OpenSSL HMAC_SHA1为b64哈希（不是base64编码）

Question

我成功获得了HMAC_SHA1，但它是十六进制的。我无法弄清楚如何获得base64字符串。这就是我得到十六进制字符串的方式：

在“message”和“key”上运行此命令会给我2088df74d5f2146b48146caf4965377e9d0be3a4的十六进制字符串，这是正确的。对此进行base64编码可以得到MjA4OGRmNzRkNWYyMTQ2YjQ4MTQ2Y2FmNDk2NTM3N2U5ZDBiZTNhNA==，这是正确的base64编码，但不是b64哈希字符串，应该是IIjfdNXyFGtIFGyvSWU3fp0L46Q=。

#include <openssl/evp.h>
#include <openssl/sha.h>
#include <openssl/hmac.h>

std::string calcHmac(std::string message, std::string key, std::string aStrType="base64") {
    char hash[41];

    // The data that we’re going to hash using HMAC
    const unsigned char *unsignedData = (unsigned char *)message.c_str();
    unsigned char *digest;
    // I have used sha1 hash engine here.
    digest = HMAC(EVP_sha1(), key.c_str(), strlen(key.c_str()), unsignedData, strlen((char *)unsignedData), NULL, NULL);

    // Length of string depends on the chosen hash engine for example with the
    // chosen hash engine  i.e SHA1 it produces a 20-byte hash value which
    // rendered as 40 characters.
    // Length of the string need to be changed as per hash engine used.
    for (int i = 0; i < 20; i++) {
      sprintf(&hash[i * 2], "% 02x", (unsigned int)digest[i]);
    }

    std::string hash_hex = std::string(hash);

    // if (aStrType == "base64") {
    //     std::string hash_b64 = "";
    //     return hash_b64;
    // }
    return hash_hex;
}