The variables are strOrig (PlainText) and strKey (Passphrase)

System.IO.MemoryStream ms = new System.IO.MemoryStream();
            System.Security.Cryptography.RijndaelManaged rj = new System.Security.Cryptography.RijndaelManaged();
            rj.Mode = System.Security.Cryptography.CipherMode.CBC;

            RandomNumberGenerator rng = RandomNumberGenerator.Create();
            byte[] salt = new byte[8];
            rng.GetBytes(salt);

            PasswordDeriveBytes pdb = new PasswordDeriveBytes(strKey, salt);
            rj.IV = pdb.GetBytes(16);
            rj.Key = pdb.GetBytes(32);

// Debug so I can see the IV, Key, and Salt Generated
            string strIV = Encoding.UTF8.GetString(rj.IV);
            string _strKey = Encoding.UTF8.GetString(rj.Key);
            string strSalt = Encoding.UTF8.GetString(salt);

            ms.Write(Encoding.UTF8.GetBytes("Salted__"), 0, Encoding.UTF8.GetByteCount("Salted__"));
            ms.Write(salt, 0, salt.Length);
            ms.Write(rj.IV, 0, rj.IV.Length);

            byte[] bOrig = Encoding.UTF8.GetBytes(strOrig);

            System.Security.Cryptography.CryptoStream cs = new System.Security.Cryptography.CryptoStream(ms, rj.CreateEncryptor(), System.Security.Cryptography.CryptoStreamMode.Write);

            cs.Write(bOrig, 0, bOrig.Length);
            cs.FlushFinalBlock();

            string bResult = Convert.ToBase64String(ms.ToArray());
            bResult = bResult.Remove(0, 10);
            ms.Flush();

            return bResult;


##</code><code>##

#!/opt/local/bin/perl -w

use IO::Socket;
use IO::Handle;
use MIME::Base64;
use Data::Dumper;
use POSIX;
use Getopt::Long;
use strict;

require Crypt::CBC;

my $msg = "Message_Generated_in_C#";
my $enc_key = "same_as_in_c#_strKey";
my $enc_alg = "Rijndael";

my $cipher = Crypt::CBC->new({
        'key'    => $enc_key,
        'cipher' => $enc_alg,
    });

my $base64_decoded_msg = decode_base64($msg);
my $decrypted_msg = $cipher->decrypt($base64_decoded_msg);

// This is for debug
        print STDERR "    Salt:\n";
        &hex_dump($cipher->salt());
        print STDERR "    Key:\n";
        &hex_dump($cipher->key());
        print STDERR "    IV:\n";
        &hex_dump($cipher->iv());
        print STDERR "    PassPhrase:\n";
        &hex_dump($cipher->passphrase());
        print STDERR "    Block Size: " . $cipher->blocksize() ."\n",
            "    Key Size:   " . $cipher->keysize(). "\n\n";

print "$decrypted_msg\n";


// Function for debug
sub hex_dump() {
    my $data = shift;

    my @chars = split //, $data;
    my $ctr = 0;
    my $ascii_str = '';
    for my $char (@chars) {
        if ($ctr % 16 == 0) {
            print STDERR " $ascii_str\n" if $ascii_str;
            printf STDERR "        0x%.4x:  ", $ctr;
            $ascii_str = '';
        }
        printf STDERR "%.2x", ord($char);

        if ((($ctr+1) % 2 == 0) and ($ctr % 16 != 0)) {
            print STDERR ' ';
        }

        if ($char =~ /[^\x20-\x7e]/) {
            $ascii_str .= '.';
        } else {
            $ascii_str .= $char;
        }
        $ctr++;
    }
    if ($ascii_str) {
        my $remainder = 1;
        if ($ctr % 16 != 0) {
            $remainder = 16 - $ctr % 16;
            if ($remainder % 2 == 0) {
                $remainder = 2*$remainder + int($remainder/2) + 1;
            } else {
                $remainder = 2*$remainder + int($remainder/2) + 2;
            }
        }
        print STDERR ' 'x$remainder, $ascii_str;
    }
    print STDERR "\n";
    return;
}