Secret Keys with Repeating Pattern

Blowfish Cipher Tutorials - Herong's Tutorial Examples

∟Secret Keys with Repeating Pattern

Tutorial examples are provided to show that secret keys with a repeating pattern will result the same ciphertext for the same given plaintext.

Another interesting feature of Blowfish algorithm is that the secret key will be used repeatedly to build array of 576 bits (72 bytes) long. So the following statement should be always true:

Encryption outputs will be the same for the same plaintext with different secret keys, if those secret keys are repeated strings of the same pattern.

For example, if you use the following secret keys to encrypt a plaintext, you will get the same ciphertext.

1122334455667788
11223344556677881122334455667788
112233445566778811223344556677881122334455667788
...

Let's use our test script, Crypt-Blowfish-Test.pl, to verify the above statement:

C:\herong>perl Crypt-Blowfish-Test.pl \
   1122334455667788 \
   0123456789abcdef

Crypt::Blowfish Test - output in Hex:
   Key in Hex            (1122334455667788)
   Plaintext in Hex      (0123456789abcdef)
   keysize()             0 bytes
   blocksize()           8 bytes
   Ciphertext in Hex     (103248f0eea98e89)
   Decrypted text in Hex (0123456789abcdef)


C:\herong>perl Crypt-Blowfish-Test.pl \
   11223344556677881122334455667788 \
   0123456789abcdef
   
Crypt::Blowfish Test - output in Hex:
   Key in Hex            (11223344556677881122334455667788)
   Plaintext in Hex      (0123456789abcdef)
   keysize()             0 bytes
   blocksize()           8 bytes
   Ciphertext in Hex     (103248f0eea98e89)
   Decrypted text in Hex (0123456789abcdef)

   
C:\herong>perl Crypt-Blowfish-Test.pl \
   112233445566778811223344556677881122334455667788 \
   0123456789abcdef

Crypt::Blowfish Test - output in Hex:
   Key in Hex            (112233445566778811223344556677881122334455667788)
   Plaintext in Hex      (0123456789abcdef)
   keysize()             0 bytes
   blocksize()           8 bytes
   Ciphertext in Hex     (103248f0eea98e89)
   Decrypted text in Hex (0123456789abcdef)