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Digitally sign PDF files in Go

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Sample Go code to use PDFTron SDK's high-level digital signature API for digitally signing and/or certifying PDF files. Learn more about our Go PDF Library and PDF Digital Signature Library.

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// Copyright (c) 2001-2021 by PDFTron Systems Inc. All Rights Reserved.
// Consult LICENSE.txt regarding license information.

//// This sample demonstrates the basic usage of the high-level digital signatures API in PDFNet.
//// The following steps reflect typical intended usage of the digital signatures API:
////  0.  Start with a PDF with or without form fields in it that one would like to lock (or, one can add a field, see (1)).
////  1.  EITHER: 
////      (a) Call doc.CreateDigitalSignatureField, optionally providing a name. You receive a DigitalSignatureField.
////      -OR-
////      (b) If you didn't just create the digital signature field that you want to sign/certify, find the existing one within the 
////      document by using PDFDoc.DigitalSignatureFieldIterator or by using PDFDoc.GetField to get it by its fully qualified name.
////  2.  Create a signature widget annotation, and pass the DigitalSignatureField that you just created or found. 
////      If you want it to be visible, provide a Rect argument with a non-zero width or height, and don't set the
////      NoView and Hidden flags. [Optionally, add an appearance to the annotation when you wish to sign/certify.]
////  [3. (OPTIONAL) Add digital signature restrictions to the document using the field modification permissions (SetFieldPermissions) 
////      or document modification permissions functions (SetDocumentPermissions) of DigitalSignatureField. These features disallow 
////      certain types of changes to be made to the document without invalidating the cryptographic digital signature once it
////      is signed.]
////  4.  Call either CertifyOnNextSave or SignOnNextSave. There are three overloads for each one (six total):
////      a.  Taking a PKCS //12 keyfile path and its password
////      b.  Taking a buffer containing a PKCS //12 private keyfile and its password
////      c.  Taking a unique identifier of a signature handler registered with the PDFDoc. This overload is to be used
////          in the following fashion: 
////          i)      Extend and implement a new SignatureHandler. The SignatureHandler will be used to add or 
////                  validate/check a digital signature.
////          ii)     Create an instance of the implemented SignatureHandler and register it with PDFDoc with 
////                  pdfdoc.AddSignatureHandler(). The method returns a SignatureHandlerId.
////          iii)    Call SignOnNextSaveWithCustomHandler/CertifyOnNextSaveWithCustomHandler with the SignatureHandlerId.
////      NOTE: It is only possible to sign/certify one signature per call to the Save function.
////  5.  Call pdfdoc.Save(). This will also create the digital signature dictionary and write a cryptographic signature to it.
////      IMPORTANT: If there are already signed/certified digital signature(s) in the document, you must save incrementally
////      so as to not invalidate the other signature(s). 
//// Additional processing can be done before document is signed. For example, UseSignatureHandler() returns an instance
//// of SDF dictionary which represents the signature dictionary (or the /V entry of the form field). This can be used to
//// add additional information to the signature dictionary (e.g. Name, Reason, Location, etc.).
//// Although the steps above describes extending the SignatureHandler class, this sample demonstrates the use of
//// StdSignatureHandler (a built-in SignatureHandler in PDFNet) to sign a PDF file.

package main
import (
	. "pdftron"

func VerifySimple(inDocpath string, inPublicKeyFilePath string) bool{
    doc := NewPDFDoc(inDocpath)
    opts := NewVerificationOptions(VerificationOptionsE_compatibility_and_archiving)

    // Add trust root to store of trusted certificates contained in VerificationOptions.

