Crypt::CBC Operation Simulation

Blowfish Cipher Tutorials - Herong's Tutorial Examples

∟Crypt::CBC Operation Simulation

A tutorial Perl example is provided to show how to use Crypt::Blowfish module to simulate the CBC operation mode. The simulation result matches the result from the Crypt::CBC module.

So far, we have learned how to use Crypt::Blowfish to encrypt a single block of plaintext. We have also learned how to use Crypt::CBC to encrypt blocks of plaintext with direct control of secret key and IV. We should be able to verify Crypt::CBC encryption results by simulating the CBC algorithm with Crypt::CBC module.

Here is my example Perl script, Crypt-CBC-Blowfish-Simulation.pl, that simulates the CBC operation by calling Crypt::Blowfish one block at time. It also verifies the ciphertext by calling the Crypt::CBC module directly.

#- Crypt-CBC-Blowfish-Simulation.pl
#- Copyright (c) 2015 HerongYang.com, All Rights Reserved.
#-
   use Crypt::CBC;
   use Crypt::Blowfish;
   my ($keyHex, $ivHex, $plainHex) = @ARGV;

   print("Crypt::CBC Blowfish Simulation - output in Hex:\n");
   $keyHex = "1122334455667788" unless $keyHex;
   $ivHex = "0000000000000000" unless $ivHex;
   $plainHex = "0123456789abcdef" unless $plainHex;
   $keyStr = pack("H*", $keyHex);
   $ivStr = pack("H*", $ivHex);
   $plainStr = pack("H*", $plainHex);
   print("   Secret Key      ($keyHex)\n");
   print("   IV              ($ivHex)\n");
   print("   Plaintext       ($plainHex)\n");

   print("Creating Crypt::Blowfish Object ...\n");
   $blowfish = new Crypt::Blowfish($keyStr);

   $count = 0;
   $cipherHexPrev = $ivHex;
   while ($plainHex) {
      $count++;
      $plainHexBlock = substr($plainHex,0,16);
      $plainHex = substr($plainHex,16);
      print("Simulating CBC on block $count...\n");
      print("   Prev ciphertext ($cipherHexPrev)\n");
      print("   Plaintext       ($plainHexBlock)\n");
      
      $cipherStrPrev = pack("H*", $cipherHexPrev);
      $plainStrBlock = pack("H*", $plainHexBlock);
      $xorStrBlock = $cipherStrPrev ^ $plainStrBlock;
      $xorHexBlock = unpack("H*", $xorStrBlock);
      print("   Plaintext XORed ($xorHexBlock)\n");
      
      $cipherStrBlock = $blowfish->encrypt($xorStrBlock);
      $cipherHexBlock = unpack("H*", $cipherStrBlock);
      print("   Ciphertext      ($cipherHexBlock)\n");

      $cipherHexPrev = $cipherHexBlock;
   }

   print("Creating Crypt::CBC Object ...\n");
   $blowfish = new Crypt::Blowfish($keyStr);
   $cipher = Crypt::CBC->new( 
      -cipher => $blowfish,
      -iv => $ivStr,
      -header => 'none',
      -padding => 'none'
      );

   print("Encrypting entire plaintext with Crypt::CBC...\n");
   $cipherStr = $cipher->encrypt($plainStr);
   $cipherHex = unpack("H*", $cipherStr);
   print("   Ciphertext      ($cipherHex)\n");

The key part of the CBC mode simulation is the following logics used in CBC mode:

Before encrypt the current block of plaintext, apply XOR operation on the plaintext with the previous block of ciphertext.
For the first block of plaintext, use IV as the previous block of ciphertext.

Now try this example script with different size plaintext:

C:\Herong>perl Crypt-CBC-Blowfish-Simulation.pl \
   1122334455667788 0000000000000000 0123456789abcdef
   
Crypt::CBC Blowfish Simulation - output in Hex:
   Secret Key      (1122334455667788)
   IV              (0000000000000000)
   Plaintext       (0123456789abcdef)
Creating Crypt::Blowfish Object ...
Simulating CBC on block 1...
   Prev ciphertext (0000000000000000)
   Plaintext       (0123456789abcdef)
   Plaintext XORed (0123456789abcdef)
   Ciphertext      (103248f0eea98e89)
Creating Crypt::CBC Object ...
Encrypting entire plaintext with Crypt::CBC...
   Ciphertext      (103248f0eea98e89)
   
