Deferred PAdES signing

Deferred signing separates PDF preparation from the private-key operation. The server creates CMS signed attributes and an opaque session; a browser, smart card, HSM, or local agent returns the raw signature. The private key never leaves that signer.

using SimpleSign.Core.Signing;
using SimpleSign.PAdES;

var profile = AdesBaselineProfile.Archive(
    new TimestampOptions(new Uri("https://tsa.example")),
    new LongTermValidationOptions());

var builder = DeferredSigner.Document(pdfBytes)
    .WithCertificate(certificate, chain)
    .WithSessionIntegrityKey(sessionIntegrityKey)
    .WithLevel(profile)
    .WithHttpClientProvider(httpClientProvider);

// Server: persist SessionData and send HashToSign to the external signer.
var prepared = await builder.PrepareAsync();
byte[] rawSignature = await SignExternallyAsync(prepared.HashToSign);

// Server: load the persisted session and apply B-T/B-LT/B-LTA enrichment.
byte[] signedPdf = await DeferredSigner.Resume(prepared.SessionData, sessionIntegrityKey)
    .WithHttpClientProvider(httpClientProvider)
    .CompleteAsync(rawSignature);

HashToSign is DER-encoded CMS signed attributes, not a document hash. The external signer must apply the resolved hash and signature scheme represented by DigestAlgorithm and SignatureAlgorithmOid. RSA-PSS callers must also honor the resolved PSS parameters.

Configuration

The deferred builder uses the same vocabulary as direct PAdES signing:

Method	Purpose
`WithCertificate(certificate, chain)`	Sets the public certificate and optional chain.
`WithHashAlgorithm` / `WithSignatureAlgorithm`	Selects a coherent signing algorithm.
`WithLevel(profile)`	Requests B-B, B-T, B-LT, or B-LTA.
`WithHttpClientProvider(provider)`	Supplies non-owned TSA/revocation clients.
`WithFieldOptions(options)`	Atomically configures or clears PDF field metadata and appearance.
`WithLogger(logger)`	Enables diagnostics.

The serialized session contains operation state, public certificate material, and the requested baseline level/endpoints. It never serializes HttpClient, loggers, private keys, or service-provider instances. It is always authenticated with HMAC-SHA256 using the mandatory server-owned key passed to WithSessionIntegrityKey and Resume. Store it only on the server and return a random one-time identifier to the browser; do not treat session bytes as a client-side token. Reattach a server-owned HTTP provider after Resume when the configured endpoint needs one.

For B-LT and B-LTA, completion requires a valid signature timestamp and complete DSS evidence for signer and TSA paths. Unsupported evidence collection fails instead of returning an artifact that falsely claims the requested level.

Table of Contents

Deferred PAdES signing

Configuration