Study Guide
Blockchain and Hash-Linked Records
What a blockchain is underneath the headlines: records grouped into blocks, each block carrying the hash of the one before, so that a change is evident; what many copies and consensus add; and why a blockchain is not necessarily public, and a central bank digital currency does not need one.
Lesson 6.11 Cryptography & PKI
A blockchain is a way of keeping a shared record that is very hard to change unnoticed. This lesson works out how it does that, what it needs besides hashing, and when it is worth using at all.
Stage: Think
A trip register
Think it through
A class keeps a paper register of who has paid for a trip. Someone could quietly change an old entry. How could you design the register so that any change to an old entry would be noticed?
- Hint 1: What would a later page need to know about an earlier one?
Show what happens
Make each page carry a fingerprint of the page before it, so that changing an old page breaks its link to the next one; and give everyone a copy, so that one person’s altered register disagrees with all the others.
Think it through
Show the answer
Answer: B. Yes, with enough effort.
Yes: with enough effort they could redo every later page. Nobody else would have a copy to compare with.
Stage: Explain
The idea
A ledger is a record of transactions: who paid whom, who owns what, what happened and in what order. Traditionally one trusted organisation, such as a bank or a land registry, keeps the ledger, and everyone relies on that organisation not to alter it.
A blockchain is a ledger made of blocks of records, in which each block contains the hash of the block before it.
- Records are collected into a block.
- The block also stores the hash of the previous block.
- The block’s own hash is computed from everything in it, including that stored hash.
Check your understanding
Show the answer
Answer: B. The previous block’s hash
The hash of the block before it, which chains it to every earlier block.
Each block is therefore chained to the one before, and through it to every earlier block. The security of the whole structure rests on one property of a hash function: change the input at all, and the digest changes completely.
Stage: Experiment
See a change break the chain
The four blocks in the panel record payments between three fictional people. Use the button to alter one earlier payment and watch what follows.
Think it through
Before pressing the button: if Block 2 is changed, which blocks will show a problem, and why?
- Hint 1: Which block stores Block 2’s hash?
Show what happens
Block 2’s hash changes, so Block 3, which still stores the old hash, no longer matches, and everything after it is unverified.
Each block stores the hash of the block before it. Changing the data in one block changes that block’s hash, and the next block is left holding a hash that no longer matches.
The chain is intact: every stored hash matches the block before it.
Block 1
Data Opening balances: Alice 50, Bob 20, Carol 0
Previous hash
none (first block)This block’s hash
1e0730f107Link: none needed for the first block.
Block 2
Data Alice pays Bob 10
Previous hash
1e0730f107This block’s hash
9893926f2dLink: matches the hash of Block 1.
Block 3
Data Bob pays Carol 5
Previous hash
9893926f2dThis block’s hash
510b9932ffLink: matches the hash of Block 2.
Block 4
Data Carol pays Alice 2
Previous hash
510b9932ffThis block’s hash
5906d79372Link: matches the hash of Block 3.
Hashes are real SHA-256 values of the previous hash followed by the data, shortened to ten characters for display. If someone changed Block 2 to read “Alice pays Bob 1”, its hash would change, and Block 3 would still hold the old hash, so the chain would no longer verify.
Stage: Explain
Why hash-linking makes a change evident
Suppose Alice wants to pretend she paid Bob 1 instead of 10, and edits Block 2.
- Block 2’s contents have changed, so Block 2’s hash changes.
- Block 3 still stores the old hash of Block 2. The two no longer match.
- To hide that, Alice would have to change the hash stored in Block 3. But that changes Block 3’s contents, so Block 3’s hash changes, and now Block 4 does not match.
- The mismatch runs forward to the newest block. She would have to recompute every block after the one she changed.
Check your understanding
Show the answer
Answer: B. Blocks 2 to 10
Blocks 2 to 10: every block after the change must be recomputed.
So an edit to old data cannot be made in one place. It forces a rewrite of everything that came after. This is why a blockchain is described as tamper-evident and append-only: new blocks are added at the end, and the past is not edited.
Stage: Explain
What hash-linking alone does not do
Step 4 of Alice’s rewrite is laborious, not impossible. If Alice were the only person holding the ledger, she could change Block 2, recompute every later hash and present a chain that verifies perfectly. Nobody would hold an older copy to compare.
Hash-linking only makes a change evident to someone who has something to compare with. On its own it is not what makes a blockchain hard to rewrite.