C:\Herong>perl Crypt-CBC-Blowfish-Simulation.pl 1122334455667788 \
 0000000000000000 0123456789abcdef0123456789abcdef
 
Crypt::CBC Blowfish Simulation - output in Hex:
   Secret Key      (1122334455667788)
   IV              (0000000000000000)
   Plaintext       (0123456789abcdef0123456789abcdef)
Creating Crypt::Blowfish Object ...
Simulating CBC on block 1...
   Prev ciphertext (0000000000000000)
   Plaintext       (0123456789abcdef)
   Plaintext XORed (0123456789abcdef)
   Ciphertext      (103248f0eea98e89)
Simulating CBC on block 2...
   Prev ciphertext (103248f0eea98e89)
   Plaintext       (0123456789abcdef)
   Plaintext XORed (11110d9767024366)
   Ciphertext      (8f5e452a60506163)
Creating Crypt::CBC Object ...
Encrypting entire plaintext with Crypt::CBC...
   Ciphertext      (103248f0eea98e898f5e452a60506163)

C:\Herong>perl Crypt-CBC-Blowfish-Simulation.pl \
   1122334455667788 1111111111111111 
   
   0123456789abcdef0123456789abcdef0123456789abcdef
Crypt::CBC Blowfish Simulation - output in Hex:
   Secret Key      (1122334455667788)
   IV              (1111111111111111)
   Plaintext       (0123456789abcdef0123456789abcdef0123456789abcdef)
Creating Crypt::Blowfish Object ...
Simulating CBC on block 1...
   Prev ciphertext (1111111111111111)
   Plaintext       (0123456789abcdef)
   Plaintext XORed (1032547698badcfe)
   Ciphertext      (c3a27e1f67d146f0)
Simulating CBC on block 2...
   Prev ciphertext (c3a27e1f67d146f0)
   Plaintext       (0123456789abcdef)
   Plaintext XORed (c2813b78ee7a8b1f)
   Ciphertext      (dd89a07b06cad2aa)
Simulating CBC on block 3...
   Prev ciphertext (dd89a07b06cad2aa)
   Plaintext       (0123456789abcdef)
   Plaintext XORed (dcaae51c8f611f45)
   Ciphertext      (2cd3a609b55defca)
Creating Crypt::CBC Object ...
Encrypting entire plaintext with Crypt::CBC...
   Ciphertext      (c3a27e1f67d146f0dd89a07b06cad2aa2cd3a609b55defca)

C:\Herong>perl Crypt-CBC-Blowfish-Simulation.pl \
   1122334455667788 ffffffffffffffff \
   0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef
   
Crypt::CBC Blowfish Simulation - output in Hex:
   Secret Key      (1122334455667788)
   IV              (ffffffffffffffff)
   Plaintext       (0123456789abcdef0123456789abcdef
                    0123456789abcdef0123456789abcdef)
Creating Crypt::Blowfish Object ...
Simulating CBC on block 1...
   Prev ciphertext (ffffffffffffffff)
   Plaintext       (0123456789abcdef)
   Plaintext XORed (fedcba9876543210)
   Ciphertext      (2b41e1b29adaa3ab)
Simulating CBC on block 2...
   Prev ciphertext (2b41e1b29adaa3ab)
   Plaintext       (0123456789abcdef)
   Plaintext XORed (2a62a4d513716e44)
   Ciphertext      (2149531ce57439d0)
Simulating CBC on block 3...
   Prev ciphertext (2149531ce57439d0)
   Plaintext       (0123456789abcdef)
   Plaintext XORed (206a167b6cdff43f)
   Ciphertext      (54061f6cddc380e2)
Simulating CBC on block 4...
   Prev ciphertext (54061f6cddc380e2)
   Plaintext       (0123456789abcdef)
   Plaintext XORed (55255a0b54684d0d)
   Ciphertext      (713fa4974566fdfd)
Creating Crypt::CBC Object ...
Encrypting entire plaintext with Crypt::CBC...
   Ciphertext      (2b41e1b29adaa3ab2149531ce57439d0
                    54061f6cddc380e2713fa4974566fdfd)