> For the complete documentation index, see [llms.txt](https://docs.zama.org/protocol/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.zama.org/protocol/solidity-guides/getting-started/overview.md).

# What is FHEVM Solidity

This document provides an overview of key features of the FHEVM smart contract library.

### Configuration and initialization

Smart contracts using FHEVM require proper configuration and initialization:

* **Environment setup**: Import and inherit from environment-specific configuration contracts
* **Relayer configuration**: Configure secure relayer access for cryptographic operations
* **Initialization checks**: Validate encrypted variables are properly initialized before use

For more information see [Configuration](/protocol/solidity-guides/smart-contract/configure.md).

### Encrypted data types

FHEVM introduces encrypted data types compatible with Solidity:

* **Booleans**: `ebool`
* **Unsigned Integers**: `euint8`, `euint16`, `euint32`, `euint64`, `euint128`, `euint256`
* **Addresses**: `eaddress`
* **Input**: `externalEbool`, `externalEaddress`, `externalEuintXX` for handling encrypted input data

Encrypted data is represented as ciphertext handles, ensuring secure computation and interaction.

For more information see [use of encrypted types](/protocol/solidity-guides/smart-contract/types.md).

### Casting types

fhevm provides functions to cast between encrypted types:

* **Casting between encrypted types**: `FHE.asEbool` converts encrypted integers to encrypted booleans
* **Casting to encrypted types**: `FHE.asEuintX` converts plaintext values to encrypted types
* **Casting to encrypted addresses**: `FHE.asEaddress` converts plaintext addresses to encrypted addresses

For more information see [use of encrypted types](/protocol/solidity-guides/smart-contract/types.md).

### Confidential computation

fhevm enables symbolic execution of encrypted operations, supporting:

* **Arithmetic:** `FHE.add`, `FHE.sub`, `FHE.mul`, `FHE.min`, `FHE.max`, `FHE.neg`, `FHE.div`, `FHE.rem`
  * Note: `div` and `rem` operations are supported only with plaintext divisors
* **Bitwise:** `FHE.and`, `FHE.or`, `FHE.xor`, `FHE.not`, `FHE.shl`, `FHE.shr`, `FHE.rotl`, `FHE.rotr`
* **Comparison:** `FHE.eq`, `FHE.ne`, `FHE.lt`, `FHE.le`, `FHE.gt`, `FHE.ge`
* **Advanced:** `FHE.select` for branching on encrypted conditions, `FHE.randEuintX` for on-chain randomness.

For more information on operations, see [Operations on encrypted types](/protocol/solidity-guides/smart-contract/operations.md).

For more information on conditional branching, see [Conditional logic in FHE](/protocol/solidity-guides/smart-contract/logics/conditions.md).

For more information on random number generation, see [Generate Random Encrypted Numbers](/protocol/solidity-guides/smart-contract/operations/random.md).

### Access control mechanism

fhevm enforces access control with a blockchain-based Access Control List (ACL):

* **Persistent access**: `FHE.allow`, `FHE.allowThis` grants permanent permissions for ciphertexts.
* **Transient access**: `FHE.allowTransient` provides temporary access for specific transactions.
* **Validation**: `FHE.isSenderAllowed` ensures that only authorized entities can interact with ciphertexts.
* **Persistent public decryption**: `FHE.makePubliclyDecryptable`, `FHE.isPubliclyDecryptable` makes a given ciphertext permanently publicly decryptable.
* **User decryption delegation**: a contract can delegate user-decryption rights via `FHE.delegateUserDecryption`; an EOA delegates its own rights by calling `IACL.delegateForUserDecryption` on the ACL directly. See [User decryption delegation](/protocol/solidity-guides/smart-contract/acl/delegation.md) for the two patterns.
* **Account deny list**: `FHE.isAccountDenied` checks whether an account is blocked from interacting with encrypted values.

For more information see [ACL](/protocol/solidity-guides/smart-contract/acl.md).


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