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Encrypt using XOR Cipher with Repeating Key
  • Last Updated : 05 Sep, 2020

Computer Architectures have predefined ASCII values & Binary forms for all printable characters, which allows us to operate bit-wise logic like XOR and most encryption/decryption algorithms depend on. The Key is XOR-operated on the plain text to produce the encrypted text. 

Only Parameters required to encrypt a plain text using this technique:

  1. Plain text (text which has to be encrypted).
  2. Key (unique byte of text which is used to encrypt, can be of any length).

Encryption Processing:

  • Finding the lengths of “Plain text” and “Key”.
  • Breaking the plain text into pieces of length equivalent to the length of Key.
  • XOR-ring the pieces of Plain text with the Key in respective order individually.
  • Store the above XOR operated outcome in an array.
  • When the looping is done, the array contains the whole encrypted text.

Note: Sometimes you may have to pad the plain text in case it is not welly aligned with the general block size. Here is how to do it https://www.geeksforgeeks.org/retaining-the-padded-bytes-of-structural-padding-in-python/

Below is the Code to encrypt Repeated-key XOR;



Python3




def repeated_key_xor(plain_text, key):
    
    # returns plain text by repeatedly xoring it with key
    pt = plain_text
    len_key = len(key)
    encoded = []
      
    for i in range(0, len(pt)):
        encoded.append(pt[i] ^ key[i % len_key])
    return bytes(encoded)
  
# Driver Code
def main():
    plain_text = b'Burning \'em, if you ain\'t quick and nimble\nI go crazy when I hear a cymbal'
    key = b'ICE'
      
    print("Plain text: ", plain_text)
    print("Encrypted as: ", repeated_key_xor(plain_text, key).hex())
  
if __name__ == '__main__':
    main()

Output:

Plain text:  b”Burning ’em, if you ain’t quick and nimble\nI go crazy when I hear a cymbal”

Encrypted as:  0b3637272a2b2e63622c2e69692a23693a2a3c6324202d623d63343c2a26226324272765272a282b2f20430a652e2c652a3124333a653e2b2027630c692b20283165286326302e27282f

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