# HKDF A standards-based implementation of an HMAC-based Key Derivation Function (HKDF). ``` struct HKDF where H : HashFunction ``` ## Overview The key derivation functions allow you to derive one or more secrets of the size of your choice from a main key or passcode. The key derivation function is compliant with IETF RFC 5869. Use one of the `deriveKey` functions, such as [`deriveKey(inputKeyMaterial:outputByteCount:)`](/documentation/CryptoKit/HKDF/deriveKey(inputKeyMaterial:outputByteCount:)) or [`deriveKey(inputKeyMaterial:salt:info:outputByteCount:)`](/documentation/CryptoKit/HKDF/deriveKey(inputKeyMaterial:salt:info:outputByteCount:)), to derive a key from a main secret or passcode in a single function. To derive a key with more fine-grained control, use `extract(inputKeyMaterial:salt:)` to create cryptographically strong key material in the form of a hashed authentication code, then call [`expand(pseudoRandomKey:info:outputByteCount:)`](/documentation/CryptoKit/HKDF/expand(pseudoRandomKey:info:outputByteCount:)) using that key material to generate a symmetric key of the length you specify. ## Topics ### Deriving a key [`deriveKey(inputKeyMaterial:outputByteCount:)`](/documentation/CryptoKit/HKDF/deriveKey(inputKeyMaterial:outputByteCount:)) Derives a symmetric encryption key from a main key or passcode using HKDF key derivation. [`deriveKey(inputKeyMaterial:info:outputByteCount:)`](/documentation/CryptoKit/HKDF/deriveKey(inputKeyMaterial:info:outputByteCount:)) Derives a symmetric encryption key from a main key or passcode using HKDF key derivation with information you specify. [`deriveKey(inputKeyMaterial:salt:outputByteCount:)`](/documentation/CryptoKit/HKDF/deriveKey(inputKeyMaterial:salt:outputByteCount:)) Derives a symmetric encryption key from a main key or passcode using HKDF key derivation with salt that you specify. [`deriveKey(inputKeyMaterial:salt:info:outputByteCount:)`](/documentation/CryptoKit/HKDF/deriveKey(inputKeyMaterial:salt:info:outputByteCount:)) Derives a symmetric encryption key from a main key or passcode using HKDF key derivation with information and salt you specify. ### Controlling key derivation [`expand(pseudoRandomKey:info:outputByteCount:)`](/documentation/CryptoKit/HKDF/expand(pseudoRandomKey:info:outputByteCount:)) Expands cryptographically strong key material into a derived symmetric key. ## Relationships ### Conforms To [`Sendable`](/documentation/Swift/Sendable) [`SendableMetatype`](/documentation/Swift/SendableMetatype) --- 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)