如何访问HashedAuthenticationCode的代码（作为字符串）？

Question

给出以下HashedAuthenticationCode：

import CryptoKit
let key = SymmetricKey(size: .bits256)
let message = "Hello world!".data(using: .utf8)!
let authenticationCode = HMAC<SHA256>.authenticationCode(for: message, using: key)
print(authenticationCode) // "HMAC with SHA256: c818a1400a9995d5faafbb5d41cc88a6701b5d1a4ded8ac169498319a2597e2a"

如何访问c818a1400a9995d5faafbb5d41cc88a6701b5d1a4ded8ac169498319a2597e2a值？显然，这必须通过URLSession进行编码和序列化，但是我认为获取HAC的“代码”部分没有明显的属性！