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Overview

One of the key aspects of writing confidential smart contracts is receiving encrypted inputs from users: 
function transfer(
    address to,
    InEuint32 memory inAmount // <------ encrypted input here
) public virtual returns (euint32 transferred) {
    euint32 amount = FHE.asEuint32(inAmount);
}
Notice in the example above the distinction between InEuint32 and euint32.

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Input Types Conversion

The input types InEuintxx (and InEbool, InEaddress) are special encrypted types that represent user input. Input types contain additional information required to authenticate and validate ciphertexts. For more on that, read about the ZK-Verifier. Before you can use an encrypted input, you need to convert it to a regular encrypted type: 
euint32 amount = FHE.asEuint32(inAmount);
Avoid storing encrypted input types in contract state. These types carry extra metadata, which increases gas costs and may cause unexpected behavior. Always convert them using FHE.asE...().
Now that amount is of type euint32, you can store or manipulate it: 
toBalance = FHE.sub(toBalance, amount);
Read more about the available FHE types and operations in the FHE Encrypted Operations guide.

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Full Example

Here’s a complete example showing how to handle encrypted inputs in a transfer function: 
function transfer(
    address to,
    InEuint32 memory inAmount
) public virtual returns (euint32 transferred) {
    euint32 amount = FHE.asEuint32(inAmount);

    toBalance = _balances[to];
    fromBalance = _balances[msg.sender];

    _updateBalance(to, FHE.add(toBalance, amount));
    _updateBalance(from, FHE.sub(fromBalance, amount));
}
For the example above to work correctly, you will also need to manage access to the newly created ciphertexts in the _updateBalance() function. Learn more about access control in the ACL Mechanism guide.

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Additional Examples

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Voting in a Poll

function castEncryptedVote(address poll, InEbool calldata encryptedVote) public {
    _submitVote(poll, FHE.asEbool(encryptedVote));
}

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Setting Encrypted User Preferences

function updateUserSetting(address user, InEuint8 calldata encryptedSetting) public {
    _applyUserSetting(user, FHE.asEuint8(encryptedSetting));
}