    result := doc.VerifySignedDigitalSignatures(opts)
    if result == PDFDocE_unsigned{
        fmt.Println("Document has no signed signature fields.")
        return false
        // e_failure == bad doc status, digest status, or permissions status
        // (i.e. does not include trust issues, because those are flaky due to being network/config-related)
    }else if result == PDFDocE_failure{
        fmt.Println("Hard failure in verification on at least one signature.")
        return false
    }else if result == PDFDocE_untrusted{
        fmt.Println("Could not verify trust for at least one signature.")
        return false
    }else if result == PDFDocE_unsupported{
        // If necessary, call GetUnsupportedFeatures on VerificationResult to check which
        // unsupported features were encountered (requires verification using 'detailed' APIs)
        fmt.Println("At least one signature contains unsupported features.")
        return false
        // unsigned sigs skipped; parts of document may be unsigned (check GetByteRanges on signed sigs to find out)
    }else if result == PDFDocE_verified{
        fmt.Println("All signed signatures in document verified.")
        return true
        fmt.Println("unrecognized document verification status")
        return false

func VerifyAllAndPrint(inDocpath string, inPublicKeyFilePath string) bool{
    doc := NewPDFDoc(inDocpath)
    opts := NewVerificationOptions(VerificationOptionsE_compatibility_and_archiving)
    // Trust the public certificate we use for signing.
    trustedCertFile := NewMappedFile(inPublicKeyFilePath)
    fileSz := trustedCertFile.FileSize()
    fileReader := NewFilterReader(trustedCertFile)
    trustedCertBuf := fileReader.Read(fileSz)
    trustedCertBytes := make([]byte, int(trustedCertBuf.Size()))
    for i := 0; i < int(trustedCertBuf.Size()); i ++ {
        trustedCertBytes[i] = trustedCertBuf.Get(i)
    opts.AddTrustedCertificate(&trustedCertBytes[0], int64(len(trustedCertBytes)))

    // Iterate over the signatures and verify all of them.
    digsigFitr := doc.GetDigitalSignatureFieldIterator()
    verificationStatus := true
    for (digsigFitr.HasNext()){
        curr := digsigFitr.Current()
        result := curr.Verify(opts)
        if result.GetVerificationStatus(){
            fmt.Printf("Signature verified, objnum: %d", curr.GetSDFObj().GetObjNum())
            fmt.Printf("Signature verification failed, objnum: %d", curr.GetSDFObj().GetObjNum())
            verificationStatus = false
        digest_algorithm := result.GetDigestAlgorithm()
        if digest_algorithm == DigestAlgorithmE_SHA1{
            fmt.Println("Digest algorithm: SHA-1")
        }else if digest_algorithm == DigestAlgorithmE_SHA256{
            fmt.Println("Digest algorithm: SHA-256")
        }else if digest_algorithm == DigestAlgorithmE_SHA384{
            fmt.Println("Digest algorithm: SHA-384")
        }else if digest_algorithm == DigestAlgorithmE_SHA512{
            fmt.Println("Digest algorithm: SHA-512")
        }else if digest_algorithm == DigestAlgorithmE_RIPEMD160{
            fmt.Println("Digest algorithm: RIPEMD-160")
        }else if digest_algorithm == DigestAlgorithmE_unknown_digest_algorithm{
            fmt.Println("Digest algorithm: unknown")
            //unrecognized document status
        fmt.Printf("Detailed verification result: \n\t%s\n\t%s\n\t%s\n\t%s", 
        changes := result.GetDisallowedChanges()
        for i := 0; i < int(changes.Size()); i++ {
            fmt.Printf("\tDisallowed change: %s, objnum: %d", changes.Get(i).GetTypeAsString(), changes.Get(i).GetObjNum())

