fhevm-solidity/docs/smart_contracts/asEXXoperators.md

asEbool, asEuintXX, asEaddress and asEbytesXX operations

This documentation covers the asEbool, asEuintXX, asEaddress and asEbytesXX operations provided by the FHE library for working with encrypted data in the fhevm. These operations are essential for converting between plaintext and encrypted types, as well as handling encrypted inputs.

The operations can be categorized into three main use cases:

1. Trivial encryption: Converting plaintext values to encrypted types
2. Type casting: Converting between different encrypted types
3. Input handling: Processing encrypted inputs with proofs

1. Trivial encryption

Trivial encryption simply put is a plain text in a format of a ciphertext.

Overview

Trivial encryption is the process of converting plaintext values into encrypted types (ciphertexts) compatible with FHE operators. Although the data is in ciphertext format, it remains publicly visible on-chain, making it useful for operations between public and private values.

This type of casting involves converting plaintext (unencrypted) values into their encrypted equivalents, such as:

• bool → ebool
• uint → euintXX
• bytes → ebytesXX
• address → eaddress

Note

: When doing trivial encryption, the data is made compatible with FHE operations but remains publicly visible on-chain unless explicitly encrypted.

Example

euint64 value64 = FHE.asEuint64(7262);  // Trivial encrypt a uint64
ebool valueBool = FHE.asEbool(true);   // Trivial encrypt a boolean

Trivial encryption of ebytesXX types

The FHE.padToBytesXX functions facilitate this trivial encryption process for byte arrays, ensuring compatibility with ebytesXX types. These functions:

• Pad the provided byte array to the appropriate length (64, 128, or 256 bytes).
• Prevent runtime errors caused by improperly sized input data.
• Work seamlessly with FHE.asEbytesXX for trivial encryption.

Important

: Trivial encryption does NOT provide any privacy guarantees. The input data remains fully visible on the blockchain. Only use trivial encryption when working with public values that need to interact with actual encrypted data.

Workflow

1. Pad Input Data: Use the padToBytesXX functions to ensure your byte array matches the size requirements.
2. Encrypt the Padded Data: Use FHE.asEbytesXX to encrypt the padded byte array into the corresponding encrypted type.
3. Grant Access: Use FHE.allowThis and FHE.allowoptionally, if you want to persist allowance for those variables for later use.

Example: Trivial Encryption with ebytesXX

Below is an example demonstrating how to encrypt and manage ebytes64, ebytes128, and ebytes256 types:

function trivialEncrypt() public {
  // Encrypt a 64-byte array
  ebytes64 yBytes64 = FHE.asEbytes64(
    FHE.padToBytes64(
      hex"19d179e0cc7e816dc944582ed4f5652f5951900098fc2e0a15a7ea4dc8cfa4e3b6c54beea5ee95e56b728762f659347ce1d4aa1b05fcc5"
    )
  );
  FHE.allowThis(yBytes64);
  FHE.allow(yBytes64, msg.sender);

  // Encrypt a 128-byte array
  ebytes128 yBytes128 = FHE.asEbytes128(
    FHE.padToBytes128(
      hex"13e7819123de6e2870c7e83bb764508e22d7c3ab8a5aee6bdfb26355ef0d3f1977d651b83bf5f78634fa360aa14debdc3daa6a587b5c2fb1710ab4d6677e62a8577f2d9fecc190ad8b11c9f0a5ec3138b27da1f055437af8c90a9495dad230"
    )
  );
  FHE.allowThis(yBytes128);
  FHE.allow(yBytes128, msg.sender);

  // Encrypt a 256-byte array
  ebytes256 yBytes256 = FHE.asEbytes256(
    FHE.padToBytes256(
      hex"d179e0cc7e816dc944582ed4f5652f5951900098fc2e0a15a7ea4dc8cfa4e3b6c54beea5ee95e56b728762f659347ce1d4aa1b05fcc513e7819123de6e2870c7e83bb764508e22d7c3ab8a5aee6bdfb26355ef0d3f1977d651b83bf5f78634fa360aa14debdc3daa6a587b5c2fb1710ab4d6677e62a8577f2d9fecc190ad8b11c9f0a5ec3138b27da1f055437af8c90a9495dad230"
    )
  );
  FHE.allowThis(yBytes256);
  FHE.allow(yBytes256, msg.sender);
}

2. Casting between encrypted types

This type of casting is used to reinterpret or convert one encrypted type into another. For example:

• euint32 → euint64
• ebytes128 → ebytes256

Casting between encrypted types is often required when working with operations that demand specific sizes or precisions.

Important

: When casting between encrypted types:

• Casting from smaller types to larger types (e.g. euint32 → euint64) preserves all information
• Casting from larger types to smaller types (e.g. euint64 → euint32) will truncate and lose information

The table below summarizes the available casting functions:

From type	To type	Function
euintX	euintX	FHE.asEuintXX
ebool	euintX	FHE.asEuintXX
euintX	ebool	FHE.asEboolXX

{% hint style="info" %} Casting between encrypted types is efficient and often necessary when handling data with differing precision requirements. {% endhint %}

Workflow for encrypted types

// Casting between encrypted types
euint32 value32 = FHE.asEuint32(value64); // Cast to euint32
ebool valueBool = FHE.asEbool(value32);   // Cast to ebool

3. Encrypted input

Overview

Encrypted input casting is the process of interpreting a handle (ciphertext reference) and its proof as a specific encrypted type. This ensures the validity of the input before it is used in computations.

Encrypted inputs is in depth explained in the following section: encrypted inputs

Example

euint64 encryptedValue = FHE.asEuint64(einputHandle, inputProof); // Interpret einputHandle as euint64

Details

Encrypted input casting validates:

1. The input handle references a valid ciphertext.
2. The accompanying proof matches the expected type.

For more information, see the Encrypetd inputs documentation

Overall operation summary

Casting Type	Function	Input Type	Output Type
Trivial encryption	FHE.asEuintXX(x)	uintX	euintX
	FHE.asEbool(x)	bool	ebool
	FHE.asEbytesXX(x)	bytesXX	ebytesXX
	FHE.asEaddress(x)	address	eaddress
Conversion between types	FHE.asEuintXX(x)	euintXX/ebool	euintYY
	FHE.asEbool(x)	euintXX	ebool
Encrypted input	FHE.asEuintXX(x, y)	einput, bytes proof	euintX
	FHE.asEbool(x, y)	einput,bytes proof	ebool
	FHE.asEbytesXX(x, y)	einput,bytes proof	ebytesXX
	FHE.asEaddress(x, y)	einput, bytes proof	eaddress