subtract corresponding coefficients
 multiply all terms by each other
 eg
let f(x) = x3 + x2 + 2 and g(x) = x2 – x + 1
f(x) + g(x) = x3 + 2x2 – x + 3
f(x) – g(x) = x3 + x + 1
f(x) x g(x) = x5 + 3x2 – 2x + 2

Polynomial Arithmetic with
Modulo Coefficients
 when computing value of each coefficient
do calculation modulo some value
 forms a polynomial ring
 could be modulo any prime
 but we are most interested in mod 2
 ie all coefficients are 0 or 1
 eg. let f(x) = x3 + x2 and g(x) = x2 + x + 1
f(x) + g(x) = x3 + x + 1
f(x) x g(x) = x5 + x2

Polynomial Division
 can write any polynomial in the form:
 f(x) = q(x) g(x) + r(x)
 can interpret r(x) as being a remainder
 r(x) = f(x) mod g(x)
 if have no remainder say g(x) divides f(x)
 if g(x) has no divisors other than itself & 1
say it is irreducible (or prime) polynomial
 arithmetic modulo an irreducible
polynomial forms a field

Polynomial GCD
 can find greatest common divisor for polys
 c(x) = GCD(a(x), b(x)) if c(x) is the poly of greatest
degree which divides both a(x), b(x)
 can adapt Euclid’s Algorithm to find it:
Euclid(a(x), b(x))
if (b(x)=0) then return a(x);
else return
Euclid(b(x), a(x) mod b(x));
 all foundation for polynomial fields as see next

Modular Polynomial
Arithmetic
 can compute in field GF(2n)
 polynomials with coefficients modulo 2
 whose degree is less than n
 hence must reduce modulo an irreducible poly
of degree n (for multiplication only)
 form a finite field
 can always find an inverse
 can extend Euclid’s Inverse algorithm to find

Computational
Considerations
 since coefficients are 0 or 1, can represent
any such polynomial as a bit string
 addition becomes XOR of these bit strings
 multiplication is shift & XOR
 cf long-hand multiplication
 modulo reduction done by repeatedly
substituting highest power with remainder
of irreducible poly (also shift & XOR)

Computational Example
 in GF(23) have (x2+1) is 1012 & (x2+x+1) is 1112
 so addition is
 (x2+1) + (x2+x+1) = x
 101 XOR 111 = 0102
 and multiplication is
 (x+1).(x2+1) = x.(x2+1) + 1.(x2+1)
= x3+x+x2+1 = x3+x2+x+1
 011.101 = (101)<<1 XOR (101)<<0 =
1010 XOR 101 = 11112
 polynomial modulo reduction (get q(x) & r(x)) is
 (x3+x2+x+1 ) mod (x3+x+1) = 1.(x3+x+1) + (x2) = x2
 1111 mod 1011 = 1111 XOR 1011 = 01002

Using a Generator
 equivalent definition of a finite field
 a generator g is an element whose
powers generate all non-zero elements
 in F have 0, g0, g1, …, gq-2
 can create generator from root of the
irreducible polynomial
 then implement multiplication by adding
exponents of generator

Summary
 have considered:
 divisibility & GCD
 modular arithmetic with integers
 concept of groups, rings, fields
 Euclid’s algorithm for GCD & Inverse
 finite fields GF(p)
 polynomial arithmetic in general and in GF(2n)

Cryptography and Network Security chapter 4.ppt

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Cryptography and Network Security chapter 4.ppt