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RSA Digital Signature Scheme using Python
• Difficulty Level : Easy
• Last Updated : 26 Mar, 2020

RSA algorithm is an asymmetric cryptography algorithm. Asymmetric actually means that it works on two different keys i.e. Public Key and Private Key. As the name describes that the Public Key is given to everyone and the Private key is kept private.

An example of asymmetric cryptography :

• A client (for example browser) sends its public key to the server and requests for some data.
• The server encrypts the data using the client’s public key and sends the encrypted data.
• Client receives this data and decrypts it.

Since this is asymmetric, nobody else except the browser can decrypt the data even if a third party has the public key of browser.

Digital signatures are used to verify the authenticity of the message sent electronically. A digital signature algorithm uses a public key system. The intended transmitter signs his/her message with his/her private key and the intended receiver verifies it with the transmitter’s public key. A digital signature can provide message authentication, message integrity and non-repudiation services.

### Algorithm

RSA Key Generation:

• Choose two large prime numbers p and q
• Calculate n=p*q
• Select public key e such that it is not a factor of (p-1)*(q-1)
• Select private key d such that the following equation is true (d*e)mod(p-1)(q-1)=1 or d is inverse of E in modulo (p-1)*(q-1)

RSA Digital Signature Scheme: In RSA, d is private; e and n are public.

• Alice creates her digital signature using S=M^d mod n where M is the message
• Alice sends Message M and Signature S to Bob
• Bob computes M1=S^e mod n
• If M1=M then Bob accepts the data sent by Alice.

Below is the implementation.

 `# Function to find gcd ``# of two numbers``def` `euclid(m, n):``     ` `    ``if` `n ``=``=` `0``:``        ``return` `m``    ``else``:``        ``r ``=` `m ``%` `n``        ``return` `euclid(n, r)``     ` `     ` `# Program to find ``# Multiplicative inverse``def` `exteuclid(a, b):``     ` `    ``r1 ``=` `a``    ``r2 ``=` `b``    ``s1 ``=` `int``(``1``)``    ``s2 ``=` `int``(``0``)``    ``t1 ``=` `int``(``0``)``    ``t2 ``=` `int``(``1``)``     ` `    ``while` `r2 > ``0``:``         ` `        ``q ``=` `r1``/``/``r2``        ``r ``=` `r1``-``q ``*` `r2``        ``r1 ``=` `r2``        ``r2 ``=` `r``        ``s ``=` `s1``-``q ``*` `s2``        ``s1 ``=` `s2``        ``s2 ``=` `s``        ``t ``=` `t1``-``q ``*` `t2``        ``t1 ``=` `t2``        ``t2 ``=` `t``         ` `    ``if` `t1 < ``0``:``        ``t1 ``=` `t1 ``%` `a``         ` `    ``return` `(r1, t1)`` ` `# Enter two large prime``# numbers p and q``p ``=` `823``q ``=` `953``n ``=` `p ``*` `q``Pn ``=` `(p``-``1``)``*``(q``-``1``)`` ` `# Generate encryption key ``# in range 1

Output:

```decryption key is:  160009
As M = M1, Accept the message sent by Alice```

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