Chapter 24 – Public Key Cryptography and the RSA Cipher 393

62. 
    

    blockSize is set to) characters from this block integer.

    
    


63. asciiNumber = blockInt // (BYTE_SIZE ** i)


64. blockInt = blockInt % (BYTE_SIZE ** i)


65. blockMessage.insert(0, chr(asciiNumber))


66. message.extend(blockMessage)


67. return ''.join(message)


68. 
    


69. 
    


70. def encryptMessage(message, key, blockSize=DEFAULT_BLOCK_SIZE):


71. 
    

    Converts the message string into a list of block integers, and then

    
    


72. 
    

    encrypts each block integer. Pass the PUBLIC key to encrypt.

    
    


73. encryptedBlocks = []


74. n, e = key


75. 
    


76. for block in getBlocksFromText(message, blockSize):


77. 
    

    ciphertext = plaintext ^ e mod n

    
    


78. encryptedBlocks.append(pow(block, e, n))


79. return encryptedBlocks


80. 
    


81. 
    


82. def decryptMessage(encryptedBlocks, messageLength, key,
    blockSize=DEFAULT_BLOCK_SIZE):


83. 
    

    Decrypts a list of encrypted block ints into the original message

    
    


84. 
    

    string. The original message length is required to properly decrypt

    
    


85. 
    

    the last block. Be sure to pass the PRIVATE key to decrypt.

    
    


86. decryptedBlocks = []


87. n, d = key


88. for block in encryptedBlocks:


89. 
    

    plaintext = ciphertext ^ d mod n

    
    


90. decryptedBlocks.append(pow(block, d, n))


91. return getTextFromBlocks(decryptedBlocks, messageLength, blockSize)


92. 
    


93. 
    


94. def readKeyFile(keyFilename):


95. 
    

    Given the filename of a file that contains a public or private key,

    
    


96. 
    

    return the key as a (n,e) or (n,d) tuple value.

    
    


97. fo = open(keyFilename)


98. content = fo.read()


99. fo.close()


100. keySize, n, EorD = content.split(',')


101. return (int(keySize), int(n), int(EorD))


102. 
     


103. 
     


104. def encryptAndWriteToFile(messageFilename, keyFilename, message,
     blockSize=DEFAULT_BLOCK_SIZE):


105. 
     

     Using a key from a key file, encrypt the message and save it to a