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Question

In a RSA cryptosystem , a participant uses two prime numbers p and Q is 17 and 11 respectively to generate his/her  public key and private keys. if the public key of participant is 7 and cipher text(C) is 11, then the original message (M) is______?

Comments

149??

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given ans is as above .bt how this one will came! .may yu explain?

Answer 1

Public Key = (n, e)    e = 7 , p = 17  ,  q  = 11 
Private Key = (n, d)  
n = pq = 187

Φ(n) = (p-1)(q-1) so, Φ(n) = 160

d *e mod Φ(n) = 1

d = 23

cipher text , c = 11

Message ,  M = c^d mod n

M = 11²³ mod 187 = 149

Comments

$11^{23}mod187 =(11^{10}∗11^{10}∗11^{3})mod187 =(11^{10}mod187∗11^{10}mod187∗11^{3}mod187)mod187 =(66∗66∗22)mod187 =88$

split big power to small small power then VC will show the correct result.

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@Shubhgupta 

thanks 

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can someone tell me how 23 calculated . I mean how actual calculation is done?

Answer 2

In RSA we encrypt using public and decrypt using private key

phi = (p-1)(q-1) = 160

Public key = 7

calculate Private key(d) comes out to be 23

let original msg be 'm'

ciphertext be 'c'

c^d mod n = m  // as c = m^e mod n and by euler toitent we know m^ed mod n = m

c = 11, d = 23 and n = p*q calculate m !