In Perl, while using RC4 I am unable to get back the encrypted text

Question

I have a file with encrypted credentials with RC4, The piece of code in charge to write such credentials to the file is the following:

sub dummyFunction() {
    # Useless stuff for the scope of the problem
    # ...
    my $dbHost = "localhost";
    my $passphrase = "123"; # For example purposes, logic is different.
    my $cipher = Crypt::RC4->new($passphrase);
    return unpack('H*',$cipher->RC4($dbHost));
}

So that piece of code would return something like: 3F9FDCE3891C6B8851 but if I try the following:

sub anotherDummyFunction() {
    my $ciphered_text = &dummyFunction();
    my $passphrase = "123";
    my $cipher = Crypt::RC4->new($passphrase);
    print $cipher->RC4(pack('H*',$ciphered_text));
}

I am expecting to see localhost but instead, I get a bunch of bytes, so how would I get back the original text?

I already checked with a RC4 decryptor online, with my passphrase and my hex encoded string and the RC4 decryptor online does return localhost so I am sure that the encrypted string is right.

Thanks!

P.S.: The above example works in an isolated environment, but when it comes to my script it doesn't. I cannot get back the original string.

Don't show us example code that works and tell us that it doesn't work in some other context. The only thing we know is that the problem *isn't* in anything you've shown us. — hobbs, May 10 '12 at 17:05
I'll use my psychic debugging skills to say that some whitespace has gotten into `$ciphered_text` in your actual script. `pack('H*',$ciphered_text)` does **not** pack just hexadecimal digits; it grabs the low 4 bits of each character in `$ciphered_text`, including whitespace characters. — cjm, May 10 '12 at 17:55
I already found the root of the problem. At first I thought it was a whitespace, but it wasn't that. I was encrypting another string right before that one, but that one was being encrypted correctly. I had an epiphany and I thought that RC4 could be caching stuff, so I undef it, and it worked. — ILikeTacos, May 10 '12 at 19:12
RC4 is a stream cipher, so of course `$cipher->RC4("encrypt-me")) x 2 ne $cipher->RC4("encrypt-me" x 2))`. If it were, a Boy Scout with a piece of paper and a pencil could break it. — cjm, May 11 '12 at 01:58

score 0 · Accepted Answer · answered Jun 06 '12 at 21:45

If anyone encounters this again:

If you have something like:

TEST.PL

 #!/usr/bin/perl
 use strict;
 use warnings;
 use Crypt::RC4;

 my $cipher = Crypt::RC4->new("passphrase123");
 print unpack('H*',$cipher->RC4("encrypt-me"))."\n";
 print unpack('H*',$cipher->RC4("encrypt-me"))."\n";

You will notice that you'll end up with two different encoded strings:

 ./test.pl
 25d2aa557cccc3951074
 1e87a5db7830a0b1cabd

In order to avoid this behavior I did:

undef $cipher

Right before trying to encrypt another string, and then instantiated the object again.

If you try this:

 my $cipher = Crypt::RC4->new("passphrase123");
 print unpack('H*',$cipher->RC4("encrypt-me"))."\n";
 undef($cipher);
 $cipher = Crypt::RC4->new("passphrase123");
 print unpack('H*',$cipher->RC4("encrypt-me"))."\n";

You'll get the same string:

./test.pl
25d2aa557cccc3951074
25d2aa557cccc3951074

this can also be wrapped in subprocedure, to avoid defining and undefining the object over and over.

sub encryptString()
{
    my ($string,$passphrase) = @_;
    my $cipher = Crypt::RC4->new($passphrase);
    return unpack('H*',$cipher->RC4($string));
}

Reading up on this stuff, came across this. A perl note: `undef($cipher);` is an unnecessary distraction. Remove it and the example runs with the same output shown (`25d2aa557cccc3951074` twice). The second `new` call is creating a "clean" `$cipher` so has the same content/output as the first. — Ashley, Sep 19 '13 at 13:20
Thanks @Ashley. I remember posting this in my second day of working with Perl. I've learnt so much since then! — ILikeTacos, Sep 19 '13 at 13:22

In Perl, while using RC4 I am unable to get back the encrypted text

1 Answers1