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OpenSignature Architecture

Engineering architecture for OpenSignature, the asynchronous digital-signature platform in this repository. Product and task sources of truth: docs/PRODUCT-SPEC.en.md, docs/TASKS.md. Code namespaces and project names use the OpenSignature.* prefix.

1. Overview / Goals

OpenSignature is a centralized enterprise signing platform built on .NET 10. Clients submit documents via REST; signing runs asynchronously on workers. Goals:

• Support PAdES, XAdES, CAdES, ASiC-S, and ASiC-E (profiles B → T → LT → LTA).
• Keep the API non-blocking: accept, persist, enqueue; never sign synchronously in the API.
• Isolate cryptography behind replaceable signing providers (PFX for demo; PKCS#11 / smart card / HSM for production).
• Store document binaries in file/object storage; PostgreSQL holds metadata, job state, and audit only.
• Transport job references over RabbitMQ — never document binaries.
• Enforce tenant isolation, idempotency, and observable end-to-end flows.

Local stack: PostgreSQL, RabbitMQ, Api, Worker, and Web via docker-compose.yml (see docker/ Dockerfiles). Solution file: OpenSignature.slnx.

2. Component Diagram

                    +-------------------+
                    |   OpenSignature.Web       |
                    |   (React / Vite)  |
                    +---------+---------+
                              |
                              v
                    +-------------------+
                    |   OpenSignature.Api       |
                    | ASP.NET Core      |
                    | Minimal API       |
                    +----+---------+----+
                         |         |
              OpenSignature.Application    |
                         |         |
              OpenSignature.Domain (pure)  |
                         |         |
              OpenSignature.Infrastructure |
                         |         |
              +----------+---------+----------+
              |          |                    |
         PostgreSQL   IFileStorage      Outbox publisher
         (metadata)   (input/signed)         |
                                             v
                                      +------------+
                                      |  RabbitMQ  |
                                      |  (refs only)|
                                      +------+-----+
                                             |
                                             v
                                      +------------+
                                      | OpenSignature.Worker|
                                      +------+-----+
                                             |
                              OpenSignature.Signing (+ Contracts)
                                             |
                    +------------------------+------------------------+
                    |            |           |                        |
                   PFX        PKCS#11    SmartCard                   HSM
                    |            |           |                        |
                    +------------+-----------+------------------------+
                                             |
                                       Signed file (storage)

Project	Role
OpenSignature.Api	HTTP boundary; validate, accept, return 202
OpenSignature.Application	Use cases, orchestration, DTOs
OpenSignature.Domain	Entities, state machine, value objects — no infrastructure
OpenSignature.Infrastructure	EF Core, PostgreSQL, storage adapters, RabbitMQ, outbox
OpenSignature.Signing.Contracts	ISigningProvider and shared signing contracts
OpenSignature.Signing	Provider implementations and format engines
OpenSignature.Validation	Certificate + signature validation (CAdES/XAdES/PAdES/ASiC crypto + certificate path) and JSON reports
OpenSignature.Worker	Consumes jobs; signs; updates state
OpenSignature.Web	React admin/demo UI

3. Async Signing Flow

Client
  |  POST /api/v1/signatures (+ Idempotency-Key)
  v
Api
  |  validate → generate signatureId / jobId
  |  store input via IFileStorage (server-generated key)
  |  persist SignatureRequest + SigningJob + OutboxMessage (same DB transaction)
  |  return 202 Accepted
  v
Outbox publisher
  |  publish small message to RabbitMQ (no binary)
  v
Worker
  |  consume → idempotent lock → load input → resolve provider → sign
  |  write signed.bin → update PostgreSQL → ACK
  v
Client
  |  GET /api/v1/signatures/{id}  (status)
  |  GET /api/v1/signatures/{id}/content  (when Completed)
  |  GET /api/v1/signatures/{id}/verification  (detailed report when Completed)

Incomplete outputs must never be exposed as completed. Storage or post-sign DB failures block or recover without falsely completing.

4. Layering & Dependency Rules

Web ──► Api ──► Application ──► Domain
                      │
                      ▼
               Infrastructure ──► Domain, Signing.Contracts
                      │
Worker ──► Application / Infrastructure / Signing
Signing ──► Signing.Contracts

Rules:

• Domain depends on nothing outside itself.
• Application depends on Domain and abstractions; not on RabbitMQ or concrete storage SDKs.
• Signing must not depend on RabbitMQ; transport stays in Infrastructure / Worker.
• Api must not perform cryptographic signing.
• Prefer dependency inversion: IFileStorage, ISigningProvider, messaging ports.

5. Signing Provider Abstraction

ISigningProvider
 ├── PfxSigningProvider          (dev/demo; PKCS#12)
 ├── Pkcs11SigningProvider
 ├── SmartCardSigningProvider
 └── HsmSigningProvider

Provider capabilities: list certificates, metadata, create digest, sign digest, health/availability, session cleanup.

