Fingerprints and signatures

The situation

A supplier emails a PDF with new bank details for their invoices. The logo is right, the tone is right, the contact name is right. Your accounts team is about to update the payment record.

How could anyone know that this document really came from the supplier, and that nobody changed a single digit on the way?

What is going on

Fingerprints: hashes

A hash function turns any data, a word or a whole film, into a short fixed-size value called a hash. People also call it a fingerprint. SHA-256, the most common one today, always produces 256 bits, usually shown as 64 hexadecimal characters.

Three properties make hashes useful:

• Same input, same hash. Anyone can recompute it and compare.
• Any change gives a completely different hash. Change one comma in a 50-page contract and the hash looks unrelated to the old one.
• One way. You cannot work back from the hash to the data, and finding two different inputs with the same hash must be infeasible. Older hashes such as MD5 and SHA-1 lost that last property, which is why they are retired for signatures.

A hash uses no key, so it is not encryption, and there is nothing to “decrypt”. It also proves nothing about who sent the data: anyone who changes the document can simply compute a new hash for it.

Signatures: proof with a private key

A digital signature adds the “who”. It builds on the key pair from lesson 2:

1. The document is hashed. Most signature algorithms do this as their first step.
2. A signature algorithm, such as ECDSA, combines that hash with the signer’s private key to produce the signature.
3. Anyone with the matching public key runs the verification algorithm on the document and the signature.

Verification passes only if the document is unchanged and the signature was made with the private key that matches that public key. Change one byte and it fails.

Swipe sideways to see the whole diagram.

What a signature proves, and what it does not

A valid signature proves two things: the data has not changed since it was signed, and it was signed with one particular private key. It does not prove who owns that key. If a fraudster sends you their own public key labelled with your supplier’s name, their signatures will verify perfectly. Lesson 4 closes that gap with certificates.

For the invoice email in the situation, that means a signature would only help if you already had the supplier’s genuine public key. Until then, the safe check is a call to a phone number you already knew, not one in the email.

Try it

Try it: Certificate decoderOpen the full tool

Runs in your browser. Nothing you paste is sent anywhere.

Paste a certificate

Try a sampleChoose a sample…A website certificate (www.example.com)An expired certificateA wildcard certificate (*.example.com)A certificate with a Common Name but no SANA self-signed root CAA complete chain: leaf, intermediate and root

-----BEGIN CERTIFICATE----- MIIC1zCCAnygAwIBAgIQWTTOzqWiPshGdQhNb6kmNjAKBggqhkjOPQQDAjB+MQsw CQYDVQQGEwJCRTEZMBcGA1UEChMQUEtJIFplcm8gdG8gSGVybzEnMCUGA1UECxMe U2FtcGxlcyAtIG5vdCBwdWJsaWNseSB0cnVzdGVkMSswKQYDVQQDEyJQS0kgWmVy byB0byBIZXJvIFNhbXBsZSBJc3N1aW5nIENBMB4XDTI2MTAwMTAwMDAwMFoXDTM2 MTAwMTAwMDAwMFowGjEYMBYGA1UEAxMPd3d3LmV4YW1wbGUuY29tMFkwEwYHKoZI zj0CAQYIKoZIzj0DAQcDQgAEw4og6Y/SOr+WrqSv+ng7tOBEsc5N3A3yZRk+9hsR XzPQvB9672aHZHTYFNlSY93RWVcTRieE+8XnIbWIYfwxIqOCAT4wggE6MAwGA1Ud EwEB/wQCMAAwDgYDVR0PAQH/BAQDAgeAMBMGA1UdJQQMMAoGCCsGAQUFBwMBMCcG A1UdEQQgMB6CD3d3dy5leGFtcGxlLmNvbYILZXhhbXBsZS5jb20wHQYDVR0OBBYE FALMipe4vcXJTpwO926S15Y/bobzMB8GA1UdIwQYMBaAFN+ph37v40SN23r6Fg/z OvSNjJqeMEgGCCsGAQUFBwEBBDwwOjA4BggrBgEFBQcwAoYsaHR0cDovL3BraS5l eGFtcGxlLmNvbS9zYW1wbGUtaXNzdWluZy1jYS5jcnQwPQYDVR0fBDYwNDAyoDCg LoYsaHR0cDovL3BraS5leGFtcGxlLmNvbS9zYW1wbGUtaXNzdWluZy1jYS5jcmww EwYDVR0gBAwwCjAIBgZngQwBAgEwCgYIKoZIzj0EAwIDSQAwRgIhAM5dJUPhL+qV artvWtQinfnykhVflJTjMXe8M6PnqQfaAiEAhOLGj1hSEkNxQoMrYNXRaIe69nq6 2unibVFSVw1AC80= -----END CERTIFICATE-----

PEM text, several PEM blocks, or a .pem, .crt, .cer or .der file (drop it on the box). Certificates are public, so they are safe to paste.

DecodeClearChoose a file

A certificate is itself a signed document. Find its signature algorithm (a hash plus a signature method, for example ECDSA with SHA-256) and the signature value at the end, then find the certificate’s fingerprint: one short hash that identifies the whole certificate.

Check

Check yourself

You change one comma in a 50-page contract and compute its SHA-256 hash again. What happens to the hash?

1. One or two characters change
2. It stays the same, because the meaning did not change
3. It becomes completely different

Show answer

Answer C It becomes completely different

What happened

The new hash looks completely unrelated to the old one.

Why

Hash functions are designed so that any change to the input, however small, changes the output unpredictably. That is what makes a hash a reliable fingerprint.

The rule

Same data gives the same hash; any change gives a different hash.

Check yourself

A document carries a valid digital signature. What does that prove?

1. That it was signed with the private key matching the public key you used, and has not changed since
2. That the document is secret
3. That the signer is honest and the content is true

Show answer

Answer A That it was signed with the private key matching the public key you used, and has not changed since

What happened

It proves integrity (nothing changed) and that the matching private key made the signature.

Why

A signature hides nothing and says nothing about honesty. It ties the document to a key, and only to that key.

The rule

A signature proves which key signed and that the data is unchanged; who owns the key is a separate question.

Check yourself

A colleague says: 'Signing just means encrypting the document with your private key.' What is the best correction?

1. That is right for all signature algorithms
2. Signing runs a signature algorithm with the private key that produces proof, not secrecy; anyone with the public key can verify, and the document stays readable
3. Signing uses the public key and verifying uses the private key

Show answer

Answer B Signing runs a signature algorithm with the private key that produces proof, not secrecy; anyone with the public key can verify, and the document stays readable

What happened

Signing and encrypting are different operations with different goals.

Why

The 'encrypt with the private key' picture comes from a shortcut sometimes used to explain RSA, where both operations use the same maths. Even there the padding and the purpose differ, algorithms such as ECDSA encrypt nothing at all, and the picture suggests the document is hidden, which it is not.

The rule

Sign with the private key, verify with the public key. A signature is not encryption.

Takeaways

• A hash is a fixed-size fingerprint: any change gives a new hash, and with no key involved it is not encryption.
• Sign with the private key, verify with the public key. A valid signature means “made with that key and unchanged since”.
• A signature proves which key signed, not who owns the key. That is the next lesson.

Go further

• Handbook: Hashes and fingerprints
• Handbook: Digital signatures
• Next lesson: Whose key is it?

In the gameLevel 5: FingerprintsScribes' Hall, Keystone. In development