stdin.pl

use constant SIG_BUFFER => 32768; # SIG_BUFFER > max(length($signature))
sub sig($) {
    open(RDF, '>', $tmp) or die "Failed to create a temporary file: $!\n";
    binmode(RDF, ':raw:utf8');
    ser(*RDF, $_[0], '', '  ', 1);
    close(RDF);
#- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    open(SIG, '-|', qq[openssl dgst -sha512 -sign "$pem"].
        ($pwd? qq[ -passin "$pwd"]: '').qq[ -binary "$tmp"])
        or die "Failed to run OpenSSL to generate the signature: $!\n";
    binmode(SIG);
    my $body;
    my $size = read(SIG, $body, SIG_BUFFER);
    close(SIG);
    $? == 0 or die "OpenSSL failed to generate the signature\n";
#- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    if (($size > 0) && ($size < SIG_BUFFER) && ($size == length($body))) {
        my $asn1 = Convert::ASN1->new(encoding => 'DER');
        $asn1->prepare(q<
            Algorithm ::= SEQUENCE {
                oid OBJECT IDENTIFIER,
                opt ANY OPTIONAL
            }
            Signature ::= SEQUENCE {
                alg Algorithm,
                sig BIT STRING
            }
        >);
        my $data = $asn1->encode(sig => $body,
            alg => {oid => sha512WithRSAEncryption()});
        if (defined($data)) {
            return encode_base64($data, '');
        } else {
            die "Failed to encode the generated signature: ".$asn1->error."\n";
        }
    } else {
        die "Failed to obtain the generated signature from OpenSSL\n";
    }
}