# init(size:initializingWith:) Creates a new key of the given size where the key contents are initialized via a callback. ``` init(size: SymmetricKeySize, initializingWith callback: @_lifetime(0: copy 0) (inout OutputRawSpan) throws(E) -> Void) throws(E) where E : Error ``` ## Parameters `size` The size of the key to generate. You can use one of the standard sizes, like [`bits256`](/documentation/CryptoKit/SymmetricKeySize/bits256), or you can create a key of custom length by initializing a [`SymmetricKeySize`](/documentation/CryptoKit/SymmetricKeySize) instance with a non-standard value. `callback` A callback that will be invoked to initialize the contents of the key. It must initialize the full set of size.bitCount / 8 bytes in the provided output span. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)