Self-Host Signal Server: Private Encrypted Messaging for Your Organization

What is Signal Server?

Signal Server is the open-source backend that powers the Signal Messenger network—handling registration, sealed-sender message relay, contacts discovery, push notifications, and storage service for attachments. Running it yourself gives enterprises, activist groups, and privacy-focused communities the ability to control metadata, federate multiple enclaves, and integrate with custom identity providers while keeping Signal’s world-class end-to-end encryption.

⚠️ Reality Check: Self-hosting Signal requires serious infrastructure (Kubernetes, Cassandra, Redis, S3, Twilio/VoIP numbers). This guide consolidates the official docs and community learnings so you can stand up a private cluster in a reproducible way.

Architecture Overview

• Account server: Java service (Spring Boot) handling registration, authentication, delivery queue, and WebSocket connections.
• Storage service: Stores encrypted attachments in S3-compatible backends.
• Contact Discovery: Privacy-preserving hashed address book lookups backed by SGX or an HSM service.
• Dependencies: Cassandra (messages & users), Redis (presence, rate limiting), Kafka (multimedia pipeline), Elasticsearch (debug/log search), AWS SNS/APNs (push).

Clients ⇄ Nginx/Envoy ⇄ Signal Service (REST+WebSocket)
                          ├── Cassandra (multi DC)
                          ├── Redis / Redis Cluster
                          ├── Kafka + S3 storage
                          ├── Contact Discovery Service
                          └── Twilio / VoIP provider

Key Features

🔒 End-to-End Encryption with Sealed Sender

• Maintains Signal’s envelope encryption, sender anonymity, and metadata reduction.
• Custom trust root so you can issue organization-specific safety numbers.

🪪 Controlled Onboarding

• Integrate with SSO, LDAP, or SCIM; map verified phone numbers to corporate identities.
• Short-lived registration tokens and device verification policies.

🌐 Federation-Ready

• Run multiple clusters (regions or business units) and bridge them using dedicated peering.
• Optionally allow outbound bridging to public Signal accounts for exec communications.

📱 Client Compatibility

• Official Android/iOS/Desktop apps can connect by pointing to your custom domain with DNS overrides or MDM-managed config.

Deployment Options

Option 1: Kubernetes (Recommended)

1. Provision Kubernetes 1.27+ with at least 6 worker nodes (8 vCPU / 32 GB each).
2. Deploy dependencies via Helm:

   helm repo add bitnami https://charts.bitnami.com/bitnami
   helm install redis bitnami/redis --set architecture=replication
   helm install cassandra bitnami/cassandra --set dbUser.password=supersecret
   helm install kafka bitnami/kafka
   

3. Deploy the official Signal charts:

   helm repo add signal https://signalapp.github.io/helm-charts
   helm install signal-server signal/signal-server \
     --values values-production.yaml
   

4. Configure ingress (NGINX or Istio) with TLS + WebSocket support.
5. Point signal.yourdomain.com to the ingress and distribute the MDM profile that rewrites textsecure-service.whispersystems.org.

Option 2: Bare Metal (Lab / PoC)

Use the docker-compose bundle maintained by the community to bootstrap everything locally:

git clone https://github.com/signalapp/Signal-Server.git
cd Signal-Server/docker
cp example.env .env
docker compose up -d

• Replace the default secrets, Twilio credentials, and Cassandra replication factor before exposing it outside localhost.
• Perfect for integration testing or building automation.

Configuration Checklist

• Push Providers: Create Apple APNs certificates and Firebase Cloud Messaging keys; load them into Kubernetes secrets.
• Phone Number Verification: Configure Twilio Verify or your SIP/SS7 provider. For landline-only deployments, enable PIN-based registration.
• Storage: Use S3/MinIO for attachments; enable lifecycle policies to purge old media.
• Contact Discovery: Deploy the CDS enclave and load salted hashes of your directory. Alternatively, disable contact discovery and rely on invitation links.
• Safety Numbers: Publish your custom safety-number verification instructions for users to validate device fingerprints.

Scaling Guidance

• Cassandra likes more RAM than CPU; aim for 3+ nodes per DC, enable incremental backups.
• Redis should run in cluster mode with at least 3 masters (no single point of failure).
• Kafka partitions should align with your attachment throughput (start with 6 partitions).
• Horizontal Pod Autoscalers keep the WebSocket tier responsive during peak hours.

Monitoring and Observability

• Scrape JVM metrics via Prometheus JMX exporter.
• Enable OpenTelemetry tracing to see registration and delivery latency.
• Ship application logs to Loki or Elasticsearch to debug push failures.
• Create alerts for:
  • Registration error spikes (>2% failure).
  • Cassandra read/write latency > 25 ms.
  • Redis memory usage > 80%.
  • Twilio SMS delivery failures > 5% in 10 minutes.

Security Best Practices

• Use mutual TLS between ingress and backend pods.
• Rotate all signing keys (sender certificates, push API keys, registration tokens) quarterly.
• Enable Hardware Security Modules or SGX for the contact discovery service in regulated environments.
• Keep the client apps updated via MDM to ensure compatibility with the server API level.

Comparing Options

Choose Signal Server when the organization already depends on Signal UX/security but needs sovereign control or air-gapped communications.

Troubleshooting

• UNAUTHORIZED responses: Check registration tokens and ensure the MDM profile points to the correct domain.
• Push notifications missing: Validate APNs/FCM credentials and confirm workers reach Apple/Google endpoints.
• Attachment upload failures: Verify S3 bucket CORS settings and ensure signed URLs use HTTPS.
• Cassandra schema mismatches after upgrades: Run the bundled Liquibase migrations before rolling out new pods.

Next Steps

• Automate onboarding with Terraform + Helm + Argo CD.
• Mirror the Signal repositories internally to review diffs before upgrades.
• Add a secondary region for disaster recovery with multi-DC Cassandra replication.

By self-hosting Signal Server you get uncompromising privacy with enterprise controls—perfect for mission-critical messaging when SaaS cannot be trusted.