user139528: I was revising for my upcoming exam and came across this slide, which I don't completely understand as I find it a bit confusing. Can someone please explain what the lecturer was trying to point out?
I'm still new to the whole topic and I appreciate your clear explanations.
Edit:Thank you all for your answers!
They really helped me to understand the slide and Bitcoin transactions in general. I really appreciate it!


user5406: In Bitcoin, all transactions are public information. While the reason for transacting and the identities of the transacting parties remain private, the public information in the transaction graph can be used to infer clusters of addresses and transactions created by the same entity. Once such a cluster is discovered, a surveillant may be able to link either the entity directly or one of the counterparties to a real-world identity to learn even more about them.
One of the heuristics used to discover clusters is the common-input-ownership heuristic described on this slide. The base assumption is that all UTXOs spent in a single transaction were owned by the same party. The common-input-ownership heuristic may be broken when multiple users contribute inputs to a transaction in a CoinJoin transaction, which may confuse a surveillant into falsely categorizing two distinct wallets as owned by one  entity. However, a naïve CoinJoin transactions may still only permit a single interpretation of the subtransactions if subsets of inputs and outputs form up to matching amounts.
Combined with other data points from heuristics for change detection, wallet fingerprints, and graph heuristics like taint analysis, a sufficiently motivated surveillant will be able to clearly delineate many wallets’ activities from each other.

user13866: It says that if two "coins" are spent in one transaction, they probably belonged to the same person, or organisation.

If that isn't clear maybe it will help to review a little of what is what in Bitcoin.
In Bitcoin, the nearest thing to a coin is an unspent transaction output (UTXO). This is an output from a prior transaction that can be used by the recipient as an input to a subsequent transaction. Thus spending it.
Every transaction output, including the unspent ones (UTXOs) has a Bitcoin locking script associated with it. The address is an abstraction of that script. It is used by a payee to tell a payer how to construct a payement to them - how to create a suitable locking script in a transaction output section. One the payee can subsequently spend as a payer.
So two addresses, when each is associated with two inputs to a transaction, can be inferred to be likely to be under the control of one person.
Of course, people deliberately use "mixers" to prevent this heuristic from being effective. Nowadays, people ony ever use any address once, also to reduce this kind of intrusion into their financial privacy.

139528: <p>I was revising for my upcoming exam and came across this slide, which I don't completely understand as I find it a bit confusing. Can someone please explain what the lecturer was trying to point out?</p>
<p>I'm still new to the whole topic and I appreciate your clear explanations.</p>
<p>Edit:Thank you all for your answers!
They really helped me to understand the slide and Bitcoin transactions in general. I really appreciate it!</p>
<p><a href="https://i.stack.imgur.com/KRpLg.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/KRpLg.png" alt="Multi Input Transaction Heuristic" /></a></p>
 5406: <p>In Bitcoin, all transactions are public information. While the reason for transacting and the identities of the transacting parties remain private, the public information in the transaction graph can be used to infer clusters of addresses and transactions created by the same entity. Once such a cluster is discovered, a surveillant may be able to link either the entity directly or one of the counterparties to a real-world identity to learn even more about them.</p>
<p>One of the heuristics used to discover clusters is the <a href="https://en.bitcoin.it/wiki/Common-input-ownership_heuristic" rel="nofollow noreferrer"><em>common-input-ownership heuristic</em></a> described on this slide. The base assumption is that all UTXOs spent in a single transaction were owned by the same party. The common-input-ownership heuristic may be broken when multiple users contribute inputs to a transaction in a <a href="https://en.bitcoin.it/wiki/CoinJoin" rel="nofollow noreferrer"><em>CoinJoin</em></a> transaction, which may confuse a surveillant into falsely categorizing two distinct wallets as owned by one  entity. However, a naïve CoinJoin transactions may still only permit a single interpretation of the subtransactions if subsets of inputs and outputs form up to matching amounts.</p>
<p>Combined with other data points from heuristics for <em>change detection</em>, <em>wallet fingerprints</em>, and graph heuristics like <em>taint analysis</em>, a sufficiently motivated surveillant will be able to clearly delineate many wallets’ activities from each other.</p>
 13866: <p>It says that if two &quot;coins&quot; are spent in one transaction, they probably belonged to the same person, or organisation.</p>
<hr />
<p>If that isn't clear maybe it will help to review a little of what is what in Bitcoin.</p>
<p>In Bitcoin, the nearest thing to a coin is an unspent transaction output (UTXO). This is an output from a prior transaction that can be used by the recipient as an input to a subsequent transaction. Thus spending it.</p>
<p>Every transaction output, including the unspent ones (UTXOs) has a Bitcoin locking script associated with it. The address is an abstraction of that script. It is used by a payee to tell a payer how to construct a payement to them - how to create a suitable locking script in a transaction output section. One the payee can subsequently spend as a payer.</p>
<p>So two addresses, when each is associated with two inputs to a transaction, can be inferred to be likely to be under the control of one person.</p>
<p>Of course, people deliberately use &quot;mixers&quot; to prevent this heuristic from being effective. Nowadays, people ony ever use any address once, also to reduce this kind of intrusion into their financial privacy.</p>
