Co-Spending Heuristics Problems in Crypto Tracing
In civil asset forfeiture cases involving cryptocurrency, law enforcement officers (LEOs) routinely rely on blockchain analytics to trace funds and attribute activity to individuals. One of the most commonly cited techniques is the “co-spend heuristic,” which is an assumption that if multiple inputs are used in a single transaction, the same person or entity controls all corresponding private keys.
That assumption is often presented in affidavits and testimony as if it were a rule, but it is not. Treating that assumption as evidence introduces analytical error, overstates evidentiary weight, and creates serious due process concerns when property is seized based on probabilistic inference rather than demonstrable fact.
As a civil asset forfeiture attorney, I recently saw an affidavit in support of a request for a search warrant to seize cryptocurrency that claimed:
“While the identity of a Virtual Currency address owner is generally anonymous, law enforcement can identify the owner of a Virtual Currency address by analyzing the blockchain.
The analysis can also reveal additional public addresses controlled by the same individual or entity. For example, when using a UTXO based virtual currency, a user or business may create many Virtual Currency addresses to receive payments from different customers.
When the user wants to transact the virtual currency that he or she has received (e.g., to exchange Bitcoin for other currency or to use virtual currency to purchase goods or services), the user may group those addresses together to send a single transaction.
This is known as the co-spend heuristic and serves to verify singular ownership of all in-put public addresses.”
Such claims are not true for the reasons explained in this article.
What the Co-Spend Heuristic Actually Does
In a UTXO-based system like Bitcoin, a transaction may aggregate multiple inputs to fund a single output. The co-spend heuristic infers that because a transaction requires signing authority over each input, the entity initiating the transaction must control all associated private keys.
Properly framed, this is a conditional inference:
- It suggests possible common control at the moment of the transaction
- It does not establish identity
- It does not establish ownership
- It does not exclude alternative explanations
That nuance is routinely lost in law enforcement presentations.
Why LEOs Treat Heuristic as “Proof”
LEOs frequently convert a working assumption into a definitive conclusion. Language like “this verifies singular ownership” or “this identifies the owner” appears in affidavits, but those statements are technically indefensible.
A heuristic is not evidence of identity—it is a pattern recognition shortcut. Without corroboration, it cannot meet even a modest evidentiary threshold, much less support seizure or forfeiture.
The co-spend heuristic breaks down in several well-documented scenarios:
- CoinJoin and Collaborative Transactions
- Privacy-enhancing transactions are explicitly designed to defeat clustering heuristics. Multiple users jointly construct a transaction with multiple inputs and outputs, making it appear as though all inputs are controlled by a single entity when they are not.
- Exchange and Custodial Wallet Behavior
- Centralized exchanges and custodians routinely batch transactions, combining inputs from multiple customers into a single transaction. The heuristic will cluster those inputs together, but the underlying reality is the opposite of singular ownership—multiple beneficial owners, one custodian.
- Shared Wallets and Multi-Party Contro
- Business wallets, escrow arrangements, and multi-signature wallets may involve multiple parties with varying degrees of control. The heuristic cannot distinguish between unilateral control and coordinated authorization.
- Payment Processors and Aggregators
- Merchants using payment processors often have funds swept or aggregated across addresses. Again, clustering reflects operational mechanics, not ownership.
The Legal Distinction Between Control and Ownership
Even if the heuristic correctly infers common control, that still does not establish legal ownership. In forfeiture litigation, that distinction is decisive for the following reasons:
- A custodian may control private keys but hold assets on behalf of customers;
- A service provider may facilitate transactions without any ownership interest; or
- An intermediary may briefly control funds without any beneficial claim
LEOs frequently collapse these distinctions, treating technical control as synonymous with ownership. That leap is not supported by property law and is vulnerable to challenge.
Attribution Requires Off-Chain Evidence
Blockchain analysis alone cannot identify a person. Attribution requires external data sources include:
- KYC records from exchanges
- Subpoenaed account information
- IP logs or device forensics
- Admissions or communications
Without that corroboration, co-spend analysis remains unattributed clustering, not identification. When affidavits imply otherwise, they are overstating the capabilities of the methodology.
Lack of Error Disclosure
Another recurring issue is the absence of any discussion of error rates or limitations. Courts are often presented with conclusions stripped of uncertainty, giving the impression of scientific precision. Civil asset forfeiture attorneys representing innocent owners often find clustering heuristics are susceptible to:
- False positives (unrelated users grouped together);
- Over-clustering (large entities absorbing unrelated addresses); or
- Misinterpretation of transaction structure.
Failure to disclose these risks deprives the court of the ability to properly weigh the evidence.
Litigation Implications in Civil Asset Forfeiture Cases
When co-spend heuristics form the backbone of probable cause in an seizure warrant affidavit, the following analytical weaknesses become legally significant:
- Probable cause built on unverified assumptions is vulnerable to suppression or dismissal;
- Overstated certainty can undermine agent credibility;
- Lack of corroboration weakens the nexus between the property and alleged criminal conduct; and
- Mischaracterization of ownership can support an innocent owner defense.
In practice, effective challenges focus on forcing the seizing agency at the local, state or federal level to concede the following:
- The heuristic is not a rule;
- It is subject to known exceptions;
- It does not establish identity or ownership; and
- Independent verification is required.
Conclusions on Co-Spending Heuristics Problems
The co-spend heuristic is a useful investigative tool when applied cautiously and corroborated with independent evidence. It becomes problematic when LEOs present it as deterministic proof of ownership or identity.
In civil asset forfeiture—where the government seeks to take property often without a criminal conviction—that distinction matters. A probabilistic inference dressed up as certainty is not just a technical error; it is a constitutional problem.
From a forfeiture-defense standpoint, that paragraph overstates certainty and collapses multiple distinct analytical steps into a single, misleading conclusion. The key problems:
1. It equates heuristic clustering with “identification.”
Saying law enforcement can “identify the owner” by analyzing the blockchain is simply inaccurate. Blockchain analysis produces probabilistic inferences, not identity attribution. At best, it yields clusters of addresses that may be commonly controlled. Actual identification requires off-chain evidence (KYC records, subpoenas, admissions, device forensics). The phrase blurs that distinction.
2. It treats the co-spend heuristic as conclusive proof of “singular ownership.”
The statement that co-spend “serves to verify singular ownership” is technically and legally wrong. The co-spend heuristic is just that—a heuristic. It assumes that if multiple inputs are used in one transaction, the same entity controls all corresponding private keys. That assumption breaks in several real-world scenarios:
CoinJoin transactions (explicitly designed to defeat this heuristic)
Collaborative custody / shared wallets
Exchange batching (multiple customers’ UTXOs spent together)
Custodial services where the “controller” is not the beneficial owner
So at most, co-spend suggests common control in that transaction context, not “singular ownership,” and certainly not legal ownership.
3. It conflates “control” with “ownership.”
Even if the heuristic were valid, control of private keys ≠ legal ownership. That distinction matters directly in forfeiture cases (e.g., innocent owner defenses). The language skips that entirely.
4. It ignores known error rates and false positives.
There’s no acknowledgment that heuristics can produce incorrect clustering. Courts are increasingly sensitive to this—especially when the government presents tracing as deterministic.
5. It uses imprecise and biased phrasing (“can identify,” “serves to verify”).
Those verbs imply certainty and validation, when the underlying methodology is inferential and conditional. That’s exactly the kind of overstatement you want to attack.
