尝试从浏览器上传图像到s3时签名不匹配

Question

我正在尝试使用概述here和here

的步骤从浏览器直接将图片上传到s3

为了对策略进行编码，我编写了一小段python代码，如下所示：

import hmac
import hashlib
import base64

def sign(key, msg):
    return hmac.new(key, msg, hashlib.sha256).digest()

post_policy = """{ "expiration": "2018-12-30T12:00:00.000Z",
  "conditions": [
    {"bucket": "<my_bucket>"},
    ["starts-with", "$key", "user/user1/"],
    {"acl": "public-read"},
    {"success_action_redirect": "http://my_bucket.s3.amazonaws.com/successful_upload.html"},
    ["starts-with", "$Content-Type", "image/"],
    {"x-amz-meta-uuid": "14365123651274"},
    {"x-amz-server-side-encryption": "AES256"},
    ["starts-with", "$x-amz-meta-tag", ""],

    {"x-amz-credential": "<my-access-id>/20151229/us-east-2/s3/aws4_request"},
    {"x-amz-algorithm": "AWS4-HMAC-SHA256"},
    {"x-amz-date": "20181229T000000Z" }
  ]
}"""

post_policy_utf8 = unicode(post_policy, "utf-8")
base64_encoded_policy = base64.b64encode(post_policy_utf8)
secret_key = '<my_secret_key>'
signed_policy = sign(("AWS4" + base64_encoded_policy), secret_key)
encoded_signed_policy = base64.b64encode(signed_policy)

我的html表单就像这样

<form action="http://<my_bucket>.s3.amazonaws.com/" method="post" enctype="multipart/form-data">
    Key to upload: 
    <input type="input"  name="key" value="user/user1/${filename}" /><br />
    <input type="hidden" name="acl" value="public-read" />
    <input type="hidden" name="success_action_redirect" value="http://<my_bucket>.s3.amazonaws.com/successful_upload.html" />
    Content-Type: 
    <input type="input"  name="Content-Type" value="image/jpeg" /><br />
    <input type="hidden" name="x-amz-meta-uuid" value="14365123651274" /> 
    <input type="hidden" name="x-amz-server-side-encryption" value="AES256" /> 
    <input type="text"   name="X-Amz-Credential" value="AKIAJ5CZ54I742JS652A/20151229/us-east-2/s3/aws4_request" />
    <input type="text"   name="X-Amz-Algorithm" value="AWS4-HMAC-SHA256" />
    <input type="text"   name="X-Amz-Date" value="20181229T000000Z" />

    Tags for File: 
    <input type="input"  name="x-amz-meta-tag" value="" /><br />
    <input type="hidden" name="Policy" value='eyAiZXhwaXJhdGlvbiI6ICIyMDE4LTEyLTMwVDEyOjAwOjAwLjAwMFoiLAogICJjb25kaXRpb25zIjogWwogICAgeyJidWNrZXQiOiAiZmlsZS51cGxvYWQuZnJvbS5icm93c2VyIn0sCiAgICBbInN0YXJ0cy13aXRoIiwgIiRrZXkiLCAidXNlci91c2VyMS8iXSwKICAgIHsiYWNsIjogInB1YmxpYy1yZWFkIn0sCiAgICB7InN1Y2Nlc3NfYWN0aW9uX3JlZGlyZWN0IjogImh0dHA6Ly9maWxlLnVwbG9hZC5mcm9tLmJyb3dzZXIuczMuYW1hem9uYXdzLmNvbS9zdWNjZXNzZnVsX3VwbG9hZC5odG1sIn0sCiAgICBbInN0YXJ0cy13aXRoIiwgIiRDb250ZW50LVR5cGUiLCAiaW1hZ2UvIl0sCiAgICB7IngtYW16LW1ldGEtdXVpZCI6ICIxNDM2NTEyMzY1MTI3NCJ9LAogICAgeyJ4LWFtei1zZXJ2ZXItc2lkZS1lbmNyeXB0aW9uIjogIkFFUzI1NiJ9LAogICAgWyJzdGFydHMtd2l0aCIsICIkeC1hbXotbWV0YS10YWciLCAiIl0sCiAgICAKICAgIHsieC1hbXotY3JlZGVudGlhbCI6ICJBS0lBSjVDWjU0STc0MkpTNjUyQS8yMDE1MTIyOS91cy1lYXN0LTIvczMvYXdzNF9yZXF1ZXN0In0sCiAgICB7IngtYW16LWFsZ29yaXRobSI6ICJBV1M0LUhNQUMtU0hBMjU2In0sCiAgICB7IngtYW16LWRhdGUiOiAiMjAxODEyMjlUMDAwMDAwWiIgfQogIF0KfQ==' />
    <input type="hidden" name="X-Amz-Signature" value="ZG0xBgY5y1Fin0D7kKAO9tuqGntG1Ptw4GDaiLvfvGU=" />
    File: 
    <input type="file"   name="file" /> <br />
    <!-- The elements after this will be ignored -->
    <input type="submit" name="submit" value="Upload to Amazon S3" />
  </form>

我们从python 2.7代码中获取的base64策略值和签名。 当我提交此信息时，AWS表示签名不匹配。我哪里出错了以及如何纠正这个问题？