Cold wallet

The variants range from the hardware wallet, the most common, to the air-gapped computer, QR-code or SD-card signing (the unsigned transaction goes in, the signed one comes out) or the seed on a physical medium of paper or metal plate. The paper wallet is the most basic form and is now discouraged for active use. For institutions, deep cold storage combines multisig (m of n) or MPC, fund segregation and safekeeping in secure facilities.

It is the de facto standard for custodying large long-term balances, whether treasuries, exchange reserves or institutional portfolios, since it removes the remote attack vector: what is not connected cannot be drained online. The trade-off is lower liquidity and operational speed, hence a custodian's typical flow, keeping a small share in a hot wallet for daily activity and the vast majority in cold storage. The ratio between the two and proof-of-reserves are trust metrics in professional custody.

Examples include the cold reserves of most regulated exchanges, institutional custody from BitGo, Anchorage or Fireblocks, and the use of Ledger or Trezor as a cold wallet by individuals. For a company custodying client assets in the EU, a documented and audited cold-storage policy is part of the custody standard that MiCA passes on to authorized providers.