RSA algorithm is an asymmetric cryptography algorithm which means, there should be two keys involve while communicating, i.e., public key and private key. There are simple steps to solve problems on the RSA Algorithm.
Example-1:
- Step-1: Choose two prime number
and
Lets takeand
- Step-2: Compute the value of
and
It is given as,and
Here in the example,
- Step-3: Find the value of
(public key)
Choose, such that
should be co-prime. Co-prime means it should not multiply by factors of
and also not divide by
Factors of
are,
so
should not multiply by
and
and should not divide by 20.
So, primes are 3, 7, 11, 17, 19…, as 3 and 11 are taken choose
as 7
Therefore,
- Step-4: Compute the value of
(private key)
The condition is given as,where y is the value of
.
To compute the value of
,
- Form a table with four columns i.e., a, b, d, and k.
- Initialize a = 1, b = 0, d =
, k = – in first row.
- Initialize a = 0, b = 1, d =
,
in second row.
- From the next row, apply following formulas to find the value of next a, b, d, and k, which is given as
As soon as,
, stop the process and check for the below condition
if
if
For a given example, the table will be,
a b d k 1 0 20 – 0 1 7 2 1 -2 6 1 -1 3 1 – As in the above table
, stop the process and check for the condition given for the
To verify that
is correct, the above condition should satisfy, i.e.
. Hence
is correct.
- Step-5: Do the encryption and decryption
Encryption is given as,
Decryption is given as,For the given example, suppose
, so
Encryption isDecryption is
Therefore in the final,
,
,
,
,
and
Example-2: GATE CS-2017 (Set 1)
In an RSA cryptosystem, a particular A uses two prime numbers p = 13 and q =17 to generate her public and private keys. If the public key of A is 35. Then the private key of A is?
and
- Compute
and
(public key)
- Compute
(private key)
a b d k 1 0 192 – 0 1 35 5 1 -5 17 2 -2 11 1 – (private key)
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