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# SymmetricKey

A symmetric cryptographic key.

```
struct SymmetricKey
```

## Overview

You typically derive a symmetric key from an instance of a shared secret
([`SharedSecret`](/documentation/CryptoKit/SharedSecret)) that you obtain through key agreement. You use a
symmetric key to compute a message authentication code like [`HMAC`](/documentation/CryptoKit/HMAC), or to
open and close a sealed box ([`ChaChaPoly.SealedBox`](/documentation/CryptoKit/ChaChaPoly/SealedBox) or
[`AES.GCM.SealedBox`](/documentation/CryptoKit/AES/GCM/SealedBox)) using a cipher like [`ChaChaPoly`](/documentation/CryptoKit/ChaChaPoly) or [`AES`](/documentation/CryptoKit/AES).

## Topics

### Creating a key

[`init(data:)`](/documentation/CryptoKit/SymmetricKey/init(data:))

Creates a key from the given data.

[`init(size:)`](/documentation/CryptoKit/SymmetricKey/init(size:))

Generates a new random key of the given size.

### Getting the key length

[`bitCount`](/documentation/CryptoKit/SymmetricKey/bitCount)

The number of bits in the key.

### Accessing underlying data


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