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    my $privatekey = pack( 'H32', $aes );       # convert from hex to binary   
    my $binary_otp = pack( 'H32', $hex );       # convert from hex to binary   

    my $cipher = Crypt::Rijndael->new( $privatekey, Crypt::Rijndael::MODE_CBC()\
 );
    my $cleartext = $cipher->decrypt( $binary_otp );

    die("Bad otp length.\n") unless ( length( $cleartext ) == 16 );

    my $ctx = Digest::CRC->new(width=>16, poly=>0x1021, init=>0xffff,
                               refin=>1, refout=>1, xorout=>0x0000, cont=>1 );

    $ctx->add($cleartext);
    my $crc2 = $ctx->hexdigest;

    die("Bad CRC.\n") unless ( $crc2 eq "f0b8" );