HKDF implementation in Swift using Apple's CryptoKit framework

hkdf_sha256.swift

func hkdf_sha256(_ seed: Data, salt: Data, info: Data, outputSize: Int = 32) -> Data? {
    // It would be nice to make this generic over <H: HashFunction> if HashFunction had byteCount instead of each hash
    // individually implementing it.
    let iterations = UInt8(ceil(Double(outputSize) / Double(SHA256.byteCount)))
    guard iterations <= 255 else {
        return nil
    }
    
    let prk = HMAC<SHA256>.authenticationCode(for: seed, using: SymmetricKey(data: salt))
    let key = SymmetricKey(data: prk)
    var hkdf = Data()
    var value = Data()
    
    for i in 1...iterations {
        value.append(info)
        value.append(i)
        
        let code = HMAC<SHA256>.authenticationCode(for: value, using: key)
        hkdf.append(contentsOf: code)
        
        value = Data(code)
    }

    return hkdf.prefix(outputSize)
}