如何用GDAX上的python中的base64对摘要输出进行编码

Question

我正在尝试使用GDAX交换上的api。在他们的网站上，他们给出了这样的代码：

# Requires python-requests. Install with pip:
#
#   pip install requests
#
# or, with easy-install:
#
#   easy_install requests

import json, hmac, hashlib, time, requests, base64
from requests.auth import AuthBase

# Create custom authentication for Exchange
class CoinbaseExchangeAuth(AuthBase):
def __init__(self, api_key, secret_key, passphrase):
    self.api_key = api_key
    self.secret_key = secret_key
    self.passphrase = passphrase

def __call__(self, request):
    timestamp = str(time.time())
    message = timestamp + request.method + request.path_url + (request.body or '')
    hmac_key = base64.b64decode(self.secret_key)
    signature = hmac.new(hmac_key, message, hashlib.sha256)
    signature_b64 = signature.digest().encode('base64').rstrip('\n')

    request.headers.update({
        'CB-ACCESS-SIGN': signature_b64,
        'CB-ACCESS-TIMESTAMP': timestamp,
        'CB-ACCESS-KEY': self.api_key,
        'CB-ACCESS-PASSPHRASE': self.passphrase,
        'Content-Type': 'application/json'
    })
    return request

api_url = 'https://api.gdax.com/'
auth = CoinbaseExchangeAuth(API_KEY, API_SECRET, API_PASS)

# Get accounts
r = requests.get(api_url + 'accounts', auth=auth)
print r.json()


# Place an order
order = {
'size': 1.0,
'price': 1.0,
'side': 'buy',
'product_id': 'BTC-USD',
}
r = requests.post(api_url + 'orders', json=order, auth=auth)
print r.json()

他们还说：“记住在将字母数字秘密字符串用作HMAC的密钥之前，首先对其进行base64解码(结果为64字节)。此外，在发送报头之前，应对摘要输出进行base64编码。”

我相信我已经修复了第一部分：

API_SECRET = base64.b64decode(b'{secret}')

然而，我不明白第二部分是什么意思。我得到了错误消息：

TypeError: Unicode-objects must be encoded before hashing