PFX (dev/demo): PfxSigningProvider loads PKCS#12 from a configured path or in-memory bytes (PfxSigningProviderOptions). Password may come from options (development) or ISigningSecretProvider via PasswordSecretName, backed by ISecretStore (ConfigurationSecretStore / EnvironmentSecretStore / RotatingSecretStore). Certificates without a usable RSA/ECDSA private key, or outside their validity window, are listed with CanSign=false unless Signing:AllowExpiredCertificates is true. Health reflects load success (Healthy / Degraded / Unavailable). Never commit PFX files or passwords.

PKCS#11 (T100–T103): Abstractions as IPkcs11Library / IPkcs11Slot / IPkcs11Session / IPkcs11LibraryFactory under OpenSignature.Signing.Pkcs11. Providers call SignDigest on-device; private keys are never exported. NativePkcs11LibraryFactory (Pkcs11Interop) loads vendor PKCS#11 modules for USB tokens, smart cards, and HSMs. MockPkcs11Library holds an in-memory RSA/ECDSA key for CI (no real hardware). SmartCardSigningProvider uses short-lived sessions (open → login → use → close) via AddSmartCardSigningProvider. HsmSigningProvider uses a bounded Pkcs11SessionPool (MaxConcurrentSessions) via AddHsmSigningProvider. API and Worker call AddHardwareSigningProviders from Signing:SmartCard / Signing:Hsm. In Development, Signing:SmartCard:AutoDetect=true probes well-known middleware DLL/SO paths so a plugged-in USB token appears on GET /api/v1/providers. Health checks detect module/token presence and PIN secret configuration; they do not log in (to avoid PIN lockout). Token PIN is resolved only via PinSecretName + ISigningSecretProvider and must never be logged.

Provider selection: ISigningProviderResolver / SigningProviderSelector resolves a registered ISigningProvider by SigningProviderType and optional provider id from the request/configuration. Providers are registered in DI as IEnumerable<ISigningProvider> (e.g. AddPfxSigningProvider, AddSmartCardSigningProvider, AddHsmSigningProvider). Unsupported or ambiguous selections throw UnsupportedSigningProviderException (SIGNING_PROVIDER_UNSUPPORTED).

Signature engine (T050–T055a, T080–T085): AddSignatureEngine registers PFX + CAdES/XAdES/PAdES/ASiC-S/ASiC-E format signers, RFC 3161 timestamping, LT data providers, profile enhancers, and SignatureOrchestrator as ISignatureCreationService. Crypto primitives live under OpenSignature.Signing.Crypto. Format signers never export private keys; digests are signed via ISigningProvider.SignDigestAsync. PAdES may include an optional visible widget (text “Digitally signed by …”, note, JPEG/PNG); appearance images are stored as appearance.bin and never placed on RabbitMQ. A later PAdES request on an already-signed PDF appends a new incremental signature (OpenSignatureN) without patching the previous /ByteRange; visible stamps are placed so they do not overlap existing widgets. Default TSA is unavailable until Timestamping:Url is set on the worker (AddRfc3161TimestampAuthority; optional HTTP Basic Auth via Timestamping:Username and PasswordSecretName). Default LT provider includes only the signing certificate; LT/LTA fail unless CRL/OCSP evidence is registered. Missing TSA or revocation evidence throws TIMESTAMP_AUTHORITY_UNAVAILABLE / SIGNATURE_VALIDATION_DATA_UNAVAILABLE — never a silent downgrade to Baseline B. Unknown profiles/formats throw SIGNATURE_PROFILE_UNSUPPORTED / SIGNATURE_FORMAT_UNSUPPORTED. See docs/SIGNATURE-PROFILES.md.

Validation (T090–T092, T160–T162): Library-first (OpenSignature.Validation). ICertificateValidator runs PRODUCT-SPEC §20 pipeline (validity → chain/trust → key usage → EKU → revocation → policy) with machine-readable codes. IRevocationChecker supports Offline/Online/SoftFail (OCSP then CRL; live fetch not enabled in MVP — stub-friendly). ISignatureValidator verifies CAdES/XAdES/PAdES crypto + certificate path, and unpacks ASiC-S/E to verify the inner CAdES. It is not a full ETSI EN 319 102-1 evaluation of T/LT/LTA evidence. IValidationReportBuilder emits JSON-serializable reports. Register via AddOpenSignatureValidation / AddSignatureVerification.

Verification APIs: GET /api/v1/signatures/{id}/verification (stored completed output) and POST /api/v1/verifications (ad-hoc upload, not persisted). The React UI shows the detailed report on the signature detail page and a dedicated Verify page. Not full ETSI EN 319 102-1 AdES conformance.

Hardware rule: for PKCS#11, smart card, and HSM providers, private keys never leave the device. Prefer:

certificate → digest → device.Sign(digest)

Do not export private keys to the API, Worker process memory as extractable material, or PostgreSQL. PFX is acceptable only for development/demo.

6. Storage Keys Pattern

Do not use client filenames as paths. Server-generated keys:

tenants/{tenantId}/signatures/{yyyy}/{MM}/{dd}/{signatureId}/input.bin
tenants/{tenantId}/signatures/{yyyy}/{MM}/{dd}/{signatureId}/signed.bin

IFileStorage: SaveAsync, OpenReadAsync, ExistsAsync, DeleteAsync, GetMetadataAsync.

