Number theory lecture (part 1)

Cryptography and Data
Security

Number Theory (1 of 2)

(presented by Aleksandr Yampolskiy)

Who is this?

Évariste Galois (1811-1832)

Divisors

n A non-zero number b divides a if 9m
s.t. a=mb (a,b,m 2 Z)
n That is, b divides into a with no
remainder
n We denote this b|a
n Example:
¡ all of 1,2,3,4,6,8,12,24 divide 24
¡ 6 | 24 (4*6 = 24), 1 | 24 (24*1 = 24),
but 5 | 24 (no m2Z such that m*5 = 24)

Divisors (cont.)

n Some axioms:
¡ a|1 ) a = §1
¡ a|b Æ b|a ) a = §b
¡ 8b?0 b|0
¡ b|g Æ b|h ) b|(mg + nh)
n A number p is prime , p ? 1 Æ 8m2
(1, p) m | p

Groups
n Def: A set G with a binary operation
?: G£ G ! G is called a group if:
1. (closure) ∀ a,b∈G, a?b∈G
2. (associativity) ∀ a,b,c∈G, (a?b)?c=a?(b?c)
4. (identity element) ∃ e∈G, ∀ a∈G, a?e=a
5. (inverse element) ∀ a∈G, ∃ a-1∈G, a?(a-1)=e

n A group is commutative (Abelian) if
∀ a,b∈G, a?b=b?a

Examples of groups
n Integers under addition, (Z, +) = {…, -2, -1, 0, +1,
+2, …}.
Identity: e = 0. Inverses: a-1 = -a
n ({Britney, Dustin}, ? ), where
¡ Britney? Britney = Britney
¡ Britney? Dustin = Dustin
¡ Dustin ? Britney = Dustin
¡ Dustin ? Dustin = Britney
Identity: e = Britney. Inverses: Britney -1 = Britney, Dustin-1 =
Dustin.

Subgroups

n Let (G, ?) be a group. (H, ?) is a sub-
group of (G, ?) if it is a group, and
H⊆G.
n Lagrange’s theorem: if G is finite and
(H, ?) is a sub-group of (G, ?) then |H|
divides |G|

Cyclic groups
n We define exponentiation as repeated
application of operator ?. For example,
¡ a3 = a?a?a
¡ we also define a0 = e and a-n = (a-1)n
n A group G is cyclic if every element is a
power of some fixed element.
n That is, G = <a> = {e, a, a2, a3,…} for some
a.
n a is said to be a generator of the group

A theorem…
Theorem: If (G, ? ) is a finite group, then
8a2 G a|G| = e.
Proof:
¡ Fix a2G. Consider <a> = {a0 = e, a, a2, …}
¡ |G| < 1 Æ <a> = G ) |<a>| < 1
¡ Hence, <a> = {e, a, a2, …, ak-1} for some k and
ak = e.
¡ By Lagrange’s Theorem, |<a>| divides |G| )
|G| = d¢|<a>| for some d2 Z.
¡ So, a|G| = ad¢|<a>| = ad¢k = {ak} d = ed. QED.

Rings
n Def: A set R together with two operations (+, ?) is a ring if
1. (R, +) is an Abelian group.
2. (R, ?) is a semi-group (just needs to be
associative)
3. ? distributes over +: a(b + c) = ab + ac and
(a + b)c = ac + bc
n We use +, ?, only for the sake of using familiar and intuitive
notation. We could instead use any symbols. We are
NOT doing regular addition/multiplication.
n In the ring R, we denote by: -a, the additive inverse of a.
On commutative rings, the multiplicative inverse of a is
denoted by a-1 (when it exists).

Rings (cont.)

n Example: set of 2x2 matrices forms a
ring under regular matrix (+, *).
n Some questions to think about:
¡ Is it always the case that A + B = B + A?
¡ What about A*B = B*A?
¡ What is the identity element?

Fields

n Def: A field is a commutative ring with
identity where each non-zero element has a
multiplicative inverse: ∀ a≠0∈F, ∃ a-1∈F,
a·a-1=1
n Equivalently, (F,+) is a commutative
(additive) group and (F {0}, ·) is a
commutative (multiplicative) group.
n Example: set of rational numbers Q

Modular arithmetic
n Def: Modulo operator a mod n = remainder
when a is divided by n
(Another notation: a % n)
n Example: 11 mod 7 = 4, 10 mod 5 = 0, 3 mod 2
= 1. n-1 0 1

.
.
.
01 n-1 n

clock
arithmetic

Modular arithmetic (cont.)

n a is congruent to b (a = b mod n) if
when divided by n, a and b give the
same remainder (a mod n = b mod n)
n a ´ b mod n if n | (a – b)
n E.g. 100 ´ 34 mod 11

Zn

n a´ b mod n defines an equivalence
relation
n set of residues Zn = {0, 1, …, n-1}
n Each integer r2 Zn actually represents
a residue class [r] = {a2 Z : a ´ r mod
n}

Zn (cont.)
E.g., Z7 = {0, 1, 2, 3, 4, 5, 6}. But in fact, we
are dealing with:
...
-21 -20 -19 -18 -17 -16 -15
-14 -13 -12 -11 -10 -9 -8
-7 -6 -5 -4 -3 -2 -1
0 1 2 3 4 5 6
7 8 9 10 11 12 13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30 31 32 33 34
...

Zn (cont.)

n Integers mod n Zn = {0, 1, …, n-1} is
an Abelian group.
n Example: What is 3+5 in Z7? What is
-6 in Z7?
n Note some peculiarities for Zn
¡ if (a+b)=(a+c) mod n then b=c mod n
¡ but (ab)=(ac) mod n then b=c mod n only
if a is relatively prime to n

Zn*

n Multiplicative integers mod n
Zn* = {x2 Zn : gcd(x, n) = 1}
n Zn* consists of all integers 0…n-1
relatively prime with n
n What is the size of this group? Euler’s
totient function φ(n) = |Zn*|

Zn* (cont.)

n What is φ(p) when p is prime?
¡ ZP* = {1, 2, …, p-1} ) φ(p) = |Zp*| = p – 1.
n What about φ(pk) where p is prime and k >
1?
¡ Zpk = {0, 1, …, pk – 1}
¡ How many multiples of p are in Zpk?
¡ Multiples are {0, p, 2p, …, (pk-1 – 1)p}. There are
pk-1 of them
¡ Hence, φ(pk) = pk – pk-1

Zn* (cont.)

n φ(mn) = φ(m)¢ φ(n)
n φ(∏i pie) = ∏i(pie – pie-1)
n Example:
¡ φ(10) = φ(2)¢φ(5) = 1¢4 = 4
¡ S = {1· n · 10 : n relatively prime to 10} =
{1. 3, 7, 9}. Notice that |S| = 4 as expected.

Number theory lecture (part 1)

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