Senior Full-Stack Developer — Build E-Signature / DocuSign Alternative From Scratch

We are looking for an experienced full-stack developer to build a production-quality electronic signature platform similar to DocuSign/DocuSeal, entirely from scratch. Critical requirement: The product must be independently developed and must not copy, modify, fork, translate, rewrite, or otherwise derive from DocuSeal, Documenso, OpenSign, DocuSign, or any other existing e-signature application codebase. Any third-party libraries used must have commercially compatible permissive licenses (e.g., MIT, Apache 2.0, BSD) and be documented. Core functionality should include: - PDF/document upload and rendering - Drag-and-drop signature, initials, date, text, checkbox and other fields - Multiple recipients and signing order - Typed, drawn and uploaded signatures - Secure signing links and recipient authentication - Templates - Email invitations, reminders and status tracking - Complete tamper-evident audit trail - Document versioning and cryptographic hashing - Final executed PDF generation - Certificate of completion - Secure document storage - APIs/webhooks - Mobile-responsive signing experience - Strong security, permissions and auditability The ideal candidate has significant experience with PDF manipulation, cryptography/digital signatures, document management, security, React/Next.js or similar frontend frameworks, backend architecture, PostgreSQL, cloud infrastructure, and production SaaS systems. Experience building e-signature, contract-management, document-management, legal-tech, fintech, or other security-sensitive systems is strongly preferred. Milestones: $1,000 — Architecture + Specification Data model, threat model, licensing/dependency plan, PDF architecture, and audit/evidence architecture. $2,000 — Document/Template Editor PDF rendering, field placement, signatures, initials, text, dates, checkboxes, and reusable templates. $2,000 — Signing Workflow Recipients, signing order, secure signing links, authentication, invitations, reminders, and completion workflows. $2,000 — Evidence/Finalization Engine Immutable audit events, document hashes, timestamps, final executed PDF generation, certificate of completion, and tamper evidence. $1,500 — Backend/Admin/Security Organizations, users, permissions, secure document storage, APIs, webhooks, and tenant isolation. $1,500 — Testing + Production Hardening Automated testing, browser/mobile testing, security review and fixes, deployment, technical documentation, and complete source-code handoff. Open-Source / Third-Party Dependency Requirement The developer must provide a complete Software Bill of Materials (SBOM) / dependency manifest identifying every third-party package, library, component, or other external code incorporated into the project, including its version, source repository, and applicable license. The project may use commercially compatible permissive licenses such as MIT, Apache 2.0, and BSD. AGPL, GPL, SSPL, source-available, or other copyleft/restrictive code may not be incorporated, copied, modified, adapted, or otherwise used without our prior written approval. The application must be independently developed and must not copy, fork, rewrite, translate, or otherwise derive from the source code of DocuSeal, Documenso, OpenSign, DocuSign, or any other existing e-signature application.