Casting and trivial encryption

This documentation covers the asEbool, asEuintXX, and asEaddress 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 two main use cases:

Trivial encryption: Converting plaintext values to encrypted types
Type casting: Converting between different encrypted types

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
address → eaddress

{% hint style="info" %} When doing trivial encryption, the data is made compatible with FHE operations but remains publicly visible on-chain unless explicitly encrypted. {% endhint %}

Example

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

2. Casting between encrypted types

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

euint32 → euint64

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

Overall operation summary

Casting Type	Function	Input Type	Output Type
Trivial encryption	`FHE.asEuintXX(x)`	`uintX`	`euintX`
	`FHE.asEbool(x)`	`bool`	`ebool`
	`FHE.asEaddress(x)`	`address`	`eaddress`
Conversion between types	`FHE.asEuintXX(x)`	`euintXX`/`ebool`	`euintYY`
	`FHE.asEbool(x)`	`euintXX`	`ebool`

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