Chapter 15 – The Affine Cipher 215

33. if keyB == 0 and mode == 'encrypt':


34. sys.exit('The affine cipher becomes incredibly weak when key B is
    set to 0. Choose a different key.')


35. if keyA < 0 or keyB < 0 or keyB > len(SYMBOLS) - 1:


36. sys.exit('Key A must be greater than 0 and Key B must be between 0
    and %s.' % (len(SYMBOLS) - 1))


37. if cryptomath.gcd(keyA, len(SYMBOLS)) != 1:


38. sys.exit('Key A (%s) and the symbol set size (%s) are not
    relatively prime. Choose a different key.' % (keyA, len(SYMBOLS)))


39. 
    


40. 
    


41. def encryptMessage(key, message):


42. keyA, keyB = getKeyParts(key)


43. checkKeys(keyA, keyB, 'encrypt')


44. ciphertext = ''


45. for symbol in message:


46. if symbol in SYMBOLS:


47. 
    

    encrypt this symbol

    
    


48. symIndex = SYMBOLS.find(symbol)


49. ciphertext += SYMBOLS[(symIndex * keyA + keyB) % len(SYMBOLS)]


50. else:


51. ciphertext += symbol # just append this symbol unencrypted


52. return ciphertext


53. 
    


54. 
    


55. def decryptMessage(key, message):


56. keyA, keyB = getKeyParts(key)


57. checkKeys(keyA, keyB, 'decrypt')


58. plaintext = ''


59. modInverseOfKeyA = cryptomath.findModInverse(keyA, len(SYMBOLS))


60. 
    


61. for symbol in message:


62. if symbol in SYMBOLS:


63. 
    

    decrypt this symbol

    
    


64. symIndex = SYMBOLS.find(symbol)


65. plaintext += SYMBOLS[(symIndex - keyB) * modInverseOfKeyA %
    len(SYMBOLS)]


66. else:


67. plaintext += symbol # just append this symbol undecrypted


68. return plaintext


69. 
    


70. 
    


71. def getRandomKey():


72. while True:


73. keyA = random.randint(2, len(SYMBOLS))


74. keyB = random.randint(2, len(SYMBOLS))