How to Achieve Pre-signed POST File Uploads to AWS S3 using Go?

Pre-Signed POST Upload to AWS S3 in Go

Problem:

How to perform a Pre-signed POST upload to upload files to an AWS S3 bucket using Go, without using the traditional Pre-signed PUT method?

Solution:

To perform a Pre-signed POST upload, follow these steps:

1. Configure the S3 Bucket for Public Downloads: Set the bucket policy to allow public downloads only.
2. Create a POST Policy: Generate a POST policy that allows upload to a specific key, bucket, and grants public-read access.
3. Generate and Sign the Policy: Use the S3 bucket owner's credentials to generate and sign the POST policy, encoding it in base64 and hex.

4. Construct and POST Multipart Form Data: Create a multipart form data request with the following fields:

   • (key) The name of the file to upload
   • (policy) The base64-encoded POST policy
   • (signature) The hex-encoded signature of the policy
   • (x-amz-credential) The AWS credentials used to sign the policy
   • (x-amz-algorithm) The algorithm used for signing the policy (AWS4-HMAC-SHA256)
   • (x-amz-date) The date used for signing the policy

Example Code in Go:

import (
    "bytes"
    "fmt"
    "io"
    "mime/multipart"
    "net/http"
    "strings"
)

// Fields represents the fields to be uploaded in the multipart form data request.
type Fields struct {
    Key, Value string
}

// Upload performs a Pre-signed POST upload using the provided URL and fields.
func Upload(url string, fields []Fields) error {
    var b bytes.Buffer
    w := multipart.NewWriter(&b)
    for _, f := range fields {
        fw, err := w.CreateFormField(f.Key)
        if err != nil {
            return err
        }
        if _, err := io.WriteString(fw, f.Value); err != nil {
            return err
        }
    }
    w.Close()

    req, err := http.NewRequest("POST", url, &b)
    if err != nil {
        return err
    }
    req.Header.Set("Content-Type", w.FormDataContentType())

    client := &http.Client{}
    res, err := client.Do(req)
    if err != nil {
        return err
    }
    if res.StatusCode != http.StatusOK {
        err = fmt.Errorf("bad status: %s", res.Status)
    }
    return nil
}

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