Decrypt and reencrypt

Decrypt

We allow explicit decryption requests for any encrypted type. The values are decrypted with the network private key (the threshold decryption protocol is in the works).

Example

function getTotalSupply() public view returns (uint32) {
  return TFHE.decrypt(totalSupply);
}

function revertIfConditionIsFalse(ebool condition) public {
  bool plaintextCondition = TFHE.decrypt(condition);
  require(plaintextCondition, "Condition was not met");
  // ... continue execution if `condition` is true
}

For now, a TFHE.decrypt is pretty cheap, making it tempting to use constructs like if(TFHE.decrypt(encryptedBool)). However, it is recommended to avoid this approach, as in the future, each decryption will trigger an external call, introducing latency and incurring gas costs. Instead, use cmux operator to handle conditions.

For the same reason, you should replace a require(TFHE.decrypt(encryptedBool1) && TFHE.decrypt(encryptedBool2)); with a TFHE.decrypt(TFHE.and(encryptedBool1, encryptedBool2)); to limit decryption calls.

Reencrypt

The reencrypt functions takes as inputs a ciphertext and a public encryption key (namely, a NaCl box).

During reencryption, the ciphertext is decrypted using the network private key (the threshold decryption protocol is in the works). Then, the decrypted result is encrypted under the user-provided public encryption key. The result of this encryption is sent back to the caller as bytes memory.

It is also possible to provide a default value to the reencrypt function. In this case, if the provided ciphertext is not initialized (i.e., if the ciphertext handle is 0), the function will return an encryption of the provided default value.

Example

TFHE.reencrypt(balances[msg.sender], publicKey, 0);

NOTE: If one of the following operations is called with an uninitialized ciphertext handle as an operand, this handle will be made to point to a trivial encryption of 0 before the operation is executed.

Handle private reencryption

In the example above (balanceOf), this view function need to validate the user to prevent anyone to reencrypt any user's balance. To prevent this, the user provides a signature of the given public key. The best way to do it is to use EIP-712 standard. Since this is something very useful, fhEVM library provide an abstract to use in your contract:

import "fhevm/abstracts/Reencrypt.sol";

contract EncryptedERC20 is Reencrypt {
  ...
}

When a contract uses Reencrypt abstract, a modifier is available to check user signature.

function balanceOf(
  bytes32 publicKey,
  bytes calldata signature
) public view onlySignedPublicKey(publicKey, signature) returns (bytes memory) {
  return TFHE.reencrypt(balances[msg.sender], publicKey, 0);
}

This signature can be generated on client side using fhevmjs library.