Stage: Predict
Many copies and consensus
Think it through
Show the answer
Answer: C. It is outvoted.
Her copy disagrees with all the others, and the agreed rules reject it.
The second ingredient is that the ledger is distributed. Many independent participants each hold a full copy, and they follow agreed rules for deciding which new block is added next. Reaching that agreement is called consensus.
Now Alice’s problem is different. She can rewrite her own copy, but everyone else still holds the original. Her version disagrees with theirs, and the rules tell honest participants to reject it. To succeed she would have to control enough of the network to outvote or out-compute everybody else.
Hash-linking makes tampering evident. Distribution and consensus make it impractical. A blockchain needs both.
Digital signatures supply the third ingredient. Each transaction is signed with its sender’s private key, so the ledger records not only what happened but evidence of who authorised it.
Go deeper How participants agree: two approaches
There are many consensus methods. Two are best known.
- Proof of work. Participants compete to solve a hard computational puzzle, and the winner adds the next block. Rewriting history means redoing that work for every later block faster than the rest of the network combined. It is secure and uses a great deal of electricity.
- Proof of stake. Participants lock up some of the system’s currency as a deposit and are chosen to add blocks in proportion to it. Cheating risks losing the deposit. It uses far less energy.
Private, permissioned blockchains often use simpler voting methods among their known members, because the participants are already identified and accountable.
Stage: Predict
Public and permissioned blockchains
A blockchain is a data structure and a set of rules. Who is allowed to take part is a separate choice.
- An open public ledger can be read by anyone, and anyone may join the network and help to validate new blocks. No permission is needed. The best-known cryptocurrencies work this way. It is the form the Security+ objectives name.
- A permissioned blockchain, also called a private blockchain, restricts who may read it, who may submit transactions and who may validate blocks. A group of banks, or the companies in one supply chain, might run one among themselves.
Do not confuse: Open public ledger and permissioned blockchain
- Who can join?Open public ledger
- Anyone can read it and take part in validating it. Participants are not identified in advance.A public cryptocurrency.
- Who can join?Permissioned blockchain
- Only approved, identified participants can read, write or validate, as the operators decide.A ledger shared among the firms in one supply chain.
Both are blockchains. A blockchain is not necessarily public.
Think it through
Show the answer
Answer: A. Not really.
Not really: every transaction is visible for ever, and addresses can often be linked to the people behind them.
A “public” ledger is also not an anonymous one. Every transaction is visible to everyone, permanently. Participants appear as addresses instead of names, which is pseudonymous at best.
Stage: Predict
Where the idea is used, and where it is not needed
Think it through
When would a shared, tamper-evident ledger be worth the trouble, and when would an ordinary database do?
- Hint 1: Who would hold the record if nobody trusted anyone else?
Show what happens
When several parties who do not fully trust each other need one shared record that none of them can alter alone. When one trusted organisation keeps the record, an ordinary database is simpler.
Blockchains suit a particular situation: several parties who do not fully trust each other need one shared record, and none of them should be able to alter it alone.
- Cryptocurrencies use an open public ledger to record who owns what without a central bank.
- Supply chains can record each handover of goods, so that every party sees the same history.
- Shared registers between organisations can avoid each one keeping its own, slightly different, copy.
When one trusted organisation already keeps the record, an ordinary database, with access control, backups and audit logs, is simpler, faster and cheaper.
A central bank digital currency does not need a blockchain
A central bank digital currency (CBDC) is digital money issued by a central bank, the electronic equivalent of banknotes. Because cryptocurrencies run on blockchains, it is often assumed that a CBDC must too.
Check your understanding
Show the answer
Answer: B. Not necessarily.
Not necessarily: it is itself the trusted central party, so a conventional database can serve.
It does not. A central bank is exactly the trusted central party that a blockchain is designed to do without, so it can keep its ledger in a conventional database. The Bank of England, for example, describes a possible digital pound as something that would not be a cryptoasset, and its technical work has treated a centralised database and distributed ledger technology as alternative ways to build the core ledger. Whether to use a blockchain is a design choice, not part of the definition.
Stage: Predict
Limits worth knowing
Think it through
Show the answer
Answer: B. Only that nobody altered it.
Only that the record has not changed since it was added, not that it was true.
- Recorded is not the same as true. A blockchain shows that a record has not changed since it was added. It cannot show that the record was accurate in the first place.
- It is not confidential by design. On a public ledger everything is visible. Confidentiality has to be added separately.
