Validating PAdES Signatures

This guide shows how to validate a digital signature with UniDoc for PAdES compatibility.

Before you begin #

You should get your API key from your UniCloud account.

If this is your first time using UniPDF SDK, follow this guide to set up a local development environment.

Project setup #

Clone the project repository #

In your terminal, clone examples repository. It contains the Go code we will be using for this guide.

git clone https://github.com/unidoc/unipdf-examples.git

Navigate to the signatures folder in the unipdf-examples directory.

cd unipdf-examples/signatures

Configure environment variables #

Replace the UNIDOC_LICENSE_API_KEY with your API credentials from your UniCloud account.

Linux/Mac #

export UNIDOC_LICENSE_API_KEY=PUT_YOUR_API_KEY_HERE

Windows #

set UNIDOC_LICENSE_API_KEY=PUT_YOUR_API_KEY_HERE

How it works #

/*
 * This example showcases how to validate a digital signature with UniDoc for PAdES compatibility.
 *
 * $ ./pdf_sign_validate_pades_b_b <INPUT_PDF_PATH>
 */
package main
 
import (
	"fmt"
	"log"
	"os"
 
	"github.com/unidoc/unipdf/v4/common/license"
	"github.com/unidoc/unipdf/v4/model"
	"github.com/unidoc/unipdf/v4/model/sighandler"
)
 
func init() {
	// Make sure to load your metered License API key prior to using the library.
	// If you need a key, you can sign up and create a free one at https://cloud.unidoc.io
	err := license.SetMeteredKey(os.Getenv(`UNIDOC_LICENSE_API_KEY`))
	if err != nil {
		panic(err)
	}
}
 
const usagef = "Usage: %s INPUT_PDF_PATH\n"
 
func main() {
	args := os.Args
	if len(args) < 2 {
		fmt.Printf(usagef, os.Args[0])
		return
	}
	inputPath := args[1]
 
	// Create reader.
	file, err := os.Open(inputPath)
	if err != nil {
		log.Fatal("Fail: %v\n", err)
	}
	defer file.Close()
 
	reader, err := model.NewPdfReader(file)
	if err != nil {
		log.Fatal("Fail: %v\n", err)
	}
 
	// Create signature handler. It is possible to validate B-T level the same way if you create NewEtsiPAdESLevelT here
	padesHandler, err := sighandler.NewEtsiPAdESLevelB(nil, nil, nil)
	if err != nil {
		log.Fatal("Fail: %v\n", err)
	}
 
	handlers := []model.SignatureHandler{
		padesHandler,
	}
 
	// Validate signatures.
	res, err := reader.ValidateSignatures(handlers)
	if err != nil {
		log.Fatal("Fail: %v\n", err)
	}
	if len(res) == 0 {
		log.Fatal("Fail: no signature fields found")
	}
 
	if !res[0].IsSigned || !res[0].IsVerified {
		log.Fatal("Fail: validation failed")
	}
 
	for i, item := range res {
		fmt.Printf("--- Signature %d\n%s\n", i+1, item.String())
	}
}

In the example code above, the import section imports the necessary UniPDF packages and other Go libraries. The init function defined in lines 18-25, loads the API keys and sets the license using license.SetMeteredKey(os.Getenv(`UNIDOC_LICENSE_API_KEY`)).

The main function in lines 29-75, contains the code that is used to validate the signature for PAdES compatibility. In lines 30-34, the input file is parsed from the command line arguments. Then a new PdfReader is created using model.NewPdfReader(file) in line 44. In line 50-53, a new signature handler is created using:

padesHandler, err := sighandler.NewEtsiPAdESLevelB(nil, nil, nil)
if err != nil {
	log.Fatal("Fail: %v\n", err)
}

The signature handler created using NewEtsiPAdESLevelB is used to validate PAdES Baseline B Signature. To validate a PAdES Baseline T Signature, use the NewEtsiPAdESLevelT function instead.

A list of handlers is instantiated in line 55. In lines 60-74, The ValidateSignatures method of the PdfReader is used to validate the signature as follows.

res, err := reader.ValidateSignatures(handlers)
if err != nil {
	log.Fatal("Fail: %v\n", err)
}
if len(res) == 0 {
	log.Fatal("Fail: no signature fields found")
}

if !res[0].IsSigned || !res[0].IsVerified {
	log.Fatal("Fail: validation failed")
}

for i, item := range res {
	fmt.Printf("--- Signature %d\n%s\n", i+1, item.String())
}

The IsSigned and IsVerified fields are checked in line 68. The for loop in line 72-74, iterates through each result and prints the validation results.

Run the code #

To run the code use the following command.

 go run pdf_sign_validate_pades_b_b.go <INPUT_PDF_PATH>