Add a Go port

It should be roughly at feature parity.

README.adoc

@@ -2,8 +2,8 @@ pdf-simple-sign
 ===============
 :compact-option:
 
-'pdf-simple-sign' is a simple open source PDF signer intended for documents
-generated by Cairo.  As such, it currently comes with some restrictions:
+'pdf-simple-sign' is a simple PDF signer intended for documents produced by
+the Cairo library.  As such, it currently comes with some restrictions:
 
  * the document may not have any forms or signatures already, as they will be
    overwitten
@@ -30,6 +30,10 @@ Runtime dependencies: libcrypto (OpenSSL 1.1 API)
  $ cd builddir
  $ ninja
 
+In addition to the C++ version, also included is a native Go port:
+
+ $ go get janouch.name/pdf-simple-sign/cmd/pdf-simple-sign
+
 Usage
 -----
 

cmd/pdf-simple-sign/main.go

@@ -0,0 +1,72 @@
+//
+// Copyright (c) 2018, Přemysl Janouch <p@janouch.name>
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+//
+
+// pdf-simple-sign is a simple PDF signer.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"janouch.name/pdf-simple-sign/pdf"
+	"os"
+)
+
+// #include <unistd.h>
+import "C"
+
+func isatty(fd uintptr) bool { return C.isatty(C.int(fd)) != 0 }
+
+func die(status int, format string, args ...interface{}) {
+	msg := fmt.Sprintf(format+"\n", args...)
+	if isatty(os.Stderr.Fd()) {
+		msg = "\x1b[0;31m" + msg + "\x1b[m"
+	}
+	os.Stderr.WriteString(msg)
+	os.Exit(status)
+}
+
+func usage() {
+	die(1, "Usage: %s [-h] INPUT-FILENAME OUTPUT-FILENAME "+
+		"PKCS12-PATH PKCS12-PASS", os.Args[0])
+}
+
+func main() {
+	flag.Usage = usage
+	flag.Parse()
+	if flag.NArg() != 4 {
+		usage()
+	}
+
+	inputPath, outputPath := flag.Arg(0), flag.Arg(1)
+	pdfDocument, err := ioutil.ReadFile(inputPath)
+	if err != nil {
+		die(1, "%s", err)
+	}
+	p12, err := ioutil.ReadFile(flag.Arg(2))
+	if err != nil {
+		die(2, "%s", err)
+	}
+	key, certs, err := pdf.PKCS12Parse(p12, flag.Arg(3))
+	if err != nil {
+		die(3, "%s", err)
+	}
+	if pdfDocument, err = pdf.Sign(pdfDocument, key, certs); err != nil {
+		die(2, "error: %s", err)
+	}
+	if err = ioutil.WriteFile(outputPath, pdfDocument, 0666); err != nil {
+		die(3, "%s", err)
+	}
+}