From 535a49bad95859d65be667b124754b8a9890e2f6 Mon Sep 17 00:00:00 2001
From: Alexander Munch-Hansen <alexmunchhansen@gmail.com>
Date: Wed, 24 Feb 2021 00:16:40 +0100
Subject: [PATCH] initial

---
 main.py | 182 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 182 insertions(+)
 create mode 100644 main.py

diff --git a/main.py b/main.py
new file mode 100644
index 0000000..9a7712e
--- /dev/null
+++ b/main.py
@@ -0,0 +1,182 @@
+#!/usr/bin/env python3
+import math
+import random
+from itertools import combinations
+
+
+def euclid(a, b):
+    """returns the Greatest Common Divisor of a and b"""
+    a = abs(a)
+    b = abs(b)
+    if a < b:
+        a, b = b, a
+    while b != 0:
+        a, b = b, a % b
+    return a
+
+
+def coPrime(l):
+    """returns 'True' if the values in the list L are all co-prime
+       otherwise, it returns 'False'. """
+    for i, j in combinations(l, 2):
+        if euclid(i, j) != 1:
+            return False
+    return True
+
+
+def extendedEuclid(a, b):
+    """return a tuple of three values: x, y and z, such that x is
+    the GCD of a and b, and x = y * a + z * b"""
+    if a == 0:
+        return b, 0, 1
+    else:
+        g, y, x = extendedEuclid(b % a, a)
+        return g, x - (b // a) * y, y
+
+
+def modInv(a, m):
+    """returns the multiplicative inverse of a in modulo m as a
+       positive value between zero and m-1"""
+    # notice that a and m need to co-prime to each other.
+    if coPrime([a, m]):
+        linearCombination = extendedEuclid(a, m)
+        return linearCombination[1] % m
+    else:
+        return 0
+
+
+def extractTwos(m):
+    """m is a positive integer. A tuple (s, d) of integers is returned
+    such that m = (2 ** s) * d."""
+    # the problem can be reduced to counting how many '0's there are in
+    # the end of bin(m). This can be done this way: m & a stretch of '1's
+    # which can be represent as (2 ** n) - 1.
+    assert m >= 0
+    i = 0
+    while m & (2 ** i) == 0:
+        i += 1
+    return i, m >> i
+
+
+def int2baseTwo(x):
+    """x is a positive integer. Convert it to base two as a list of integers
+    in reverse order as a list."""
+    # repeating x >>= 1 and x & 1 will do the trick
+    assert x >= 0
+    bitInverse = []
+    while x != 0:
+        bitInverse.append(x & 1)
+        x >>= 1
+    return bitInverse
+
+
+def millerRabin(n, k):
+    """
+    Miller Rabin pseudo-prime test
+    return True means likely a prime, (how sure about that, depending on k)
+    return False means definitely a composite.
+    Raise assertion error when n, k are not positive integers
+    and n is not 1
+    """
+    assert n >= 1
+    # ensure n is bigger than 1
+    assert k > 0
+    # ensure k is a positive integer so everything down here makes sense
+
+    if n == 2:
+        return True
+    # make sure to return True if n == 2
+
+    if n % 2 == 0:
+        return False
+    # immediately return False for all the even numbers bigger than 2
+
+    extract2 = extractTwos(n - 1)
+    s = extract2[0]
+    d = extract2[1]
+    assert 2 ** s * d == n - 1
+
+    def tryComposite(a):
+        """Inner function which will inspect whether a given witness
+        will reveal the true identity of n. Will only be called within
+        millerRabin"""
+        x = pow(a,d,n)
+        if x == 1 or x == n - 1:
+            return None
+        else:
+            for j in range(1, s):
+                x = pow(x,2,n)
+                if x == 1:
+                    return False
+                elif x == n - 1:
+                    return None
+            return False
+
+    for i in range(0, k):
+        a = random.randint(2, n - 2)
+        if tryComposite(a) == False:
+            return False
+    return True  # actually, we should return probably true.
+
+
+def findAPrime(a, b, k):
+    """Return a pseudo prime number roughly between a and b,
+    (could be larger than b). Raise ValueError if cannot find a
+    pseudo prime after 10 * ln(x) + 3 tries. """
+    x = random.randint(a, b)
+    for i in range(0, int(10 * math.log(x) + 3)):
+        if millerRabin(x, k):
+            return x
+        else:
+            x += 1
+    raise ValueError
+
+
+def newKey(a, b, k):
+    """ Try to find two large pseudo primes roughly between a and b.
+    Generate public and private keys for RSA encryption.
+    Raises ValueError if it fails to find one"""
+    try:
+        p = findAPrime(a, b, k)
+        while True:
+            q = findAPrime(a, b, k)
+            if q != p:
+                break
+    except:
+        raise ValueError
+
+    n = p * q
+    m = (p - 1) * (q - 1)
+
+    while True:
+        e = random.randint(1, m)
+        if coPrime([e, m]):
+            break
+
+    d = modInv(e, m)
+    return (n, e, d)
+
+
+def encrypt(message, modN, e, blockSize):
+    """given a string message, public keys and blockSize, encrypt using
+    RSA algorithms."""
+    return pow(message, e, modN)
+
+
+def decrypt(secret, modN, d, blockSize):
+    """reverse function of encrypt"""
+    return pow(secret, d, modN)
+
+if __name__ == '__main__':
+
+    n, e, d = newKey(10 ** 100, 10 ** 101, 50)
+    message = 35274764
+
+    print("original message is {}".format(message))
+    print("-"*80)
+    cipher = encrypt(message, n, e, 15)
+    print("cipher text is {}".format(cipher))
+    print("-"*80)
+    deciphered = decrypt(cipher, n, d, 15)
+    print("decrypted message is {}".format(deciphered))
+