Signing PDF files with Go

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)
}