        // Get and print all the detailed trust-related results, if they are available.
        if result.HasTrustVerificationResult(){
            trustVerificationResult := result.GetTrustVerificationResult()
            var msg string
            if trustVerificationResult.WasSuccessful(){
                msg = "Trust verified."
            } else {
                msg = "Trust not verifiable."
            tmpTimeTv := trustVerificationResult.GetTimeOfTrustVerification()
            trustVerificationTimeEnum := trustVerificationResult.GetTimeOfTrustVerificationEnum()
            if trustVerificationTimeEnum == VerificationOptionsE_current{
                fmt.Println("Trust verification attempted with respect to current time (as epoch time): " + strconv.Itoa(int(tmpTimeTv)))
            }else if trustVerificationTimeEnum == VerificationOptionsE_signing{
                fmt.Println("Trust verification attempted with respect to signing time (as epoch time): " + strconv.Itoa(int(tmpTimeTv)))
            }else if trustVerificationTimeEnum == VerificationOptionsE_timestamp{
                fmt.Println("Trust verification attempted with respect to secure embedded timestamp (as epoch time): " + strconv.Itoa(int(tmpTimeTv)))
                //unrecognized time enum value

            if trustVerificationResult.GetCertPath().Size() == 0{
                fmt.Println("Could not print certificate path.")
                fmt.Println("Certificate path:")
                certPath := trustVerificationResult.GetCertPath()
                for i := 0; i < int(certPath.Size()); i ++{
                    fmt.Println("\t\tIssuer names:")
                    issuerDn := certPath.Get(i).GetIssuerField().GetAllAttributesAndValues()
                    for j := 0; j < int(issuerDn.Size()); j ++{ 
                        fmt.Println("\t\t\t" + issuerDn.Get(j).GetStringValue())
                    fmt.Println("\t\tSubject names:")
                    subjectDn := certPath.Get(i).GetSubjectField().GetAllAttributesAndValues()
                    for k := 0; k < int(subjectDn.Size()); k ++{
                        fmt.Println("\t\t\t" + subjectDn.Get(k).GetStringValue())
                    for m := 0; m < int(certPath.Get(i).GetExtensions().Size()); m ++{
                        fmt.Println("\t\t\t" + certPath.Get(i).GetExtensions().Get(m).ToString())
            fmt.Println("No detailed trust verification result available.")
            unsupportedFeatures := result.GetUnsupportedFeatures()
            if unsupportedFeatures.Size() > 0 {
                fmt.Println("Unsupported features:")
                for i := 0; i < int(unsupportedFeatures.Size()); i ++{
                    fmt.Println("\t" + unsupportedFeatures.Get(i))
    return verificationStatus

func CertifyPDF(inDocpath string,
    inCertFieldName string ,
    inPrivateKeyFilePath string,
    inKeyfilePassword string,
    inAppearanceImagePath string,
    inOutpath string){
    fmt.Println("Certifying PDF document")

    // Open an existing PDF
    doc := NewPDFDoc(inDocpath)

    if doc.HasSignatures(){
        fmt.Println("PDFDoc has signatures")
        fmt.Println("PDFDoc has no signatures")
    page1 := doc.GetPage(1)

    // Create a text field that we can lock using the field permissions feature.
    annot1 := TextWidgetCreate(doc, NewRect(50.0, 550.0, 350.0, 600.0), "asdf_test_field")

    // Create a new signature form field in the PDFDoc. The name argument is optional;
    // leaving it empty causes it to be auto-generated. However, you may need the name for later.
    // Acrobat doesn"t show digsigfield in side panel if it's without a widget. Using a
    // Rect with 0 width and 0 height, or setting the NoPrint/Invisible flags makes it invisible. 
    certificationSigField := doc.CreateDigitalSignatureField(inCertFieldName)
    widgetAnnot := SignatureWidgetCreate(doc, NewRect(0.0, 100.0, 200.0, 150.0), certificationSigField)

    // (OPTIONAL) Add an appearance to the signature field.
    img := ImageCreate(doc.GetSDFDoc(), inAppearanceImagePath)

    // Add permissions. Lock the random text field.
    fmt.Println("Adding document permissions.")
    // Prepare to lock the text field that we created earlier.
    fmt.Println("Adding field permissions.")
    testField := NewVectorString()
    certificationSigField.SetFieldPermissions(DigitalSignatureFieldE_include, testField)

    certificationSigField.CertifyOnNextSave(inPrivateKeyFilePath, inKeyfilePassword)

    // (OPTIONAL) Add more information to the signature dictionary.
    certificationSigField.SetLocation("Vancouver, BC")
    certificationSigField.SetReason("Document certification.")

