A server-blind, browser-native encrypted form poster.

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Form submissions are encrypted in the browser using X25519 sealed boxes before reaching any endpoint. The backend receives and stores opaque ciphertext only. Decryption is operator-controlled.

formseal is not a hosted service, dashboard, or SaaS product. It is a drop-in client-side utility.

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Installation

Via npm (recommended)

npm install -g @formseal/embed
fse init

Via GitHub release (zero toolchain)

1. Download the latest release artifact
2. Unzip → drop formseal-embed/ into your project
3. Edit fse.config.js manually

Prefer not to install globally? npx @formseal/embed init works, but subsequent fse commands won't be available — you'll need to edit fse.config.js manually.

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Configure

fse configure quick

You'll be prompted for your POST endpoint and public key. See Getting started for key generation.

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Security guarantee

If the POST endpoint is fully compromised, seized, or maliciously operated, previously submitted form data remains confidential.

Encryption happens in the browser. The backend stores ciphertext only. Decryption keys never exist in the backend environment. A backend compromise yields no recoverable plaintext.

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Threat model

formseal is for environments where:

• The hosting provider or backend may be compromised
• The backend must be treated as hostile
• Data seizure is a realistic concern
• Retroactive disclosure must be prevented

The priority is backward confidentiality — protecting already-submitted data — not convenience or real-time administration.

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How it works

On submit, formseal:

1. Collects field values from your form by name attribute
2. Validates them against your field rules (in fields.jsonl)
3. Seals the payload with crypto_box_seal (Curve25519 + XSalsa20-Poly1305)
4. POSTs raw ciphertext to your configured endpoint

Your endpoint stores the ciphertext. Only the holder of the private key can decrypt it.

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Wire up your HTML

After fse init, files live in ./formseal-embed/. Reference them via your server's static path (e.g. /formseal-embed/globals.js).

<form id="contact-form">

  <!-- honeypot — hide off-screen with CSS -->
  <input type="text" name="_hp" tabindex="-1" autocomplete="off"
    style="position:absolute;left:-9999px;opacity:0;height:0;">

  <input type="text"  name="name">
  <span data-fse-error="name"></span>

  <input type="email" name="email">
  <span data-fse-error="email"></span>

  <textarea name="message"></textarea>
  <span data-fse-error="message"></span>

  <button type="submit" id="contact-submit">Send message</button>
</form>

<div id="contact-status"></div>

<script>
  window.fseCallbacks = {
    onSuccess: () => document.getElementById('contact-status').textContent = 'Sent securely.',
    onError:   (err) => console.error('formseal error:', err),
  };
</script>

<script src="/formseal-embed/globals.js"></script>

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Payload format

{
  "version": "fse.v1.0",
  "origin": "contact-form",
  "id": "<uuid>",
  "submitted_at": "<iso8601>",
  "data": {
    "name": "...",
    "email": "...",
    "message": "..."
  }
}

The entire object is sealed with crypto_box_seal. Your endpoint receives raw ciphertext as the request body.

No IP, no timezone, no fingerprints — just the data you explicitly collect.

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Field configuration

Fields are defined in fields.jsonl (one JSON object per line):

{"name": {"required": true, "maxLength": 100}}
{"email": {"required": true, "type": "email"}}
{"message": {"required": true, "maxLength": 1000}}

Use the CLI to manage fields:

fse configure field add phone type:tel required:false
fse configure field required name true
fse configure field maxLength message 500
fse configure field remove company

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CSS hooks

Selector	When
[data-fse-error="field"]	Populated with a validation error
[aria-invalid="true"]	Set on invalid inputs
[data-fse-status="success"]	Set on status element on success
[data-fse-status="error"]	Set on status element on error

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What formseal does not do

• No admin dashboard or inbox UI
• No hosted service
• No bundled decryption tools (yet)
• No npm dependencies at runtime

These are intentional.

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Documentation

• Getting started
• Concepts → How it works
• Concepts → Security
• Integration → HTML
• Integration → Fields
• Integration → JavaScript
• Deployment → Endpoint
• Deployment → Decryption
• Deployment → Versioning

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License

MIT