MVP: local filesystem. Production adapters: S3-compatible, Azure Blob, S3. PostgreSQL does not store document binaries or private keys by default.

7. RabbitMQ Topology

Exchange:     esign.signature (durable, direct)
Routing key:  signature.created
Queue:        esign.signature.worker (x-dead-letter-exchange → esign.signature.dlx)
DLX:          esign.signature.dlx (durable, fanout)
DLQ:          esign.signature.dlq

Publisher: ISigningJobPublisher / RabbitMqSigningJobPublisher publishes JSON metadata only (SigningJobMessage) to the exchange.

Messages carry job metadata and storage references only (e.g. jobId, tenantId, signatureId, inputPath, format/profile, attempt). No document binaries in messages.

Processing: deserialize → validate → acquire job lock → verify input → sign → store output → persist final state → ACK.

Retry / DLQ (worker):

• Classify failures as transient vs permanent (SigningJobFailureClassifier; processors may throw TransientSigningJobException / PermanentSigningJobException).
• Transient: bounded retries with exponential backoff (SigningJobRetry options; attempt tracked via x-attempt and SigningJobMessage.Attempt).
• After MaxAttempts or on permanent failure: explicit publish to esign.signature.dlx → esign.signature.dlq, then ACK the original delivery.
• Worker queue DLX args are a safety net for NACK-without-requeue fallbacks.

8. Outbox Pattern

Reliable publication without dual-write loss:

1. In one PostgreSQL transaction: persist signature metadata/job state and an OutboxMessage (Id, Type, Payload, OccurredAt, PublishedAt, RetryCount, Error).
2. A publisher drains unpublished outbox rows to RabbitMQ (DI scope per poll; EF Core DbContext pool + Npgsql connection pool — not a new TCP session every cycle).
3. Mark published only after successful broker handoff.

Worker consumption must be idempotent so at-least-once delivery does not create duplicate successful signatures:

1. Skip when the signature request or job is already terminal (Completed / Cancelled / Rejected).
2. Atomically try-acquire a job lock (ISigningJobLockService / conditional PostgreSQL UPDATE): only one worker may transition Pending/Failed → Locked, or reclaim Locked/Processing when LockedUntil is missing or in the past.
3. If the lock is not acquired (duplicate delivery or another worker holds a non-expired lock), log and ACK without signing.
4. After a retryable signing failure, release the lock (job → Failed, LockedUntil cleared, request → RetryScheduled) before the message is retried so the next delivery can acquire it. Permanent failures (including unreadable PDF structure) mark both request and job Failed.
5. After crash/restart, expired Locked/Processing jobs may be reclaimed; active (non-expired) locks must not be stolen. Lock duration must exceed expected signing time.
6. Before writing signed output, re-check terminal state so a late worker never double-writes after another completion.

9. Multi-Tenancy Notes

• Domain entities and queue messages include TenantId.
• Storage keys are tenant-prefixed.
• Idempotency is scoped as TenantId + IdempotencyKey.
• Application port ISignatureRequestIdempotencyStore (EF implementation) looks up by tenant + key, creates when missing, and recovers from unique-constraint races so concurrent callers observe one SignatureRequest row.
• At most one SigningJob per signature request (IX_SigningJobs_SignatureRequestId unique); GetOrCreateSigningJobAsync uses the same conflict-recovery pattern.
• Auth roadmap: MVP API keys; production OAuth2/OIDC + JWT with tenant isolation and RBAC (Administrator, Signer, Operator, Auditor, Developer).
• Queries, authorization, and indexes must enforce tenant boundaries; never cross-tenant file or job access by ID alone.

10. Observability

OpenTelemetry for traces, metrics, and structured logs.

Every operation should carry:

correlationId · traceId · signatureId · jobId

Key metrics: signature_requests_total, signature_completed_total, signature_failed_total, signature_duration_seconds, signature_queue_wait_seconds, rabbitmq_messages_total, rabbitmq_retry_total, provider_sign_operations_total, provider_sign_failures_total, storage_operations_total.

Never log private keys, passwords, PINs, secrets, or document contents. Prefer machine-readable error codes at the API; keep sensitive crypto/device detail in secure diagnostics only.

11. Current Repo Status

Area	Status
Phase 0 (T001–T003)	Done — solution bootstrap (OpenSignature.slnx), engineering standards, docker-compose.yml (Postgres + RabbitMQ)
Phase 14 (T140–T141)	Done — CI/CD (build, test, GHCR Api/Worker/Web images) and full-stack Compose
Domain / persistence	WIP — domain model and later Phase 1 tasks
Signing / providers	WIP — contracts and engines not yet complete
API async pipeline, outbox, worker	Outbox writer/processor + RabbitMQ publisher/worker skeleton in place; full API pipeline still planned
Docs	This architecture doc (T150); API/security/ops docs follow their tasks

This document describes the target architecture. Implementation progresses task-by-task in docs/TASKS.md; do not assume runtime behavior exists until the corresponding tasks are DONE.