Multi-party computation.

Custody used to mean one private key in one place, guarded by whatever wall you built around it. Multi-party computation, or MPC, removes the single place. The key is split into shares held by separate parties, and a transaction is signed by those shares working together, without the full key ever being assembled.

Picture a safe that opens only when three people turn their own dials at once, and no single dial is the combination. An attacker who steals one share gets a fragment that signs nothing. Forging a signature means compromising several holders at the same moment.

MPC moves the risk into the coordination between share holders. Security depends on who holds a share, how each one authorizes a signature, and whether the shares are truly independent.

If those shares sit on the same server or answer to the same admin, the math splits the key but the setup still gives an attacker one door.