Homophonic Substitution Cipher

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Presentation transcript:

Homophonic Substitution Cipher Each letter in the secret message is replaced with a number which represents the position of a word in an assistant text which start with this letter

Beale cipher We assign a set of numbers to each letter in the plain text alphabet by using a specific text, each letter in the plain text will be replaced by number that represent the location of some ward in the text that start with this letter

Beale cipher

Beale cipher Ex Plain text = A L M A M O N C O L L E G E Cipher text = 19 11 17 21 47 44 62 63 58 16 40 20 12 20 Decryption Cipher text = 19 11 17 21 47 44 62 63 58 16 40 12 20 Plain text = A L M A M O N C O L L G E

Beale cipher If the assistant text = zainab muhammed and P= zahzuh The assistant text will be as follows after dispose the repeated letters from it. Assistant text = zainbmuhed

Beale cipher

Beale cipher

Polyalphabetic Substitution Cipher 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

Vigenère Cipher simplest polyalphabetic substitution 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

Vigenère Cipher Encryption C= E(p) = (p+ki) mod (26) Decryption p= E(C) = (C - ki) mod (26)

Vigenère Cipher Example :We can encrypt the message “She is listening” using the 6-character keyword “PASCAL”.

Vigenère Cipher

Vigenère Cipher A Vigenere Table

Vigenère Cipher Plaintext = the forth class Key = car

Vigenère Cipher C(f) = (5+2) mod 26 = 7 → h C=(P+K) mod 26 C(f) = (5+2) mod 26 = 7 → h C(o) = (14+0) mod 26 = 14 → o C(r) = (17+17) mod 26 = 8 → i C(t) = (19+2) mod 26 = 21 → v C(h) = (7+0) mod 26 = 7 → h C(c) = (2+17) mod 26 =19 → t C(l) = (11+2) mod 26 =13 → n C(a) = (0+0) mod 26 = 0 → a C(s) = (18+17) mod 26 = 9 → j C(s) = (18+2) mod 26 = 20 → u ∴ C = vhvhoivhtnaju

Vigenère Cipher P=(C-K) mod 26 P(v) =(21-2) mod 26 = 19 → t P(h) =(7-0) mod 26 = 7 → h P(v) =(21-17) mod 26 = 4 → e P(h) =(7-2) mod 26 = 5 → f P(o) = (14-0) mod 26 = 14 → o P(i) = (8-17) mod 26 = (26-9) mod 26 = 17 → r

Vigenère Cipher P(v) = (21-2) mod 26 = 19 → t P(h) = (7-0) mod 26 = 7 →h P(t) = (19-17) mod 26 =2 → c P(n) = (13-2) mod 26 = 11 →l P(a) =(0-0) mod 26 =0 → a P(j) = (9-17) mod 26 =(26-8) mod 26 =18 →s P(u) = (20-2) mod 26 =18 →s ∴ P= theforthclass

Vigenère Cipher Plaintext = she is listening Key = pascal