Crypto Custody: How It Works and How to Choose a Custodian

By the Guardian team Updated November 2026 6 min read

Quick answer. Custody means controlling the keys that move client assets, so no one else can move them alone. Evaluate a custodian on its key lifecycle, signing controls, segregation, sub-custodians and dated independent assurance, not its marketing.

Watch: Crypto Custody: How It Works and How to Choose a Custodian 1:47

Video transcript

Every custody model answers one question. Who can move the assets, and what stops them?

Assets move when someone signs with the right key. US bank regulators say control means no other party, including the customer, can move them alone.

Four models decide who holds the keys: self-custody, a qualified custodian, sub-custody through a delegate, or shared control. Each carries its own main risk.

An HSM keeps keys in dedicated hardware. MPC shares sign jointly offchain. Multisig lets the chain enforce the threshold. None removes the signing path.

Protect the key across its life. Generate it in certified hardware where practicable. Distribute backups. Let no one person hold a whole key. Show signers full details. Plan for compromise.

Two documented losses came through the signing path, not the cryptography. In February 2025, Bybit lost about 1.5 billion dollars when a compromised Safe Wallet developer machine proposed a disguised transaction.

SOC 2 and CCSS help, within their scope. Ask for an opinion that controls are operating effectively, with key management covered. Neither is a code review.

Score custodians by evidence. Can anyone move assets alone? Is recovery tested? What does a signer actually see? Are assets segregated, who else holds keys, and can you leave?

Guardian reviews custodians as concentrations of risk, not logos on a slide. Read the full guide, or talk to Guardian at guardian.security.

Every custody model answers one question: who can move the assets, and what stops them?

US bank regulators put it plainly. Safekeeping crypto-assets "entails controlling the cryptographic keys".

Below: the models, the key lifecycle and a scorecard. No vendor rankings.

This guide explains the technical and security side of the regulatory texts it cites. It is not legal advice; confirm how it applies to you with counsel.

What Crypto Custody Is

A crypto-asset moves when someone signs with the right key. Whoever controls signing controls the asset.

In their July 14, 2025 joint statement, the OCC, Federal Reserve and FDIC describe control. A bank has it when it can show no other party, including the customer, has information enough to transfer the asset alone.

Taking possession of existing keys may not be control. The customer may have kept copies.

From October 2027, the FCA's CASS 17 will protect the same thing, the "means of access": a key, part of one, or anything else needed to transfer.

Custody Models Compared

First, who holds the keys.

Model	Who controls the keys	Main risk
Self-custody	Your own staff and systems	Your key operations, insiders and recovery
Qualified custodian	A regulated bank, trust company or broker-dealer	Custodian failure, commingling, slow access
Sub-custody	A third party appointed by your custodian	A delegate you never assessed
Shared control	You and a provider, each holding keys or shares	Recovery that depends on the provider

Qualified custodians

Under the SEC's custody rule, a qualified custodian is a bank or savings association, a registered broker-dealer, a futures commission merchant or certain foreign financial institutions.

On September 30, 2025, SEC staff granted no-action relief to advisers treating state trust companies as banks for crypto. Before engaging one, and yearly, the adviser checks that:

its state banking authority has authorized it to custody crypto;
it has written policies on private key management and cybersecurity;
its financial statements are audited, and an auditor's control report, such as SOC 1 or SOC 2, finds controls "operating effectively".

The custody agreement must bar lending or rehypothecation without written consent, and keep client assets segregated.

Sub-custody

In May 2025 the OCC confirmed that banks may use crypto sub-custodians, "subject to appropriate third-party risk management practices".

The joint statement says the bank remains responsible for the sub-custodian, subject to the customer agreement, and applies the same control standards to it.

Under the UK's CASS 17.6, the third party's jurisdiction must specifically regulate and supervise crypto safeguarding. The agreement must segregate assets, exclude set-off and require consent for further delegation.

Shared control

In shared models, client and provider each hold part of the signing power. In BitGo's self-custody multisig wallets, the customer can create the user and backup keys; BitGo "always creates and stores the BitGo key".

Recovery is the dividing line. Under Hong Kong's proposed custodian licence, an MPC provider would need none where clients can transfer their assets and can rebuild the key, or retrieve access, without it.

Key Technology: HSM, MPC, Multisig

Next, how the keys are protected.

Technology	How it works	Where it fails
HSM	Keys generated and used inside dedicated hardware	Whoever can instruct the HSM can sign
MPC	Key shares sign jointly offchain; one onchain signature	Share holders and the policy engine
Multisig	Separate keys sign; the chain enforces the threshold	Signers approving what they cannot see

NIST's FIPS 140-3 sets security requirements for cryptographic modules such as HSMs, at four increasing levels. The SFC expects HSM vendors to keep a patched HSM validated, with its certification updated promptly.

