Signing PDF files with Go

This PDF signing library is written in Go. The library is in development, might not work for all PDF files and the API might change, bug reports, contributions and suggestions are welcome.

From the command line

Usage of ./pdfsign:
  -certType string
        Type of the certificate (CertificationSignature, ApprovalSignature, UsageRightsSignature, TimeStampSignature) (default "CertificationSignature")
  -contact string
        Contact information for signatory
  -location string
        Location of the signatory
  -name string
        Name of the signatory
  -reason string
        Reason for signing
  -tsa string
        URL for Time-Stamp Authority (default "https://freetsa.org/tsr")

Example usage:
        ./pdfsign -name "Jon Doe" sign input.pdf output.pdf certificate.crt private_key.key [chain.crt]
        ./pdfsign -certType "CertificationSignature" -name "Jon Doe" sign input.pdf output.pdf certificate.crt private_key.key [chain.crt]
        ./pdfsign -certType "TimeStampSignature" input.pdf output.pdf
        ./pdfsign verify input.pdf

As library

import "github.com/digitorus/pdf"

input_file, err := os.Open(input)
if err != nil {
    return err
}
defer input_file.Close()

output_file, err := os.Create(output)
if err != nil {
    return err
}
defer output_file.Close()

finfo, err := input_file.Stat()
if err != nil {
    return err
}
size := finfo.Size()

rdr, err := pdf.NewReader(input_file, size)
if err != nil {
    return err
}

err = sign.Sign(input_file, output_file, rdr, size, sign.SignData{
    Signature: sign.SignDataSignature{
        Info: sign.SignDataSignatureInfo{
            Name:        "John Doe",
            Location:    "Somewhere on the globe",
            Reason:      "My season for siging this document",
            ContactInfo: "How you like",
            Date:        time.Now().Local(),
        },
        CertType:   sign.CertificationSignature,
        DocMDPPerm: sign.AllowFillingExistingFormFieldsAndSignaturesPerms,
    },
    Signer:            privateKey,         // crypto.Signer
    DigestAlgorithm:   crypto.SHA256,      // hash algorithm for the digest creation
    Certificate:       certificate,        // x509.Certificate
    CertificateChains: certificate_chains, // x509.Certificate.Verify()
    TSA: sign.TSA{
        URL: "https://freetsa.org/tsr",
        Username: "",
        Password: "",
    },

    // The follow options are likely to change in a future release
    //
    // cache revocation data when bulk signing
    RevocationData:     revocation.InfoArchival{}, 
    // custom revocation lookup
    RevocationFunction: sign.DefaultEmbedRevocationStatusFunction,
})
if err != nil {
    log.Println(err)
} else {
    log.Println("Signed PDF written to " + output)
}

Signature Appearance with Text and / or Images

You can add an image (JPG or PNG) to the visible signature appearance. This is useful for including a handwritten signature or a company logo in the signature field.

Supported image formats: JPG and PNG.

Example: Signing a PDF with a visible signature and image

// Read the signature image file
signatureImage, err := os.ReadFile("signature.jpg")
if err != nil {
    log.Fatal(err)
}

err := sign.Sign(inputFile, outputFile, rdr, size, sign.SignData{
    Signature: sign.SignDataSignature{
        Info: sign.SignDataSignatureInfo{
            Name:        "John Doe",
            Location:    "Somewhere",
            Reason:      "Signed with image",
            ContactInfo: "None",
            Date:        time.Now().Local(),
        },
        CertType:   sign.ApprovalSignature,
        DocMDPPerm: sign.AllowFillingExistingFormFieldsAndSignaturesPerms,
    },
    Appearance: sign.Appearance{
        Visible:     true,
        LowerLeftX:  400,
        LowerLeftY:  50,
        UpperRightX: 600,
        UpperRightY: 125,
        Image:       signatureImage, // JPG or PNG image bytes
        // ImageAsWatermark: true,   // Optional: set to true to draw text over the image
    },
    DigestAlgorithm: crypto.SHA512,
    Signer:          privateKey,
    Certificate:     certificate,
})
if err != nil {
    log.Fatal(err)
}

Key Features:

1. Image Support: Both JPG and PNG formats are supported
2. Flexible Positioning: Control signature placement with LowerLeftX/Y and UpperRightX/Y coordinates
3. Watermark Mode: Optional ImageAsWatermark setting allows drawing text over the image
4. Transparency Support: PNG images with alpha channel (transparency) are properly handled

Notes:

• The image will be scaled to fit the signature rectangle while maintaining its aspect ratio
• For optimal results, prepare your image with the desired dimensions and transparency before using it
• Only visible approval signatures can include images