Guarantees & Limitations

What Veylent Guarantees

1. Tamper Evidence

Each signer's document hash is the SHA-256 of the exact bytes they saved, and every signed version is a byte prefix of the final file. Desktop Verify cuts the file at each signer's layer, rehashes it, and compares the result with the sealed record anchored on-chain. Changing any byte a signer covered produces a mismatch, and anything added after the last signature is reported as changed-after-signing. The blockchain record cannot be altered to match the tampered document.

2. Temporal Proof

The blockchain transaction includes a block timestamp. This proves the signature existed at or before that time. The timestamp cannot be backdated.

3. Signer Privacy

Signer personal data is encrypted with a unique per-signature key. The API server cannot read it. Only someone with physical or digital access to the QR code can decrypt the signer details.

4. Signature Chaining

Multiple signatures on the same document form a verifiable chain. Each signature references the previous signer's anchor ID, creating an ordered, immutable sequence. The contract rejects a reused anchor ID and a previous anchor ID that was never anchored, so a chain can only be extended, never forked onto made-up predecessors.

5. Offline Document Handling

The PDF bytes never leave the signer's machine. The API receives only hashes and encrypted metadata. This is particularly important for confidential documents.

What Each Check Proves

	QR scan (web or mobile)	Desktop Verify
Who signed and when	Yes, from the sealed record checked against the on-chain composite hash	Yes, same check for every signer
The record belongs to this QR code	Yes, the record names the scanned anchor ID	Yes
The file in hand is what was signed	No. A scan never sees the document	Yes, each signer's layer is rehashed and compared
Nothing was added after the last signature	No	Yes (changed-after-signing otherwise)
Signature order	Shows chain length only	Yes, each record must link to the previous anchor ID

A verified scan means a Veylent signer sealed those details at that time. It does not mean the paper or file in front of the verifier is unchanged. For that, open the file in the desktop app.

What Veylent Does NOT Guarantee

1. Identity Verification

Veylent authenticates signers via magic link (email verification), Google OAuth, or Microsoft OAuth. Each signature records a trust anchor indicating the authentication method (e.g. "google", "email"). However, this confirms account ownership, not real-world identity. Veylent does not perform KYC -- it does not verify that "Jane Doe" is who she says she is beyond what the auth provider confirms. Future integrations with government ID providers (e.g. Singpass, eID) will offer stronger identity assurance.

2. Document Content Validation

Veylent proves a document was signed, not that the document's content is accurate, legal, or binding. The legal weight of a Veylent signature depends on jurisdiction.

3. Revocation

Once a signature is anchored on-chain, it cannot be revoked. If a signer wants to repudiate a signature, they must do so through external means (legal process, counter-signature, etc.).

4. Document Integrity from a QR Scan Alone

A QR scan confirms the signer and the time, but does not check that the PDF content matches the anchored document hash. Someone could copy a genuine QR code onto a different document, and the scan would still show the original signer. Full integrity checking requires the desktop app.

5. Files Re-Saved by Other Apps

Desktop Verify relies on each signer's layer still being in the file byte for byte. Re-saving a signed PDF in another app (an editor, a "Save as" from a viewer, a PDF optimiser) usually rewrites the whole file. The signers' layers can then no longer be found, and Verify reports the file as rewritten. It cannot confirm the content in that case, even if it looks the same. Keep and share the file exactly as Veylent saved it.

6. Legacy Files

Files signed with earlier Veylent formats did not anchor a hash of the saved file. Verify reports them as legacy and does not check them, and they can't be signed further.

7. Key Recovery

The key exists only in the QR code and in the signed file's signing history. If both are lost (for example, the only copy was printed and the paper is destroyed), the key is gone. The anchor remains on-chain, but the sealed record cannot be opened, so neither the signer details nor the document hash can be checked.

Comparison Matrix

Property	Veylent	PKI / X.509	Centralized Services	Notary
Tamper evidence	Yes	Yes	Yes	No
No trusted third party	Yes	No (CA)	No	No
Survives vendor shutdown	Yes	Partial	No	Yes
Privacy from platform	Yes	Yes	No	No
Identity verification	No	Yes (CA)	Yes	Yes
Revocation	No	Yes (CRL)	Yes	N/A
Offline signing	Yes	Yes	No	No
Legal recognition	Varies	Broad	Broad	Broad