Initial

Author: harshildarji

ciphers/affine_cipher.py

@@ -0,0 +1,76 @@
+import sys, random, cryptomath_module as cryptoMath
+
+SYMBOLS = """ !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~"""
+
+def main():
+    message = input('Enter message: ')
+    key = int(input('Enter key [2000 - 9000]: '))
+    mode = input('Encrypt/Decrypt [E/D]: ')
+
+    if mode.lower().startswith('e'):
+              mode = 'encrypt'
+              translated = encryptMessage(key, message)
+    elif mode.lower().startswith('d'):
+              mode = 'decrypt'
+              translated = decryptMessage(key, message)
+    print('\n%sed text: \n%s' % (mode.title(), translated))
+
+def getKeyParts(key):
+    keyA = key // len(SYMBOLS)
+    keyB = key % len(SYMBOLS)
+    return (keyA, keyB)
+
+def checkKeys(keyA, keyB, mode):
+    if keyA == 1 and mode == 'encrypt':
+        sys.exit('The affine cipher becomes weak when key A is set to 1. Choose different key')
+    if keyB == 0 and mode == 'encrypt':
+        sys.exit('The affine cipher becomes weak when key A is set to 1. Choose different key')
+    if keyA < 0 or keyB < 0 or keyB > len(SYMBOLS) - 1:
+        sys.exit('Key A must be greater than 0 and key B must be between 0 and %s.' % (len(SYMBOLS) - 1))
+    if cryptoMath.gcd(keyA, len(SYMBOLS)) != 1:
+        sys.exit('Key A %s and the symbol set size %s are not relatively prime. Choose a different key.' % (keyA, len(SYMBOLS)))
+
+def encryptMessage(key, message):
+    '''
+    >>> encryptMessage(4545, 'The affine cipher is a type of monoalphabetic substitution cipher.')
+    'VL}p MM{I}p~{HL}Gp{vp pFsH}pxMpyxIx JHL O}F{~pvuOvF{FuF{xIp~{HL}Gi'
+    '''
+    keyA, keyB = getKeyParts(key)
+    checkKeys(keyA, keyB, 'encrypt')
+    cipherText = ''
+    for symbol in message:
+        if symbol in SYMBOLS:
+            symIndex = SYMBOLS.find(symbol)
+            cipherText += SYMBOLS[(symIndex * keyA + keyB) % len(SYMBOLS)]
+        else:
+            cipherText += symbol
+    return cipherText
+
+def decryptMessage(key, message):
+    '''
+    >>> decryptMessage(4545, 'VL}p MM{I}p~{HL}Gp{vp pFsH}pxMpyxIx JHL O}F{~pvuOvF{FuF{xIp~{HL}Gi')
+    'The affine cipher is a type of monoalphabetic substitution cipher.'
+    '''
+    keyA, keyB = getKeyParts(key)
+    checkKeys(keyA, keyB, 'decrypt')
+    plainText = ''
+    modInverseOfkeyA = cryptoMath.findModInverse(keyA, len(SYMBOLS))
+    for symbol in message:
+        if symbol in SYMBOLS:
+            symIndex = SYMBOLS.find(symbol)
+            plainText += SYMBOLS[(symIndex - keyB) * modInverseOfkeyA % len(SYMBOLS)]
+        else:
+            plainText += symbol
+    return plainText
+
+def getRandomKey():
+    while True:
+        keyA = random.randint(2, len(SYMBOLS))
+        keyB = random.randint(2, len(SYMBOLS))
+    if cryptoMath.gcd(keyA, len(SYMBOLS)) == 1:
+        return keyA * len(SYMBOLS) + keyB
+
+if __name__ == '__main__':
+    import doctest
+    doctest.testmod()
+    main()

ciphers/cryptomath_module.py

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+def gcd(a, b):
+    while a != 0:
+        a, b = b % a, a
+    return b
+
+def findModInverse(a, m):
+    if gcd(a, m) != 1:
+        return None
+    u1, u2, u3 = 1, 0, a
+    v1, v2, v3 = 0, 1, m
+    while v3 != 0:
+        q = u3 // v3
+        v1, v2, v3, u1, u2, u3 = (u1 - q * v1), (u2 - q * v2), (u3 - q *v3), v1, v2, v3
+    return u1 % m