How to sign a PDF with PAdES in .NET (C#)

This how-to signs a PDF document in C# with an advanced electronic signature in the PAdES format (ETSI EN 319 142), the format required for eIDAS signatures in PDF. It covers the four PAdES baseline levels: B-B, B-T (time-stamp), B-LT (validation data) and B-LTA (archive time-stamp). The library is miPDFlib (Apache-2.0); the signature is built with the CMS support in .NET, no BouncyCastle is needed.

dotnet add package miPDFlib

1. Sign with a certificate (PAdES B-B)

using MiPdf;
using MiPdf.Signatures;

var signer = CertificateSigner.FromPfx("signer.pfx", "password");   // private key + chain from a PKCS#12 file
byte[] signed = PdfSigner.Sign(File.ReadAllBytes("contract.pdf"), signer, new PdfSignatureOptions
{
    Reason = "Approved",
    Location = "Vienna",
});
File.WriteAllBytes("contract-signed.pdf", signed);

The signature is appended as an incremental update, so existing signatures in the document stay valid. The default format is PAdES (ETSI.CAdES.detached with the signing-certificate-v2 attribute) with SHA-256. RSA (PKCS#1 v1.5 or RSASSA-PSS) and ECDSA keys (NIST and Brainpool curves) are supported.

A certificate from the Windows certificate store or a smart card works the same way: pass an X509Certificate2 with private key instead of the CertificateSigner.

2. Visible signature

var options = new PdfSignatureOptions
{
    PageNumber = 1,
    Rectangle = new PdfRectangle(350, 50, 550, 110),                  // points, origin bottom left
    Appearance = new PdfSignatureAppearance
    {
        Text = "Signed by {name}\n{date}\nReason: {reason}",
        Image = MiPdf.Images.PdfImage.FromFile("signature.png"),      // optional: handwritten signature image
    },
};

On rotated pages the appearance is drawn upright automatically. Without Rectangle the signature is invisible.

3. Add a time-stamp (PAdES B-T)

A time-stamp from a time-stamp authority (RFC 3161) proves when the document was signed. Several authorities can be combined; the next one is tried if one fails:

var tsa = FailoverTimestampClient.FromUrls(new[]
{
    "http://timestamp.digicert.com",
    "http://timestamp.sectigo.com",
});
byte[] signed = PdfSigner.Sign(pdf, signer, new PdfSignatureOptions { TimestampClient = tsa });

For qualified time-stamps use the authority of your trust service provider.

4. Long-term validation (PAdES B-LT and B-LTA)

A signature has to remain verifiable after the signer certificate has expired or the CA has stopped answering revocation requests. B-LT embeds the certificates, OCSP responses and CRLs in the document (Document Security Store); B-LTA adds a document time-stamp over everything:

LtvResult lta = PdfLtv.MakeLta(signed, tsa);                      // validation data + document time-stamp
if (!lta.IsComplete)
    Console.WriteLine(string.Join(Environment.NewLine, lta.Warnings));
File.WriteAllBytes("contract-lta.pdf", lta.Pdf);                  // lta.Level == PadesLevel.BLTA

PdfLtv.AddValidationData(signed) stops at B-LT. Both also work for signatures created by other software, and MakeLta can be called again before the last time-stamp expires to extend the archive period.

5. Keys in an HSM, smart card or cloud signing service

If the private key cannot be exported, implement IExternalSigner. Only the hash is passed to it:

public sealed class HsmSigner : IExternalSigner
{
    public X509Certificate2 Certificate { get; init; } = null!;                       // signer certificate
    public IReadOnlyList<X509Certificate2> CertificateChain { get; init; } = [];       // intermediates to embed
    public byte[] SignHash(byte[] hash, HashAlgorithmName algorithm) => hsm.Sign(hash, algorithm);
}

For services that sign in a separate step (remote signing, mobile signatures), split signing in two: PdfSigner.PrepareExternalSignature returns the digest to sign, and PdfSigner.CompleteExternalSignature embeds the CMS container returned by the service. See Signatures.

6. Verify the signature

foreach (var result in PdfSignatureValidator.Validate(File.ReadAllBytes("contract-lta.pdf")))
    Console.WriteLine($"{result.FieldName}: integrity {result.IsIntegrityValid}, level {result.Level}, " +
                      $"whole document {result.CoversWholeDocument}, time-stamp {result.TimestampTime}");

For a full assessment against the EU trusted lists use a validation service based on the EU DSS library, for example miPDFvalidator.

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