Blowfish Cipher Tutorials - Herong's Tutorial Examples - v2.03, by Herong Yang
Crypt::CBC Prepending IV to Ciphertext
A tutorial Perl example is provided to show how to use Crypt::CBC to prepend the IV value to the ciphertext, if IV is specified by the user. This saves the effort to pass the IV to receiver separately from the ciphertext.
When using Crypt::CBC with the "Literal Key" or "Crypt::Blowfish Object" option, we need to pass the IV value together with the ciphertext to the receiver, so he/she can decrypt the ciphertext with the secret key.
Passing the IV value each time we send a ciphertext in not very convenient, so Crypt::CBC offers the feature to prepend the IV to the ciphertext string. Here is how it can be done, using the "Crypt::Blowfish Object" option as an example:
To help us understanding the -header => 'randomiv' parameter, I wrote the following example Perl script, Crypt-CBC-Blowfish-Prepend-IV.pl:
#- Crypt-CBC-Blowfish-Prepend-IV.pl #- Copyright (c) 2015 HerongYang.com, All Rights Reserved. #- use Crypt::CBC; use Crypt::Blowfish; my ($keyHex, $ivHex, $plainHex) = @ARGV; print("Crypt::CBC Prepend IV Test - 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::CBC with Blowfish Object ...\n"); $blowfish = new Crypt::Blowfish($keyStr); $cipher = Crypt::CBC->new( -cipher => $blowfish, -iv => $ivStr, -header => 'randomiv', -padding => 'none' ); print("Encrypting plaintext...\n"); $cipherStr = $cipher->encrypt($plainStr); $cipherHex = unpack("H*", $cipherStr); print(" Ciphertext ($cipherHex)\n"); print("Decrypting ciphertext...\n"); $decryptStr = $cipher->decrypt($cipherStr); $decryptHex = unpack("H*", $decryptStr); print(" Decrypted text ($decryptHex)\n"); $passStr = $cipher->passphrase(); $passHex = unpack("H*", $passStr); print(" Pass Phrase ($passHex)\n"); $ivStr = $cipher->iv(); $ivHex = unpack("H*", $ivStr); print(" IV ($ivHex)\n");
If you run this example script with different IV values and see what blocks are prepended to the ciphertext:
C:\herong>perl Crypt-CBC-Blowfish-Prepend-IV.pl \ 1122334455667788 0000000000000000 0123456789abcdef Crypt::CBC Prepend IV Test - output in Hex: Secret Key (1122334455667788) IV (0000000000000000) Plaintext (0123456789abcdef) Creating Crypt::CBC with Blowfish Object ... Encrypting plaintext... Ciphertext (52616e646f6d49560000000000000000103248f0eea98e89) Decrypting ciphertext... Decrypted text (0123456789abcdef) Pass Phrase () IV (0000000000000000) C:\herong>perl Crypt-CBC-Blowfish-Prepend-IV.pl \ 1122334455667788 1111111111111111 0123456789abcdef Crypt::CBC Prepend IV Test - output in Hex: Secret Key (1122334455667788) IV (1111111111111111) Plaintext (0123456789abcdef) Creating Crypt::CBC with Blowfish Object ... Encrypting plaintext... Ciphertext (52616e646f6d49561111111111111111c3a27e1f67d146f0) Decrypting ciphertext... Decrypted text (0123456789abcdef) Pass Phrase () IV (1111111111111111)
The output confirms that our understanding is correct:
Exercise: Write a Perl script to use Crypt::CBC Literal Key option with -header => 'randomiv'.
Table of Contents
Installing Crypt::CBC 2.33 with ActivePerl
Crypt::CBC Encryption with Literal Keys
Crypt::CBC Literal Key Error Cases
Crypt::CBC Encryption with Crypt::Blowfish Objects
Crypt::CBC Operation Simulation
Crypt::CBC Encryption Verification
Blowfish CBC 2-Block Test Vectors
►Crypt::CBC Prepending IV to Ciphertext
Crypt::CBC Encryption with Salted Keys
Crypt::CBC Salted Key Test Cases
Crypt::CBC Secret Key and IV Algorithm
Crypt::CBC Encryption with Random Salt
Crypt::CBC Padding Option Tests
Crypt::CBC Blowfish Encryption Summary
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