Time-lock is a cryptographic primitive that can be used as part of a commit & reveal scheme, to provide a guarantee that the information associated to the commit phase is eventually revealed.

For more background information on time-locks and their uses, see Timelocks in the Tezos documentation.

Cryptographic design

The time-lock features are supported in Octez by the Tezos_crypto.Timelock library .

Users first generate a RSA modulus and a symmetric encryption key. They use authenticated encryption to encrypt a packed Michelson value (an array of bytes computed with PACK) and encrypt that encryption key using a time-lock puzzle. They then combine the RSA modulus, the time-locked symmetric key, the constant T and the encrypted value as a single value as well (called chest in our library).

A proof of decryption can be the symmetric key itself. However, a malicious user could propose an authenticated ciphertext that does not yield a valid value even when decrypted with the symmetric key that was indeed time locked. To avoid this threat, an opening (called chest_key in our library) includes the symmetric key and a proof that the symmetric key proposed is indeed the one hidden in the time-lock puzzle. In this way one can differentiate whether the chest or the chest_key was proposed by a malicious user.

Opcode and types

To expose the features of this library, the Michelson language provides the following types:

  • chest, which represents time-locked arbitrary bytes with the necessary public parameters to open it.

  • chest_key, which represents the decryption key, alongside with a proof that the key is correct.

and the following opcode:

open_chest ::  chest_key chest time bytes option

open_chest takes a chest and a chest_key, and produces either the underlying plaintext or indicates that the chest_key is malicious. If the chest is not well-formed (which is possible since we use authenticated encryption), the empty Byte is returned.

Implementation of the time-lock puzzle

The implementation of the time-lock puzzle and proof scheme is located in src/lib_crypto/timelock.ml.

To facilitate the use of time-locks, commands have also been implemented in Octez client to generate a chest and chest_key as well as to open and verify them. An additional command precompute was implemented to fasten the time-lock chest generation.

For more information on the client commands, see cli-commands.


The built-in coin flip contract gives an example of using time-lock. Beware this contract is for educational purpose only and is not secure.