Its a group project of a program which is a special kind of cryptosystem where same plaintext can generate different cipher text also same Cipher text can generate different plain text.
RBSAPSS Cipher
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First we take the plain text as input [Think it that its “ ABCDAB”]
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Now we need the key which contains 3 random index value a
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We can see that the whole text is combined with 6 letter. So if the key is [1] [2] [3] [random]
As we divide it 6 / 3 = 2 [for a easy way]
Then the plain text will be divided by 2 part as “ABC” “DAB”
And after every 3index value the key will be repeated.
A-->1-->ENCRYPT-->B-->1-->DECRYPT-->A
B-->2-->ENCRYPT-->D-->2-->DECRYPT-->B
C-->3-->ENCRYPT-->F-->3-->DECRYPT-->C
D-->1-->ENCRYPT-->E-->1-->DECRYPT-->D
A-->2-->ENCRYPT-->C-->2-->DECRYPT-->A
B-->3-->ENCRYPT-->E-->3-->DECRYPT-->B
ENCRYPTION FORMULA
E=(P1+K1) mod 26
here ,P1 First index of the plain text;
k1= randomly taken first key; [here, P1 = I(A) = 0; k1 = 1]
E=(P2+K2) mod 26
here B = 1 which is plain text (index 1)
for P2 the key will be K2 which is 2(randomly taken)
[B-->2-->ENCRYPT-->D-->2]
E=(P3+K3) mod 26
here C = 0 which is plain text (index 2)
for P3 the key will be K3 which is 3(randomly taken)
[C-->3-->ENCRYPT-->F-->3]
E=(P4+K1) mod 26
here C = 0 which is plain text (index 3)
for P4 the key will be K3 which is 1(randomly taken)
[D-->1-->ENCRYPT-->E-->1]
E=(P5+K2) mod 26
here C = 0 which is plain text (index 4)
for P5 the key will be K3 which is 2(randomly taken)
[A-->2-->ENCRYPT-->C-->2]
E=(P6+K3) mod 26
here C = 0 which is plain text (index 5)
for P6 the key will be K3 which is 3(randomly taken)
[B-->3-->ENCRYPT-->E-->3]
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then use the decryption formula
D1 = (E1-K1) mod 26