Crypt::CBC how does the header/salt work?

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

I've noticed that when I encrypt something using Crypt::CBC in the resulting string are always the same over multiple executions (using the same passphrase). The remainder of the string is always random. What is going on here? :)

#!/usr/bin/perl

use Crypt::CBC;

my $cipher = Crypt::CBC->new(
    -key => 'my secret key blah blah',
    -cipher => 'Blowfish',
);

my $ciphertext = $cipher->encrypt_hex("This data is hush hush blah bla
+h");
my $plaintext  = $cipher->decrypt_hex($ciphertext);

print "$ciphertext\n$plaintext\n";
[download]

$ perl t.pl
53616c7465645f5f27aad01783d47f045a963683bb06e3aa959fc83d7c94e1f5aad894
+d37c8df0d93abaaa80fb9e073b9555761f85204457
This data is hush hush blah blah
$ perl t.pl
53616c7465645f5fe7e7d95a83e54fb5f8bfc966d07bff9c3db436a17693bad1c31a85
+766026d5f107d5c17adb017b0eeb46afa6f2d6e462
This data is hush hush blah blah
$ perl t.pl
53616c7465645f5fec4cc8f6fc8ecbd4ebdeb938b7d4fe0cf028d3eed7a4dce694e001
+0e8cd99a94e799e3241a41d466a6755999bdb58e5f
This data is hush hush blah blah
$ perl t.pl
53616c7465645f5f639d36c6f916256557b485abb9c26837b235764faa361d2df38484
+42e58ea52bbbff329a3deb6d182d2b4cf04573e016
This data is hush hush blah blah
$ perl t.pl
53616c7465645f5fda138a9a74e5c8c3081430fd3b895de307c1c303fbf48afa2d12bd
+8c7097d2a3d600819f4c46fb16702536e97d27211c
This data is hush hush blah blah
[download]

Comment on Crypt::CBC how does the header/salt work? Select or Download Code

Replies are listed 'Best First'.
Re: Crypt::CBC how does the header/salt work? by jethro (Monsignor) on Nov 19, 2012 at 15:47 UTC
If I set the salt to something constant, I get something constant out. If I then change the plaintext, everything after the first 16 bytes changes. This seems to indicate that salt=>'1' is really the default and that the first 8 bytes are a constant prefix that probably defines the crypto algorithm. The next 8 bytes after the prefix are the salt.	[reply]
Re: Crypt::CBC how does the header/salt work? by zentara (Archbishop) on Nov 19, 2012 at 16:18 UTC
See ikegami's responses in Customer data encryption I'm not really a human, but I play one on earth. Old Perl Programmer Haiku ................... flash japh	[reply]
Re: Crypt::CBC how does the header/salt work? by Anonymous Monk on Nov 19, 2012 at 15:00 UTC
It works like that , see also http://search.cpan.org/perldoc/Crypt::CBC#new()	[reply]
Re: Crypt::CBC how does the header/salt work? by Anonymous Monk on Nov 19, 2012 at 14:59 UTC
Another 2 questions. If I always want the resulting encrypted string to be unique/random what must I "keep" in addition to the passphrase so that I can decrypt the completely random bytes? And how must I call the Crypt::CBC->new() method to support that encryption and decryption?	[reply]
Re^2: Crypt::CBC how does the header/salt work? by Anonymous Monk on Nov 19, 2012 at 15:13 UTC
Take a look at how its actually used http://deps.cpantesters.org/depended-on-by.pl?dist=Crypt-CBC-2.31 looks to me you only need passphrase/cipher ... see Crypt::CBCeasy	[reply]
Re: Crypt::CBC how does the header/salt work? by Anonymous Monk on Nov 19, 2012 at 16:16 UTC
Think I've got it figured out: `my $cipher = Crypt::CBC->new( -cipher => 'Blowfish', -key => "blahblahblah", -iv => "01234567", -header => 'none' );` [download] this gives you a random encrypt_hex result with every run because the header isn't included.	[reply] [d/l]


Just another Perl shrine
	PerlMonks