    // Save the PDFDoc. Once the method below is called, PDFNet will also sign the document using the information provided.
    doc.Save(inOutpath, uint(0))


func SignPDF(inDocpath string, 
    inApprovalFieldName string, 
    inPrivateKeyFilePath string, 
    inKeyfilePassword string, 
    inAppearanceImgPath string, 
    inOutpath string){
    fmt.Println("Signing PDF document")

    // Open an existing PDF
    doc := NewPDFDoc(inDocpath)

    // Retrieve the unsigned approval signature field.
    foundApprovalField := doc.GetField(inApprovalFieldName)
    foundApprovalSignatureDigsigField := NewDigitalSignatureField(foundApprovalField)
    // (OPTIONAL) Add an appearance to the signature field.
    img := ImageCreate(doc.GetSDFDoc(), inAppearanceImgPath)
    foundApprovalSignatureWidget := NewSignatureWidget(foundApprovalField.GetSDFObj())

    // Prepare the signature and signature handler for signing.
    foundApprovalSignatureDigsigField.SignOnNextSave(inPrivateKeyFilePath, inKeyfilePassword)

    // The actual approval signing will be done during the following incremental save operation.
    doc.Save(inOutpath, uint(SDFDocE_incremental))


func ClearSignature(inDocpath string,
    inDigsigFieldName string,
    inOutpath string){

    fmt.Println("Clearing certification signature")

    doc := NewPDFDoc(inDocpath)

    digsig := NewDigitalSignatureField(doc.GetField(inDigsigFieldName))
    fmt.Println("Clearing signature: " + inDigsigFieldName)

    if !digsig.HasCryptographicSignature(){
        fmt.Println("Cryptographic signature cleared properly.")
    // Save incrementally so as to not invalidate other signatures from previous saves.
    doc.Save(inOutpath, uint(SDFDocE_incremental))


func PrintSignaturesInfo(inDocpath string){
    fmt.Println("Reading and printing digital signature information")

    doc := NewPDFDoc(inDocpath)
    if !doc.HasSignatures(){
        fmt.Println("Doc has no signatures.")
        fmt.Println("Doc has signatures.")

    fitr := doc.GetFieldIterator()
    for fitr.HasNext(){
        current := fitr.Current()
        if (current.IsLockedByDigitalSignature()){
            fmt.Println("==========\nField locked by a digital signature")
            fmt.Println("==========\nField not locked by a digital signature")
        fmt.Println("Field name: " + current.GetName())

    fmt.Println("====================\nNow iterating over digital signatures only.\n====================")

    digsigFitr := doc.GetDigitalSignatureFieldIterator()
    for digsigFitr.HasNext(){
        current := digsigFitr.Current()
        fmt.Println("Field name of digital signature: " + NewField(current.GetSDFObj()).GetName())

        digsigfield := current
        if !digsigfield.HasCryptographicSignature(){
            fmt.Println("Either digital signature field lacks a digital signature dictionary, " +
                "or digital signature dictionary lacks a cryptographic Contents entry. " +
                "Digital signature field is not presently considered signed.\n" +
        certCount := digsigfield.GetCertCount()
        fmt.Println("Cert count: " + strconv.Itoa(int(certCount)))
        for i := uint(0); i < certCount; i ++{
            cert := digsigfield.GetCert(i)
            fmt.Println("Cert //" + strconv.Itoa(int(i)) + " size: " + strconv.Itoa(int(cert.Size())))
        subfilter := digsigfield.GetSubFilter()

        fmt.Println("Subfilter type: " + strconv.Itoa(int(subfilter)))

