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)

The output indeed confirms that those 3 different secret keys give the same ciphertext 0x103248f0eea98e89.

Last update: 2015.

Table of Contents

 About This Book

 Blowfish Cipher Algorithm

Perl Crypt::Blowfish Module

 What Is Crypt::Blowfish?

 Installing Crypt::Blowfish 2.14 with ActivePerl

 Crypt::Blowfish Behavior Tests

Secret Keys with Repeating Pattern

 Crypt::Blowfish Verification with Test Vectors

 Blowfish Test Vectors with 16-Byte Keys

 Perl Crypt::ECB Perl Module

 Perl Crypt::CBC Module

 Perl Crypt::CFB Perl Module

 OpenSSL "enc -bf-ecb" for Blowfish/ECB Encryption

 OpenSSL "enc -bf-cbc" for Blowfish/CBC Encryption

 OpenSSL "enc -bf-cfb" for Blowfish/CFB Encryption

 OpenSSL "enc -bf-ofb" for Blowfish/OFB Encryption

 References

 PDF Printing Version