This document introduces the affine cipher encryption technique. It explains how to encrypt and decrypt messages using an affine cipher by: 1. Converting letters to numbers using their position in the alphabet. 2. Using the formula Y=(aP+k)%26 to encrypt numbers, where a is the multiplier, k is the shift, and p is the original letter's number. 3. Decrypting using the inverse of the multiplier and the same shift in the formula P=[(a^-1)(Y-k)]%26. Worked examples are provided to illustrate encrypting the letter 'g' to 's' and decrypting it back. Flowcharts outline the full encryption and decryption processes

1. AFFINE CIPHER
Ammar Amjad
Kainat Khalid
Zakriya Ali Sabir
Syed Mohammad Ali
Hamza Tariq
Qazi Talha Hamid

2. INTRODUCTION
BACKGROUND

3. ENCODING
Convert into a coded form.

5. ENCRYPT
Convert (information or data) into a cipher or code,
especially to prevent unauthorized access.

8. JEAN-FRANCOIS
CHAMPOLLION
French soldier
Philologist
Orientalist
DECIPHER of the EGYPTIAN
HIEROGLYPHS

10. ENCRYPTION
TECHNIQE

11. Y=(aP+k)%26

12. a=multiplier
k=shift
p=data

13. gcd(a,26)

14. Why 26??

15. g=6
Suppose we want to encrypt ‘g’
Checking corresponding numeric value

16. ( 7, 2)
using

17. Plugging the values
in formula:
=(7*6+2)%26
=44%26
=18

18. as s=18
we get :
‘ g ’  ‘ s ’

19. DECRYPTION TECHNIQUE

21. Y=(aP+k)%26

23. ( a , k )KEY 

24. P=[(a^-1)(Y-k)] % 26
Here this (a^-1) is not the multiplicative
inverse of a but it is the modular inverse of a.

26. a = 7
k = 2
As we had encrypted :
‘ g ’  ‘ s ’
Using :

27. ( a . a inverse) % 26 = 1
If we take :
1 <= a <= 25
εa ^ -1 { 1,3,5,7,9,11,15,17,19,21,23,25}

28. key( a , k )=( 7,2 )
a inverse = 15
Y = 18
P = [a inverse ( Y – k) ] % 26
P = [ 15 ( 18 - 2 ) ] % 26
P = 6

29. key( a , k )=( 7,2 )
a inverse = 15
Y = 18
P = [a inverse ( Y – k) ] % 26
P = [ 15 ( 18 - 2 ) ] % 26
P = 6
Its CORRECT !

31. ALGORITHM & CODE

39. FLOW-CHART
PROCESS OF ENCRYPTION
PROCESS OF DECRYPTION

40. START
TAKE INPUT FROM USER
CONVERT ALPHABETS TO
NUMBERS
ENCRYPT NUMBERS

41. CONVERT ENCRYPTED
NUMBERS TO LETTERS
CONVERT LETTERS
BACK TO NUMBERS
DECRYPT NUMBERS
CONVERT NUMBERS TO
LETTERS
END

42. LIMITATIONS
 Case of the alphabets
 Spaces
 Shift
 Multiplier

43. CONCLUSION
Thank-you!