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# RSA Cipher
# (BSD Licensed)
import sys
# IMPORTANT: The block size MUST be less than or equal to the key size!
# (Note: The block size is in bytes, the key size is in bits. There
# are 8 bits in 1 byte.)
DEFAULT_BLOCK_SIZE = 128 # 128 bytes
BYTE_SIZE = 256 # One byte has 256 different values.
def main():
# Runs a test that encrypts a message to a file or decrypts a message
# from a file.
filename = 'encrypted_file.txt' # the file to write to/read from
mode = 'encrypt' # set to 'encrypt' or 'decrypt'
if mode == 'encrypt':
message = '''"Journalists belong in the gutter because that is where the ruling classes throw their guilty secrets." -Gerald Priestland "The Founding Fathers gave the free press the protection it must have to bare the secrets of government and inform the people." -Hugo Black'''
pubKeyFilename = 'al_sweigart_pubkey.txt'
print('Encrypting and writing to %s...' % (filename))
encryptedText = encryptAndWriteToFile(filename, pubKeyFilename, message)
print('Encrypted text:')
elif mode == 'decrypt':
privKeyFilename = 'al_sweigart_privkey.txt'
print('Reading from %s and decrypting...' % (filename))
decryptedText = readFromFileAndDecrypt(filename, privKeyFilename)
print('Decrypted text:')
def getBlocksFromText(message, blockSize=DEFAULT_BLOCK_SIZE):
# Converts a string message to a list of block integers. Each integer
# represents 128 (or whatever blockSize is set to) string characters.
messageBytes = message.encode('ascii') # convert the string to bytes
blockInts = []
for blockStart in range(0, len(messageBytes), blockSize):
# Calculate the block integer for this block of text
blockInt = 0
for i in range(blockStart, min(blockStart + blockSize, len(messageBytes))):
blockInt += messageBytes[i] * (BYTE_SIZE ** (i % blockSize))
return blockInts
def getTextFromBlocks(blockInts, messageLength, blockSize=DEFAULT_BLOCK_SIZE):
# Converts a list of block integers to the original message string.
# The original message length is needed to properly convert the last
# block integer.
message = []
for blockInt in blockInts:
blockMessage = []
for i in range(blockSize - 1, -1, -1):
if len(message) + i < messageLength:
# Decode the message string for the 128 (or whatever
# blockSize is set to) characters from this block integer.
asciiNumber = blockInt // (BYTE_SIZE ** i)
blockInt = blockInt % (BYTE_SIZE ** i)
blockMessage.insert(0, chr(asciiNumber))
return ''.join(message)
def encryptMessage(message, key, blockSize=DEFAULT_BLOCK_SIZE):
# Converts the message string into a list of block integers, and then
# encrypts each block integer. Pass the PUBLIC key to encrypt.
encryptedBlocks = []
n, e = key
for block in getBlocksFromText(message, blockSize):
# ciphertext = plaintext ^ e mod n
encryptedBlocks.append(pow(block, e, n))
return encryptedBlocks
def decryptMessage(encryptedBlocks, messageLength, key, blockSize=DEFAULT_BLOCK_SIZE):
# Decrypts a list of encrypted block ints into the original message
# string. The original message length is required to properly decrypt
# the last block. Be sure to pass the PRIVATE key to decrypt.
decryptedBlocks = []
n, d = key
for block in encryptedBlocks:
# plaintext = ciphertext ^ d mod n
decryptedBlocks.append(pow(block, d, n))
return getTextFromBlocks(decryptedBlocks, messageLength, blockSize)
def readKeyFile(keyFilename):
# Given the filename of a file that contains a public or private key,
# return the key as a (n,e) or (n,d) tuple value.
fo = open(keyFilename)
content =
keySize, n, EorD = content.split(',')
return (int(keySize), int(n), int(EorD))
def encryptAndWriteToFile(messageFilename, keyFilename, message, blockSize=DEFAULT_BLOCK_SIZE):
# Using a key from a key file, encrypt the message and save it to a
# file. Returns the encrypted message string.
keySize, n, e = readKeyFile(keyFilename)
# Check that key size is greater than block size.
if keySize < blockSize * 8: # * 8 to convert bytes to bits
sys.exit('ERROR: Block size is %s bits and key size is %s bits. The RSA cipher requires the block size to be equal to or less than the key size. Either increase the block size or use different keys.' % (blockSize * 8, keySize))
# Encrypt the message
encryptedBlocks = encryptMessage(message, (n, e), blockSize)
# Convert the large int values to one string value.
for i in range(len(encryptedBlocks)):
encryptedBlocks[i] = str(encryptedBlocks[i])
encryptedContent = ','.join(encryptedBlocks)
# Write out the encrypted string to the output file.
encryptedContent = '%s_%s_%s' % (len(message), blockSize, encryptedContent)
fo = open(messageFilename, 'w')
# Also return the encrypted string.
return encryptedContent
def readFromFileAndDecrypt(messageFilename, keyFilename):
# Using a key from a key file, read an encrypted message from a file
# and then decrypt it. Returns the decrypted message string.
keySize, n, d = readKeyFile(keyFilename)
# Read in the message length and the encrypted message from the file.
fo = open(messageFilename)
content =
messageLength, blockSize, encryptedMessage = content.split('_')
messageLength = int(messageLength)
blockSize = int(blockSize)
# Check that key size is greater than block size.
if keySize < blockSize * 8: # * 8 to convert bytes to bits
sys.exit('ERROR: Block size is %s bits and key size is %s bits. The RSA cipher requires the block size to be equal to or less than the key size. Did you specify the correct key file and encrypted file?' % (blockSize * 8, keySize))
# Convert the encrypted message into large int values.
encryptedBlocks = []
for block in encryptedMessage.split(','):
# Decrypt the large int values.
return decryptMessage(encryptedBlocks, messageLength, (n, d), blockSize)
# If is run (instead of imported as a module) call
# the main() function.
if __name__ == '__main__':