What Is CLTV (CheckLockTimeVerify)?
CLTV (CheckLockTimeVerify) is a Bitcoin script opcode (BIP 65) that prevents a UTXO from being spent until a specific block height or Unix timestamp is reached. It enables time-locked transactions, ensuring funds remain locked until a predetermined future date.
Absolute Timelocks
CLTV creates absolute timelocks specified in either block height (e.g., block 800,000) or Unix time (e.g., January 1, 2025). Once that point is reached, the funds can be spent according to other conditions in the script.
Use Cases
Inheritance & Vaults
CLTV enables time-delayed recovery mechanisms. For example, funds could require multiple signatures normally, but allow a single backup key to spend after 6 months of inactivity.
Payment Channels
The Lightning Network uses CLTV in HTLCs to ensure that payment routes have deadlines, preventing funds from being locked indefinitely.
CLTV works alongside CSV (CheckSequenceVerify), which provides relative timelocks. Together, they enable sophisticated smart contracts and security mechanisms. Tools like Miniscript make CLTV easier to use safely in complex spending conditions.
Quick Facts
- •Introduced via BIP65 in 2015
- •Enables absolute time locks
- •Used in Lightning Network and smart contracts