crypto-013, exam 1, question 7

Question 7

Suppose you are told that the one time pad encryption of the message "attack at dawn" is 6c73d5240a948c86981bc294814d (the plaintext letters are encoded as 8-bit ASCII and the given ciphertext is written in hex). What would be the one time pad encryption of the message "attack at dusk" under the same OTP key?

Code

pt = 'attack at dawn'
print 'plain text: '+pt
pt_hex = pt.encode('hex')
print 'plain text (hexa): '+pt_hex+', length: '+str(len(pt_hex))
pt1 = 'attack at dusk'
print 'plain text modified: '+pt1
pt1_hex = pt1.encode('hex')
print 'plain text modified (hexa): '+pt1_hex+', length: '+str(len(pt1_hex))

ct = '6c73d5240a948c86981bc294814d'
print 'ciphered text (hexa): '+ct+', length: '+str(len(ct))

pt_hex_xor_ct = hex(int(pt_hex, 16) ^ int(ct, 16))
key_hex = '0'+pt_hex_xor_ct[2:-1]
print 'key (hexa): '+key_hex+', length: '+str(len(key_hex))

ct_1 = hex(int(key_hex, 16) ^ int(pt1_hex, 16))
ct_1_fmt = ct_1[2:-1]
print 'ciphered text modified (hexa): '+ct_1_fmt+', length: '+str(len(ct_1_fmt))

References

• https://class.coursera.org/crypto-013
• https://www.google.com/