- Permanence cuts both ways. Data that cannot be changed cannot be corrected or deleted either, which sits awkwardly with data protection rights. Personal data should not be written into a blockchain without careful thought.
- Keys still matter. Control of an entry rests on a private key. A lost key means lost access, with no administrator to reset it, and a stolen key means stolen assets.
Stage: Explain
Security+ terminology
The CompTIA Security+ SY0-701 exam objectives (objective 1.4) list Blockchain and Open public ledger as separate bullets.
The objectives do not list permissioned blockchains, consensus methods or central bank digital currencies; they are included here because “blockchain” and “public ledger” are so often treated as the same thing.
Stage: Apply
Apply it in a new situation
These questions use the lesson in situations it has not shown. For the strongest test of your understanding, come back to them on another day and answer each one at the first attempt.
Apply it
Show the answer
Answer: B. It cannot be erased.
Data that cannot be changed cannot be corrected or deleted either, which sits awkwardly with data-protection rights.
Apply it
Show the answer
Answer: C. Nobody
Nobody: control rests on the key, and there is no administrator to reset it.
Apply it
Show the answer
Answer: B. An ordinary database
An ordinary database with access control, backups and audit logs: one trusted party keeps the record.
Stage: Check
Check your understanding
These questions come from the approved revision bank. They were approved under owner-authorised delegated review: the site owner authorised the review and did not read each question individually.
Show answer
Answer: Each block contains the hash of the previous block
A blockchain is a ledger made of blocks of records in which each block includes the hash of the block before it. The link is a hash, not encryption: the records are not hidden by it. No central authority is needed to sign blocks, and a block stores a short hash of its predecessor, not a copy.
Revision sources: Session 9: Cryptographic Algorithms, Public Key Infrastructure and Cryptographic Solutions (Classroom session record) · Blockchain and Hash-Linked Records (Supplementary Study Guide explanation)
Show answer
Answer: That block’s hash changes, so the next block’s stored hash no longer matches
Each block includes the hash of the block before it, so changing an earlier block changes its hash and breaks every later link. The mismatch is what makes the alteration evident. Hash-linking involves no encryption or revocation, and a block’s hash is computed from its own contents.
Revision sources: Blockchain and Hash-Linked Records (Supplementary Study Guide explanation)
Show answer
Answer: True
True. Hash-linking alone only makes a change evident; it is the many independently held copies, kept in agreement by consensus, that make tampering hard. A sole holder could alter a block and recompute every later hash, and nobody would have an older copy to compare.
Revision sources: Blockchain and Hash-Linked Records (Supplementary Study Guide explanation)
Show answer
Answer: Many participants hold copies and agree on new blocks by consensus
Copies of the ledger are held by many participants who agree on new blocks by a consensus process, which is what makes tampering detectable and hard without central control. Someone who rewrites their own copy is simply out of step with everyone else. Encryption, deleting old blocks and secrecy play no part in this.
Revision sources: Blockchain and Hash-Linked Records (Supplementary Study Guide explanation)
Show answer
Answer: Anyone may read and take part in the first; the second restricts who may do so
An open public ledger can be read and joined by anyone, while a permissioned or private blockchain restricts who may read, write or validate. Both use hash-linked blocks and both are normally held by several participants; the difference is who is allowed in.
Revision sources: Session 9: Cryptographic Algorithms, Public Key Infrastructure and Cryptographic Solutions (Classroom session record) · Blockchain and Hash-Linked Records (Supplementary Study Guide explanation)
Show answer
Answer: False
False. An open public ledger can be read and joined by anyone, while a permissioned or private blockchain restricts who may read, write or validate. A blockchain is therefore not necessarily public: the hash-linked structure is one thing, and who is allowed to take part is another.
Revision sources: Blockchain and Hash-Linked Records (Supplementary Study Guide explanation)
Show answer
Answer: False
False. A central bank digital currency does not inherently require a blockchain. A central bank is a trusted central operator, so it can keep its ledger in a conventional database; whether to use distributed ledger technology is a design choice, not a requirement.
Revision sources: Blockchain and Hash-Linked Records (Supplementary Study Guide explanation)
Study and revision
Practise through the Cryptography & Data Protection topic. Questions cover what links the blocks, what happens when an early block is altered, what hash-linking does not do alone, the role of consensus, open and permissioned ledgers, and whether a central bank digital currency needs a blockchain; three flashcards recall the main points. Most are learning-only. The questions that have been verified against CompTIA Security+ SY0-701 objective 1.4 are also in the Security+ practice track; that does not mean the lesson covers the whole of an objective, and the lesson does not establish practical experience.
