How to encode a POST Policy in golang - browser based uploads to amazon S3?

I am trying to upload a image file to amazon s3. The setup is as follows:

Web server: golang<br/>Front-end: simple html form for testing

In reference with this document : http://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-post-example.html

I referred to the example provided in the above document and tried this:<br/> http://play.golang.org/p/3zn5fSDasK

  package main

  import "fmt"
  import "encoding/base64"

  func main() {
         bytePolicy := []byte(`{ 
                    "expiration": "2013-08-06T12:00:00.000Z",
                    "conditions": [
                             {"bucket": "examplebucket"},
                             ["starts-with", "$key", "user/user1/"],
                             {"acl": "public-read"},
                             {"success_action_redirect": "http://acl6.s3.amazonaws.com/successful_upload.html"},
                             ["starts-with", "$Content-Type", "image/"],
                             {"x-amz-meta-uuid": "14365123651274"},
                             ["starts-with", "$x-amz-meta-tag", ""],
                             {"x-amz-credential":"AKIAIOSFODNN7EXAMPLE/20130806/us-east-1/s3/aws4_request"},
                             {"x-amz-algorithm": "AWS4-HMAC-SHA256"},
                             {"x-amz-date": "20130806T000000Z" }
                      ]
                  }`)
        fmt.Println(base64.StdEncoding.EncodeToString(bytePolicy))
 }

The result I got with this<br/>

   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

The base 64 encoded policy which is on link of amazon tutorial:<br/>

   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

Why my base64 encoded policy doesn't match with that of Amazon's?

Simply because the 2 JSON source texts which produce those base64 strings have different indentation and different content as seen below (they are not equal char-by-char).

Decode both of the base64 strings, and you will see the differences. You can do that either with a program (not necessarily Go), or simply use an online service like this one.

You shouldn't worry about indentation, it doesn't matter in JSON. But when you encode a text using Base64, that encodes all characters of the source including spaces and tabs used to indent, so different indentation will result in different Base64 encoded forms.

But comparing the 2 decoded JSON, there are also other differences:

First one:

"expiration": "2013-08-06T12:00:00.000Z"
"success_action_redirect": "http://acl6.s3.amazonaws.com/successful_upload.html"

2nd one:

"expiration": "2013-08-07T12:00:00.000Z"
"success_action_redirect": "http://examplebucket.s3.amazonaws.com/successful_upload.html"

The complete decoded JSON texts:

The first one:

{ 
                        "expiration": "2013-08-06T12:00:00.000Z",
                        "conditions": [
                                 {"bucket": "examplebucket"},
                                ["starts-with", "$key", "user/user1/"],
                                 {"acl": "public-read"},
                                 {"success_action_redirect": "http://acl6.s3.amazonaws.com/successful_upload.html"},
                                ["starts-with", "$Content-Type", "image/"],
                                {"x-amz-meta-uuid": "14365123651274"},
                                ["starts-with", "$x-amz-meta-tag", ""],

                                {"x-amz-credential":"AKIAIOSFODNN7EXAMPLE/20130806/us-east-1/s3/aws4_request"},
                                {"x-amz-algorithm": "AWS4-HMAC-SHA256"},
                               {"x-amz-date": "20130806T000000Z" }
                          ]
                    }

And the 2nd one:

{ "expiration": "2013-08-07T12:00:00.000Z",
  "conditions": [
    {"bucket": "examplebucket"},
    ["starts-with", "$key", "user/user1/"],
    {"acl": "public-read"},
    {"success_action_redirect": "http://examplebucket.s3.amazonaws.com/successful_upload.html"},
    ["starts-with", "$Content-Type", "image/"],
    {"x-amz-meta-uuid": "14365123651274"},
    ["starts-with", "$x-amz-meta-tag", ""],

    {"x-amz-credential": "AKIAIOSFODNN7EXAMPLE/20130806/us-east-1/s3/aws4_request"},
    {"x-amz-algorithm": "AWS4-HMAC-SHA256"},
    {"x-amz-date": "20130806T000000Z" }
  ]
}