The study of encryption methods is called cryptography.
The study of principles/ methods of
deciphering cipher text without knowing key is called crypto-analysis (code breaking). Study of
both cryptography and crypto-analysis is known as cryptology. The simple text is known as
plane text. The encoded message is known as cipher text. The algorithm for transforming
plaintext to cipher text. The information used in cipher known only to sender/receiver is known
as key. If the sender and receiver uses same key then it is said to be symmetric key (or) single
key (or) conventional encryption. If the sender and receiver use different keys then it is said to be
public key encryption.
ENCIPHER (ENCRYPT) - converting plaintext to cipher text
DECIPHER (DECRYPT) - recovering cipher text from plaintext
All the encryption algorithms are a based on two general principles:
Substitution, in which each element in the plaintext is mapped into another element, and
transposition, in which elements in the plaintext are rearranged.
CLASSICAL ENCRYPTION TECHNIQUES
There are two basic building blocks of all encryption techniques: substitution and
transposition.
SUBSTITUTION TECHNIQUES
A substitution technique is one in which the letters of plaintext are replaced by other
letters or by numbers or symbols. If the plaintext is viewed as a sequence of bits, then
substitution involves replacing plaintext bit patterns with cipher text bit patterns.
Cesar cipher
The earliest known use of a substitution cipher and the simplest was by Julius
Caesar. The Caesar cipher involves replacing each letter of the alphabet with the
letter standing 3 places further down the alphabet. e.g., plain text : pay more money
Cipher text: SDB PRUH PRQHB
Note that the alphabet is wrapped around, so that letter following „z‟ is „a‟.
For each plaintext letter p, substitute the cipher text letter c such that C = E(p) =
(p+3) mod 26
A shift may be any amount, so that general Caesar algorithm is C = E (p) = (p+k) mod
26 Where k takes on a value in the range 1 to 25. The decryption algorithm is simply P =
D(C) = (C-k) mod 26
ENCRYPTION
NESOACADEMY
C = E (p) = (p + k) mod 26
i. N = (13+3) % 26 =16=Q
ii. E = (O4+3) % 26=07=H
iii. S = (18+3) % 26 =21=V
iv. O = (14+3) % 26 =17=R
v. A = (00+3) % 26 =03=D
vi. C = (02+3) % 26 =05=F
vii. A = (00+3) % 26 =03=D
viii. D = (03+3) % 26 =06=G
ix. E = (04+3) % 26 =07=H
x. M = (12+3) % 26 =15=P
xi. Y = (24+3) % 26 =27=01=B
QHVRDFDGHPB
DECRYPTION
QHVRDFDGHPB
p = E (C) = (C - k) mod 26
i. Q = (16-3) % 26 =13=N
ii. H = (O7-3) % 26=04=E
iii. V = (21-3) % 26 =18=S
iv. R = (17-3) % 26 =14=O
v. D = (03-3) % 26 =00=A
vi. F = (05-3) % 26 =02=C
vii. D = (03-3) % 26 =00=A
viii. G = (06-3) % 26 =03=D
ix. H = (07-3) % 26 =04=E
x. P = (15-3) % 26 =12=M
xi. B = (27-3) % 26 =24=Y
NESOACADEMY
