Secret Key Padding and Truncation

Blowfish Cipher Tutorials - Herong's Tutorial Examples

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

∟Secret Key Padding and Truncation

A tutorial example is provided to show you how OpenSSL 'enc' apply padding or truncation to get a 16-byte secret key from the value specified in the '-K' option.

In the previous tutorial, we learned how to use "bf-ecb" cipher in the "Literal Key" way to control the secret key and the IV. In this tutorial, we will do some tests on the secret key is specified using the -K option as described below:

"-K value" - Specifies the secret key in 32 hex digits (16 bytes, or 128 bits). If the specified value is not enough, it will be padded with 0x00. If the specified value is too long, it will be truncated.

One nice feature of the OpenSSL "enc" command is that has the "-p" option to display what are the values of secret key, IV and salt actually used in during the encryption process. So we are going to use in our tests from now on.

Test 1. "-K" value with 16 bytes. No padding and truncation should happen.

C:\herong>perl -e "binmode(STDOUT); print pack('H*', '0000000000000000')" \
   > 0000000000000000.txt

C:\herong>C:\local\gnuwin32\bin\openssl enc -bf-ecb -e \
   -K 0123456789ABCDEF0123456789ABCDEF -iv 0000000000000000 \
   -in 0000000000000000.txt -out cipher.txt -nopad -p
   
salt=0200000040265701
key=0123456789ABCDEF0123456789ABCDEF
iv =0000000000000000

C:\herong>perl -e "while (read(STDIN,$_,1)){print unpack('H*',$_);}" \
   < cipher.txt
   
245946885754369a

The ciphertext, 0x4ef997456198dd78, is correct, because it matches the test vector published at https://www.schneier.com/code/vectors.txt. Note that secret key 0x0123456789ABCDEF0123456789ABCDEF is the same as 0x0123456789ABCDEF for Blowfish algorithm, because the secret key is repeatedly concatenated to reach the size of 72 bytes.

key bytes               clear bytes             cipher bytes
0123456789ABCDEF        0000000000000000        245946885754369A

OpenSSL generated a salt value of 0x0200000040265701. But it was not used.

Test 2. "-K" value with 8 bytes. It will be padded with 0x00.

C:\herong>C:\local\gnuwin32\bin\openssl enc -bf-ecb -e \
   -K 0123456789ABCDEF -iv 0000000000000000 \
   -in 0000000000000000.txt -out cipher.txt -nopad -p
   
salt=0200000000267E00
key=0123456789ABCDEF0000000000000000
iv =0000000000000000

C:\herong>perl -e "while (read(STDIN,$_,1)){print unpack('H*',$_);}" \
   < cipher.txt
   
9713e3a4c20e0746

The output confirms that the actual secret will have 8 bytes of 0x00 padded, if only 8 bytes of value specified in the "-K" option. So don't "-K" option with less than 16 bytes.

Test 3. "-K" value with 24 bytes. It will be truncated to 16 bytes.

C:\herong>C:\local\gnuwin32\bin\openssl enc -bf-ecb -e \
   -K 0123456789ABCDEF0123456789ABCDEF1122334455667788 \
   -iv 0000000000000000 -in 0000000000000000.txt -out cipher.txt \
   -nopad -p
   
salt=0200000050264401
key=0123456789ABCDEF0123456789ABCDEF
iv =0000000000000000

C:\herong>perl -e "while (read(STDIN,$_,1)){print unpack('H*',$_);}" \
   < cipher.txt
   
245946885754369a