Why can I encrypt data with one DES key and successfully decrypt with another?

Question

I tried to implement the DES algorithm using the pyDes and Crypto.Cipher.DES modules. I found that when I encrypt the key 82514145 and then decrypt the cipher with 93505044 , I can get the decrypted text. I found 256 keys that behave like this. This is a violation of cryptography. My code is as follows:

  from Crypto.Cipher import DES plain_text = 'asdfghij' print 'plain Text: ', plain_text des = DES.new('82514145', DES.MODE_ECB) cipher_text = des.encrypt(plain_text) print 'the cipher text is ', cipher_text des = DES.new('93505044', DES.MODE_ECB) print 'the decrypted text is: ', des.decrypt(cipher_text) 

Exit:

 plain Text: asdfghij the cipher text is @ Z     the decrypted text is: asdfghij 

Is there any mistake in my work? I also got results with pyDes.

Answer 1

DES keys are 56 bits long, but they expand to 64 bits thanks to the parity bit. The eighth bit of each byte must be set to ensure odd parity .

Many crypto libraries ignore parity bits, which means that there are many ways to represent the same 56-bit key in a 64-bit key string. Actually there are 2 ⁸ different ways, which explains why you found 256 matching keys.

Your example includes two key values ​​that differ only in parity bits. See below: the parity bit is in [] :

 82514145 = 0x3832353134313435 = 0011100[0] 0011001[0] 0011010[1] 0011000[1] 0011010[0] 0011000[1] 0011010[0] 0000000[0] 93505044 = 0x3933353035303434 = 0011100[1] 0011001[1] 0011010[1] 0011000[0] 0011010[1] 0011000[0] 0011010[0] 0000000[0] 

None of the keys are truly valid. The correct representation of this key is: 0x3832343134313401 .

Answer 2

This is a great example of why you should never use a password provided by a user as a key in a key in a cipher . Instead, use the key derivation function .

In addition, you should not use DES for purposes other than education, as this is usually considered unsafe. The key is currently considered too short, and some known attacks reduce its complexity.