        if subfilter != DigitalSignatureFieldE_ETSI_RFC3161{
            fmt.Println("Signature's signer: " + digsigfield.GetSignatureName())

            signingTime := digsigfield.GetSigningTime()
            if signingTime.IsValid(){
                fmt.Println("Signing time is valid.")
            fmt.Println("Location: " + digsigfield.GetLocation())
            fmt.Println("Reason: " + digsigfield.GetReason())
            fmt.Println("Contact info: " + digsigfield.GetContactInfo())
            fmt.Println("SubFilter == e_ETSI_RFC3161 (DocTimeStamp; no signing info)")
        if digsigfield.HasVisibleAppearance(){
            fmt.Println("Not visible")
        digsigDocPerms := digsigfield.GetDocumentPermissions()
        lockedFields := digsigfield.GetLockedFields()
        for i := 0; i < int(lockedFields.Size()); i ++{
            fmt.Println("This digital signature locks a field named: " + lockedFields.Get(i))
        if digsigDocPerms == DigitalSignatureFieldE_no_changes_allowed{
            fmt.Println("No changes to the document can be made without invalidating this digital signature.")
        }else if digsigDocPerms == DigitalSignatureFieldE_formfilling_signing_allowed{
            fmt.Println("Page template instantiation, form filling, and signing digital signatures are allowed without invalidating this digital signature.")
        }else if digsigDocPerms == DigitalSignatureFieldE_annotating_formfilling_signing_allowed{
            fmt.Println("Annotating, page template instantiation, form filling, and signing digital signatures are allowed without invalidating this digital signature.")
        }else if digsigDocPerms == DigitalSignatureFieldE_unrestricted{
            fmt.Println("Document not restricted by this digital signature.")
            fmt.Println("Unrecognized digital signature document permission level.")


func TimestampAndEnableLTV(inDocpath string, 
    inTrustedCertPath string, 
    inAppearanceImgPath string,
    inOutpath string) bool{
    doc := NewPDFDoc(inDocpath)
    doctimestampSignatureField := doc.CreateDigitalSignatureField()
    tstConfig := NewTimestampingConfiguration("")
    opts := NewVerificationOptions(VerificationOptionsE_compatibility_and_archiving)
//   It is necessary to add to the VerificationOptions a trusted root certificate corresponding to 
//   the chain used by the timestamp authority to sign the timestamp token, in order for the timestamp
//   response to be verifiable during DocTimeStamp signing. It is also necessary in the context of this 
//   function to do this for the later LTV section, because one needs to be able to verify the DocTimeStamp 
//   in order to enable LTV for it, and we re-use the VerificationOptions opts object in that part.

//       By default, we only check online for revocation of certificates using the newer and lighter 
//   OCSP protocol as opposed to CRL, due to lower resource usage and greater reliability. However, 
//   it may be necessary to enable online CRL revocation checking in order to verify some timestamps
//   (i.e. those that do not have an OCSP responder URL for all non-trusted certificates).


    widgetAnnot := SignatureWidgetCreate(doc, NewRect(0.0, 100.0, 200.0, 150.0), doctimestampSignatureField)

    // (OPTIONAL) Add an appearance to the signature field.
    img := ImageCreate(doc.GetSDFDoc(), inAppearanceImgPath)

    fmt.Println("Testing timestamping configuration.")
    configResult := tstConfig.TestConfiguration(opts)
    if configResult.GetStatus(){
        fmt.Println("Success: timestamping configuration usable. Attempting to timestamp.")
        // Print details of timestamping failure.
        if configResult.HasResponseVerificationResult(){
            tstResult := configResult.GetResponseVerificationResult()
            fmt.Println("CMS digest status: "+ tstResult.GetCMSDigestStatusAsString())
            fmt.Println("Message digest status: " + tstResult.GetMessageImprintDigestStatusAsString())
            fmt.Println("Trust status: " + tstResult.GetTrustStatusAsString())
        return false

    doctimestampSignatureField.TimestampOnNextSave(tstConfig, opts)

    // Save/signing throws if timestamping fails.
    doc.Save(inOutpath, uint(SDFDocE_incremental))

    fmt.Println("Timestamping successful. Adding LTV information for DocTimeStamp signature.")

