The security architecture of a cloud-to-letterbox API.

The architectural boundary: Extending zero trust from the cloud to the physical world

Threat modeling the print pipeline: STRIDE analysis across the physical boundary

The cryptographic lifecycle: Ingest, volatile rasterization, and ephemeral memory purging

Physical cleanroom security: ISO 27001, biometric zoning, and closed-loop optical matching

The 3-step zero-trust postal integration blueprint

Code walkthrough: Implementing client-side envelope encryption and audit hashing

src/security/zeroTrustPostalClient.ts — typescript
import crypto from 'crypto';
​
interface SecureLetterRequest {
  recipient: {
    name: string;
    addressLine1: string;
    city: string;
    postcode: string;
  };
  documentHtml: string;
  auditReference: string;
}
​
export class ZeroTrustPostalClient {
  private apiKey: string;
  private baseUrl: string = 'https://api.letterapi.uk/v1';
​
  constructor(apiKey: string) {
    this.apiKey = apiKey;
  }
​
  public async dispatchSecureLetter(req: SecureLetterRequest): Promise<{ letterId: string; clientChecksum: string }> {
    // 1. Compute SHA-256 payload checksum prior to transmission
    const clientChecksum = crypto
      .createHash('sha256')
      .update(req.documentHtml, 'utf8')
      .digest('hex');
​
    const payload = {
      recipient: {
        name: req.recipient.name,
        address_line_1: req.recipient.addressLine1,
        city: req.recipient.city,
        postcode: req.recipient.postcode,
        country: 'GB'
      },
      content: {
        type: 'html',
        html: req.documentHtml
      },
      postage: 'first_class',
      security: {
        ephemeral_processing: true,
        zero_retention: true,
        retention_window_hours: 0,
        disable_debug_storage: true,
        enforce_optical_validation: true
      },
      metadata: {
        client_audit_reference: req.auditReference,
        client_sha256_checksum: clientChecksum,
        dispatched_timestamp: new Date().toISOString()
      }
    };
​
    const res = await fetch(`${this.baseUrl}/letters`, {
      method: 'POST',
      headers: {
        'Authorization': `Bearer ${this.apiKey}`,
        'Content-Type': 'application/json',
        'X-LetterAPI-Zero-Retention': 'enforce',
        'X-Client-Payload-Checksum': clientChecksum
      },
      body: JSON.stringify(payload)
    });
​
    if (!res.ok) {
      const error = await res.json();
      throw new Error(`Zero-trust dispatch failed [${res.status}]: ${error.message}`);
    }
​
    const data = await res.json();
    return {
      letterId: data.id,
      clientChecksum
    };
  }
}

DL window aperture geometry and preventing accidental PII window exposure

Royal Mail 2D Mailmark carrier telemetry and cryptographic proof-of-dispatch

SIEM and compliance audit integration: Streaming dispatch telemetry into Datadog, Splunk, and AWS CloudWatch

Disaster recovery, high availability, and sovereign UK data residency

Multi-tenant sandboxing and WASM memory isolation: Defeating side-channel and SSRF exploits

Automated pre-flight security linting: Disabling dynamic script execution and asset leaks

The architectural transformation: Extending zero trust across the digital-physical divide

Deploy zero-trust physical mail architecture with LetterAPI today