Signature verification API: what to look for
In short Look for an API that compares signatures with each person’s own references, returns a verdict with reasons rather than a bare score, keeps your data where you need it, and prices a clear unit. Test it on your own documents.
Integration
The best API is the one your team can wire in within a day and trust in production. Look for:
- A documented REST API, ideally described in OpenAPI, so you can generate a client.
- Idempotency keys, so a retried request never checks or bills the same document twice.
- Versioning you control, so a breaking change never reaches you before you choose it.
- Signed webhooks, so results come back to you and you can prove they came from the provider.
- A batch route that needs no code, for back offices and archives.
Unforged Nib has all five: a REST API described in OpenAPI 3.1, an Idempotency-Key header, a pinned Unforged-Version header, webhooks signed with HMAC-SHA256, and a bucket drop for files plus a manifest. The API docs show each call.
Verdict design
A score from 0 to 1 is only half an answer. Ask what the API tells you to do, and why.
- Is each signature compared with the person’s own references?
- Is the answer a clear verdict with reasons you could show an auditor?
- Is there a band that shows how sure it is?
- Can you set the thresholds, and does each result record the policy and version used?
- What happens to poor images and missing signatures?
Unforged Nib returns one of six verdicts: AUTO APPROVE, APPROVE, FLAG, FAILED, UNREADABLE and NO SIGNATURE. Each comes with a calibrated probability, a band and plain-language reasons.
Data handling
Signatures are personal data, and can be biometric data. Ask where documents are processed and stored, how long they are kept, and whether they train the product.
Unforged Nib is hosted in London (AWS eu-west-2). Documents are used only to run your checks and are never used for training.
Uploads and results are kept for your history-retention period: 90 days by default, or anything from 30 minutes to 10 years, or indefinitely.
Our security page covers access control, keys and the audit log. For the legal side, see signatures and UK GDPR.
Pricing models
Providers price in different units: per signature, per page, per document or by subscription. The unit matters more than the headline number.
- What exactly counts as one billable unit?
- Are unreadable images and unsigned pages charged?
- Is enrolling references charged?
- Is there a minimum, a commitment or a setup fee?
Unforged Nib costs $0.50 per check, pay as you go. A check is one document checked against one person’s references, however many signatures it holds.
You pay only when it returns AUTO APPROVE, APPROVE, FLAG or FAILED. UNREADABLE and NO SIGNATURE results, and enrolling references, are free. See pricing.
Limits and scale
Check the file types, the size limits and the rate limit before you build.
Unforged Nib takes forms, scans, phone photos, PDFs, TIFFs and ZIP archives, up to 50 MB and 200 pages. Its default rate limit is 20 requests a second, burst 50, per organisation.
A checklist for any provider
- Run a trial on your own documents, including known forgeries and genuine signatures that vary.
- Look at the unclear cases: where they go and what a reviewer sees.
- Read a result aloud. Could you explain it to a customer or an auditor?
- Confirm the data region, the retention setting and the training policy in writing.
- Work out the cost per document, including the ones that come back unreadable or unsigned.
- Try the failure paths: a retried request, a webhook you missed, a version upgrade.
To try Unforged Nib on your own documents, see pricing. New organisations get $10 of credit, no card needed.