    // Add LTV information for timestamp signature to document.
    timestampVerificationResult := doctimestampSignatureField.Verify(opts)
    if !doctimestampSignatureField.EnableLTVOfflineVerification(timestampVerificationResult){
        fmt.Println("Could not enable LTV for DocTimeStamp.")
        return false
    doc.Save(inOutpath, uint(SDFDocE_incremental))
    fmt.Println("Added LTV information for DocTimeStamp signature successfully.")

    return true

func main(){
    // Initialize PDFNet
    result := true
    inputPath := "../../TestFiles/"
    outputPath := "../../TestFiles/Output/"
    //////////////////////////////////////// TEST 0:
    // Create an approval signature field that we can sign after certifying.
    // (Must be done before calling CertifyOnNextSave/SignOnNextSave/WithCustomHandler.)
    // Open an existing PDF
    doc := NewPDFDoc(inputPath + "tiger.pdf")
    widgetAnnotApproval := SignatureWidgetCreate(doc, NewRect(300.0, 300.0, 500.0, 200.0), "PDFTronApprovalSig")
    page1 := doc.GetPage(1)
    doc.Save(outputPath + "tiger_withApprovalField_output.pdf", uint(SDFDocE_remove_unused))
    CertifyPDF(inputPath + "tiger_withApprovalField.pdf",
            inputPath + "pdftron.pfx",
            inputPath + "pdftron.bmp",
            outputPath + "tiger_withApprovalField_certified_output.pdf")
    PrintSignaturesInfo(outputPath + "tiger_withApprovalField_certified_output.pdf")
    //////////////////////////////////////// TEST 2: sign a PDF with a certification and an unsigned signature field in it.
    SignPDF(inputPath + "tiger_withApprovalField_certified.pdf",
            inputPath + "pdftron.pfx",
            inputPath + "signature.jpg",
            outputPath + "tiger_withApprovalField_certified_approved_output.pdf")
    PrintSignaturesInfo(outputPath + "tiger_withApprovalField_certified_approved_output.pdf")
    //////////////////////////////////////// TEST 3: Clear a certification from a document that is certified and has an approval signature.
    ClearSignature(inputPath + "tiger_withApprovalField_certified_approved.pdf",
            outputPath + "tiger_withApprovalField_certified_approved_certcleared_output.pdf")
    PrintSignaturesInfo(outputPath + "tiger_withApprovalField_certified_approved_certcleared_output.pdf")

    //////////////////////////////////////// TEST 4: Verify a document"s digital signatures.
    if !VerifyAllAndPrint(inputPath + "tiger_withApprovalField_certified_approved.pdf", inputPath + "pdftron.cer"){
        result = false
    //////////////////////////////////////// TEST 5: Verify a document's digital signatures in a simple fashion using the document API.
    if !VerifySimple(inputPath + "tiger_withApprovalField_certified_approved.pdf", inputPath + "pdftron.cer"){
       result = false
    //////////////////////////////////////// TEST 6: Timestamp a document, then add Long Term Validation (LTV) information for the DocTimeStamp.
    if !TimestampAndEnableLTV(inputPath + "tiger.pdf",
        inputPath + "GlobalSignRootForTST.cer",
        inputPath + "signature.jpg",
        outputPath+ "tiger_DocTimeStamp_LTV.pdf"){
        result = false
    //////////////////////////////////////// End of tests. ////////////////////////////////////////

    if !result{
        fmt.Println("Tests FAILED!!!\n==========")
    fmt.Println("Tests successful.\n==========")

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