---
title: "Hashes, Signatures and Certificates"
description: "A cryptographic hash gives a short fingerprint that is impractical to forge a match for, which makes tampering detectable. Signing a hash with a private key proves who produced it, and a certificate i"
canonical: https://lightmysky.com/learn/computing/hashes-signatures-and-certificates-mt_qLHm767U87
source: https://lightmysky.com/learn/computing/hashes-signatures-and-certificates-mt_qLHm767U87.md
retrieved: 2026-11-12
---

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# Hashes, Signatures and Certificates

A cryptographic hash gives a short fingerprint that is impractical to forge a match for, which makes tampering detectable. Signing a hash with a private key proves who produced it, and a certificate is that signature applied to the claim that a key belongs to a name.

Subject: Computing · Area: Networks & Security · Ages 21 to 22
Page: https://lightmysky.com/learn/computing/hashes-signatures-and-certificates-mt_qLHm767U87

## Ready when they can

- Say what property of a hash makes it usable for detecting tampering
- Explain what a signature proves and what it does not
- Follow a certificate chain up to a root and say what trust is being placed where

## Lesson: Fingerprints, seals, and chains of trust

A hash is a short fingerprint of some data. Change one letter and the fingerprint comes out completely different. It is impractical to craft a different input with the same fingerprint, so when you recompute it, tampering stands out.

*(drawing: The posted print froze the original. A different recomputed print means the data changed.)*

A signature is that fingerprint signed with someone's private key, which anyone checks with the matching public key. It proves who produced the statement. It does not prove the statement is true or the site is safe. A certificate applies that signature to the claim that a key belongs to a name.

Your browser climbs the chain link by link: each signature, the requested name, and the dates. Trust lands on the root stored in your computer, not on the website alone.

*(drawing: Trust flows down from a root you hold. Each link vouches for the next, ending at the root keeper.)*

A signature names its author and no one else. Only the private-key holder could have signed it, checked with the public key, so the signature proves who produced the statement. It says nothing about truth.

A certificate signs one claim: a key belongs to a name. Design and price are never signed. Read the signed line to see the binding.

**Recap.** Hashes expose change, signatures name the author, and the chain moves your trust up to a root you already hold.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Public-Key Cryptography and Key Exchange](https://lightmysky.com/learn/computing/public-key-cryptography-and-key-exchange-mt_AHCtrer4eh)
- [Modular Arithmetic and Congruence Classes](https://lightmysky.com/learn/mathematics/modular-arithmetic-and-congruence-classes-mt_Lt4_Aazx2j)

## Optional connections

- [Hash Tables and Near-Constant Lookup](https://lightmysky.com/learn/computing/hash-tables-and-near-constant-lookup-mt_7v94MJEdmJ)
- [Rings, Fields and Their First Properties](https://lightmysky.com/learn/mathematics/rings-fields-and-their-first-properties-mt_BmhIook13o)

## Opens up

- [TLS: Putting the Pieces Together](https://lightmysky.com/learn/computing/tls-putting-the-pieces-together-mt_JzIwi-BYG0)
- [Authentication, Authorisation and Secure Defaults](https://lightmysky.com/learn/computing/authentication-authorisation-and-secure-defaults-mt_ofIiD4Bbpk)