BitGo's docs class MPC as offchain cryptography and multisig as onchain. Fireblocks says its MPC distributes key shares "across environments".

Contract multisigs add surface. Safe's docs say modules could "allow the execution of transactions without requiring all confirmations".

None of these removes the signing path. The SFC says recent incidents point to vulnerabilities "regardless of the custody solutions used", naming HSMs, MPC and multisig.

The Key Lifecycle

NIST SP 800-57 splits a key's life into four phases: pre-operational, operational, post-operational and destroyed. A key never moves back a phase.

Controls by stage:

Stage	What good looks like	Source
Generation	Non-deterministic; where practicable, offline and in a certified HSM or similar	SFC VATP Guidelines 10.8(a)
Storage	Encrypted at rest; backups access-controlled	CCSS 1.03
Backup	Distributed; no key rebuilt from one location	SFC 10.8(d)
Access	No one person holds a whole seed, key or backup passphrase	SFC 10.8(c)
Use	Whitelists; full details human-readable before manual signing	SFC 25EC44
Compromise	Documented policy (Level I); tested yearly (Level III)	CCSS 2.04
Destruction	Recorded, with when and why	FCA CASS 17.4.8R

CASS 17.4.4R requires protection "throughout the entire life cycle". Our CASS 17.4 checklist and private key management guide cover each stage.

How Custody Gets Attacked

Two well-documented losses came through the signing path, not the cryptography.

On February 21, 2025, Bybit lost about $1.5 billion from its ETH multisig cold wallet. The FBI attributed the theft to North Korea.

The Safe Ecosystem Foundation said a compromised Safe{Wallet} developer machine proposed the disguised transaction. External forensic review found no vulnerabilities in the Safe smart contracts.

WazirX's July 18, 2024 preliminary report cited a "mismatch" between what Liminal's interface displayed and what was signed. Losses exceeded $230 million.

The SFC drew four weaknesses from overseas incidents:

malicious code injected into a third-party wallet interface;
weak access controls on approval devices;
inadequate systematic, independent transaction verification;
signers approving forged transactions without checking them.

Add insiders, lost backups and commingling sub-custodians.

Assurance: SOC 2 and CCSS

The AICPA's SOC 2 reports on controls relevant to security, availability, processing integrity, confidentiality or privacy.

Ask for an opinion that controls are "operating effectively", with key management in scope.

The CryptoCurrency Security Standard (CCSS), from the CryptoCurrency Certification Consortium, certifies systems, not companies, at Level 1, 2 or 3.

Neither is a code review. The SFC expects independent code review of third-party wallet software before onboarding and material changes.

How to Evaluate a Custodian

Score answers by evidence.

Area	Ask	Strong evidence
Control	Can anyone, including us, move assets alone?	Quorum map and signing policy; a reasoned no
Generation	How and where were the keys made?	Ceremony records, entropy source, HSM certificate
Recovery	Can you recover from backup alone?	A dated, timed recovery test
Signing path	What does a signer actually see?	Signer view against raw transaction, per type
Segregation	Are our assets separate and unencumbered?	No rehypothecation without consent; no set-off
Sub-custodians	Who else holds keys or shares?	Named delegates, our consent rights, their diligence
Assurance	What does each report cover, and when?	SOC 2 on operating effectiveness; scope; dates
Exit	Can we leave with our assets?	A tested withdrawal path

Add notice terms: US bank regulators suggest sub-custody contracts cover key compromise, use of backups and irreversible errors.

Our vendor due diligence questionnaire has every question.

How Guardian Helps

Guardian's institutional practice advises on the custody and signing model, and reviews custodians "as concentrations of risk rather than as logos on a slide".

Its OpSec audit covers custody and key ceremony: where keys live, who holds each share, and what a quorum requires in practice.

Its regulatory assessments level key management against CCSS, and assess custody against the rules your license carries.

Frequently Asked Questions

What is a qualified custodian for crypto?

Under the SEC custody rule, a bank or savings association, registered broker-dealer, futures commission merchant or certain foreign financial institution. Since September 30, 2025, SEC staff no-action relief lets advisers treat state trust companies as banks for crypto, with conditions.

Is MPC safer than multisig?

Neither wins outright. MPC signs offchain; multisig enforces the threshold onchain. The SFC says recent weaknesses apply whatever the technology, so judge the signing path and recovery.

What is crypto sub-custody?

Your custodian appoints a third party to hold the keys. US bank regulators say the bank stays responsible for its activities, subject to the customer agreement, and should assess its key management first.

Does a SOC 2 report prove a custodian is secure?

No. It covers the controls in its scope, for its period. Ask for an operating effectiveness opinion with key management in scope, then add code review and a recovery test.

Sources

Test your custody model before you commit to it

Guardian reviews custodians, key ceremonies and signing paths, and maps the evidence to the custody rules you answer to.

Talk to Guardian Explore Guardian for institutions