---
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: 2026-09-12
---

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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.

**Example.** You download a file plus its posted fingerprint. You hash the download and the two match, so nothing changed on the way. If one byte had flipped, your fingerprint would differ and you would know.

A signature is that fingerprint locked with someone's private key, which 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.

**Tip.** 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.

**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

- [Hash Tables and Near-Constant Lookup](https://lightmysky.com/learn/computing/hash-tables-and-near-constant-lookup-mt_7v94MJEdmJ)
- [Public-Key Cryptography and Key Exchange](https://lightmysky.com/learn/computing/public-key-cryptography-and-key-exchange-mt_AHCtrer4eh)
- [Rings, Fields and Their First Properties](https://lightmysky.com/learn/mathematics/rings-fields-and-their-first-properties-mt_BmhIook13o)
- [Modular Arithmetic and Congruence Classes](https://lightmysky.com/learn/mathematics/modular-arithmetic-and-congruence-classes-mt_Lt4_Aazx2j)

## 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)