Time-lock¶
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 by the Mavryk_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 Mavkit 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.
Example¶
The coin flip contract gives an example of using time-lock. Beware this contract is for educational purpose only and is not secure.