Digital signatures · C# and .NET
Digitally sign a PDF in C#
Add a cryptographic PKCS#7 or PAdES signature to a PDF from C#. rust-pdf signs through a non-destructive incremental update, so the original bytes are preserved and the signature stays verifiable in Adobe Reader, pdfsig and any PAdES validator.
Why C# and .NET needs this
.NET's mature options (iText, Aspose) are powerful but expensive and carry AGPL or per-server licensing, which is exactly what teams want to avoid.
A real digital signature gives a document legal weight: it proves who signed it and that nothing changed afterwards. rust-pdf builds the detached CMS by hand to control the ByteRange, supports PAdES B-B, B-LT and B-LTA for long-term validation, and lets you supply your own key and certificate as DER.
- PKCS#7 detached and PAdES B-B, with B-LT and B-LTA for long-term validation.
- Incremental update: the original file is preserved byte for byte, so earlier signatures stay valid.
- Bring your own key and X.509 certificate (PKCS#8 DER), or chain to a TSA for timestamps.
Sign a PDF in C# with rust-pdf
Install the package, then call the same idiomatic API every rust-pdf binding shares. The snippet below is real C# code from the reference docs.
using RustPdf;
byte[] pdf = File.ReadAllBytes("contract.pdf");
byte[] keyDer = File.ReadAllBytes("signing-key.pkcs8.der"); // PKCS#8 private key (DER)
byte[] certDer = File.ReadAllBytes("signing-cert.der"); // X.509 certificate (DER)
byte[] signed = Pdf.Sign(pdf, keyDer, certDer,
reason: "Approved", location: "New York",
name: "Jane Doe", pades: true);
File.WriteAllBytes("contract.signed.pdf", signed);
// Verify in a shell: pdfsig contract.signed.pdf → "Signature is Valid."C# basic generation is free. Signing is a corporate feature, unlocked by one offline license token. See pricing & licensing.
Full C# reference in the documentation.
Signing in C#: FAQ
Is the signature legally valid?
rust-pdf produces standards-compliant PKCS#7 and PAdES signatures. Legal validity depends on the certificate you sign with (for example an eIDAS qualified certificate or an ICP-Brasil certificate). The library handles the cryptography and the PDF structure correctly, which is what validators such as pdfsig and Adobe Reader check.
Does it support long-term validation (LTV)?
Yes. After signing you can append a Document Security Store with certificates and CRLs (PAdES B-LT) and an RFC 3161 document timestamp (PAdES B-LTA), all offline. A trusted external TSA and live OCSP fetching are the only parts that need network infrastructure.
Do I need a license to sign in C#?
Signing is a corporate feature, so it requires an active license token. Basic PDF generation in C# is free. The same offline Ed25519 token unlocks signing across every language.
Digitally Sign a PDF in C# (PAdES)
One Rust core, the same output across every language. Prototype for free, license the corporate features when you ship.