Signing and encryption

Create a password-protected document#

using KillerPdf.Engine.Authoring;
using KillerPdf.Engine.Security;
 
var encryption = new PdfPasswordEncryptionOptions
{
    UserPassword = "reader password",
    OwnerPassword = "owner password",
    AllowContentCopying = false,
    AllowDocumentModification = false,
    AllowHighQualityPrinting = true
};
 
byte[] protectedPdf = new PdfDocumentBuilder()
    .SetPasswordEncryption(encryption)
    .AddBlankPage()
    .Build();

New documents use PDF 2.0 AES-256 protection. The engine can also read supported RC4 and AES-128 documents and preserve authenticated security during incremental updates and rewrites.

Open encrypted input#

using KillerPdf.Engine.Documents;
 
PdfDocument document = PdfDocument.Open(source, password);
Console.WriteLine(document.PasswordAuthenticationRole);
Console.WriteLine(document.DeclaredPermissions?.AllowContentCopying);

Honor the authenticated permission set in your application workflow. An owner password grants unrestricted access. A user password may limit printing, copying, assembly, forms, annotations, or general modification.

Sign with detached CMS#

PdfDetachedSignatureWriter prepares the PDF signature revision and calls your application back to produce the detached CMS value. Your application owns certificate selection, private-key access, timestamp service policy, and trust configuration.

using KillerPdf.Engine.Signing;
 
var options = new PdfSignatureOptions
{
    FieldName = "Signature1",
    SignerName = "Example signer",
    Reason = "Approved",
    DigestMethod = PdfSignatureDigestMethod.Sha256
};

Use the Sign overload that matches your certificate and CMS provider. Do not copy a private key into the PDF or into application logs.

Read and verify signatures#

IReadOnlyList<PdfSignatureInfo> signatures = PdfSignatureReader.Read(document);
 
foreach (PdfSignatureInfo signature in signatures)
{
    PdfSignatureVerificationResult integrity =
        PdfSignatureVerifier.VerifyIntegrity(document, signature);
    Console.WriteLine(integrity.IsCryptographicallyValid);
}

Integrity verification answers whether the signed bytes match the signature. Trust verification additionally evaluates the certificate chain under the trust policy supplied by your application.

Analyze later revisions#

PdfSignedRevisionAnalyzer identifies changes after a signature and evaluates them against certification permissions and field locks. This is different from a simple cryptographic pass or fail result. A signature can remain cryptographically intact while later revisions add changes that violate the declared policy.

Security boundaries#

• Treat PDF passwords as sensitive data and avoid retaining them longer than needed.
• Keep private keys in the operating system certificate store, hardware, or another protected signing provider.
• Make trust policy explicit. Do not treat any embedded certificate as trusted by default.
• Validate signed output with an independent CMS implementation before release.