Digital signature

Not an image of a signature

The most common use of the phrase is the one it does not mean. A scanned handwritten signature dropped into a document is an image. It can be copied out of one certificate and into another in under a minute, and it says nothing about whether the document around it has been altered. Nearly every forged certificate carries a perfectly convincing signature image.

A digital signature is not a picture and is not visible. It is a value, typically a long string of characters, computed from two inputs: the exact contents being signed, and a private key that only the signer holds. Change either input and you get a different value.

How the check works

Three steps, and the third is the interesting one.

• Signing. The issuer takes the credential's contents in a fixed, unambiguous form and computes a signature over it using their private key. The signature is stored with the credential.
• Publishing. The matching public key is published. It can verify signatures but cannot produce them, so publishing it is safe and is the point.
• Verifying. Anyone reconstructs the credential's contents, takes the signature, and checks the pair against the public key. It either confirms or it does not. There is no partial result and no judgement involved.

That asymmetry is what lets a total stranger check a credential without being trusted with anything. One key signs, a different key verifies, and you cannot work out the first from the second.

What a valid signature proves

Exactly two things, and it is worth being strict about them.

• Integrity. The contents have not changed since signing. Not one character. A signature over a certificate naming Adaeze covers the name, the award, the issuer, and the date together, so altering any of them breaks it.
• Authenticity. The signature was produced by whoever holds the private key. If that key belongs to the issuing organisation, the credential came from them.

And what it does not

This is where the term is oversold, so the limits deserve as much space as the capabilities.

• It says nothing about the issuer's standards. A diploma mill can sign its certificates perfectly. The signature confirms the certificate is genuinely theirs; whether that is worth anything is a separate question.
• It does not identify the holder. Verification confirms the record, not the person presenting it. Spotting a fake certificate covers impersonation, which is a different problem with a different answer.
• It depends on key custody. A signature is only as meaningful as the secrecy of the private key. A stolen key produces valid signatures on fraudulent credentials, which is why how keys are stored matters as much as which algorithm signs with them.

What we use

Credentials here are signed with Ed25519, a modern elliptic-curve scheme chosen for small signatures, fast verification, and a design that avoids several implementation mistakes older schemes invite. Private keys are held in an encrypted vault, separate from the credential data they sign, and are never present in anything sent to a browser. The security page sets out the scheme, the key custody arrangement, and its limits in full, including the parts we would rather were stronger.

The signature is what makes a credential a verifiable credential rather than a web page asserting something.