Encrypted types

The TFHE library provides encrypted integer types and a type system that is checked both at compile time and at run time.

Encrypted integers behave as much as possible as Solidity's integer types. Currently, however, behaviour such as "revert on overflow" is not supported as this would leak some information about the encrypted value. Therefore, arithmetic on e(u)int types is unchecked, i.e. there is wrap-around on overflow.

Encrypted integers with overflow checking are coming soon to the TFHE library. They will allow reversal in case of an overflow, but will leak some information about the operands.

In terms of implementation in the fhEVM, encrypted integers take the form of FHE ciphertexts. The TFHE library abstracts away that and, instead, exposes ciphertext handles to smart contract developers. The e(u)int types are wrappers over these handles.

The following encrypted data types are defined:

type	supported
ebool	yes
euint4	yes
euint8	yes
euint16	yes
euint32	yes
euint64	yes
euint128	no, coming soon
euint256	no, coming soon
eaddress	yes
ebytes64	no, coming soon
ebytes128	no, coming soon
ebytes256	yes
eint8	no, coming soon
eint16	no, coming soon
eint32	no, coming soon
eint64	no, coming soon
eint128	no, coming soon
eint256	no, coming soon

Higher-precision integers are supported in the TFHE-rs library and can be added as needed to fhEVM.

Casting

You can cast types with asEuint/asEbool methods.

euint64 value64 = TFHE.asEuint64(7262);
euint32 value32 = TFHE.asEuint32(value64);
ebool valueBool = TFHE.asEbool(value32);

Contract state variables with encrypted types

If you require a state variable that utilizes these encrypted types, you cannot assign the value with immutable or constant keyword. If you're using these types, the compiler attempts to ascertain the value of TFHE.asEuintXX(yy) during compilation, which is not feasible because asEuintXX() invokes a precompiled contract. To address this challenge, you must not declare your encrypted state variables as immutable or constant. Still, you can use the following methods to set your variables:

euint64 private totalSupply = TFHE.asEuint64(0);

euint64 private totalSupply;
constructor() {
  totalSupply = TFHE.asEuint64(0);
}