Stage: Consolidate
What to remember
- A blockchain is a ledger of blocks, each storing the hash of the one before.
- Changing an old block breaks every later link; hash-linking makes tampering evident.
- Many copies and consensus make tampering impractical; signatures add evidence of who acted.
- Open public ledgers let anyone join; permissioned ones restrict who takes part; public is not anonymous.
- Use a blockchain only when untrusting parties share a record; recorded is not the same as true.
Key terms
- Blockchain
A ledger made of blocks of records in which each block includes the hash of the block before it.
- Open public ledger
A ledger that anyone can read and anyone may join to help validate new entries, with no permission needed.
- Hash function
A function that turns input of any size into a fixed-length digest. It is one-way and uses no key.
- Consensus
The agreed process by which the participants in a distributed ledger decide which new block is added, so that every honest copy stays the same.
Common misunderstandings
An open public ledger is one kind. A permissioned blockchain restricts who may read, write and validate.
Blocks are linked by hashes, not hidden by encryption. On a public ledger the records are readable by everyone.
The links make a change evident. What makes rewriting impractical is that many independent participants hold copies and agree by consensus.
A central bank digital currency does not inherently require a blockchain. A central bank can keep its ledger in a conventional database.
Further reading
- NIST IR 8202: Blockchain Technology Overview: a vendor-neutral explanation of blocks, hashing, consensus and permission models.
- Bank of England: The digital pound: the Bank’s description of a possible UK central bank digital currency.
- CompTIA Security+ SY0-701 exam objectives (PDF): objective 1.4.
Lesson sources and acknowledgements
Sources and scope
- Covered in Session 9 (see the Session 9 Learning Log): blockchain and hash-linked records.
- Accuracy qualifications: a blockchain does not necessarily use a public ledger; a central bank digital currency does not inherently require a blockchain; hash-linking makes a change evident and does not by itself prevent rewriting.
- Independently checked: the structure of a blockchain and the distinction between permissionless and permissioned systems against NIST IR 8202; the position on a digital pound against the Bank of England’s own publications; and the Security+ wording against the published SY0-701 objectives. These are external references, not course evidence.
- Supplementary explanation: every definition, the interactive four-block chain, the step-by-step account of an edit, the point about a sole holder, distribution and consensus, the two consensus methods, the comparison of public and permissioned ledgers, the uses, the central bank digital currency section, the limits and the misunderstandings. These were developed afterwards and are my explanations, not the tutor’s words. The diagram is original.
- Guided version (published 1 November 2026, awaiting the site owner’s editorial review): the trip-register opening questions, every question asked in the lesson with its feedback and hints, the application questions and the closing summary are supplementary explanation written for the Study Guide. The settings are fictional. The questions in the lesson’s steps are not revision-bank items; those in “Check your understanding” are, unchanged.
- No practical work is claimed. Alice, Bob, Carol and their payments are fictional. I have not built, operated or analysed a blockchain as part of this lesson, and nothing here is financial advice.
Something an organisation values, such as information, a service, equipment or a business capability.
A record of which account did what and when on a system, such as sign-ins, changes and approvals.
A ledger made of blocks of records in which each block includes the hash of the block before it.
Digital money issued by a central bank: the electronic equivalent of banknotes.
The security objective of restricting information to authorised people, systems or processes.
The agreed process by which the participants in a distributed ledger decide which new block is added, so that every honest copy stays the same.
A measure intended to modify risk, for example by restricting access, detecting misuse or supporting recovery.
A value produced with a private key that only the signer should hold, which anyone can check with the matching public key to confirm who signed some content and that it has not changed since.
Transforming data with a cipher and a key so that it cannot be read without the right key. Decryption reverses it.
A function that turns input of any size into a fixed-length digest. It is one-way and uses no key.
A record of transactions: who paid whom, who owns what, and in what order things happened.
The fixed-length output of a hash function: a short fingerprint of the input.
A ledger that anyone can read and anyone may join to help validate new entries, with no permission needed.
A blockchain that restricts who may read it, submit transactions or validate blocks, as its operators decide.
Information relating to an identified or identifiable living individual.
The half of a key pair that its owner keeps secret. It decrypts data sent to the owner and creates the owner’s signatures.
Built so that any change shows. A tamper-evident record does not prevent alteration; it makes alteration detectable.
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