Authenticating Requests in Browser-Based Uploads Using POST (AWS Signature Version 2) for golang http://docs.aws.amazon.com/ja_jp/AmazonS3/latest/dev/UsingHTTPPOST.html

main.go

package main

import (
	"flag"
	"fmt"
	"log"
	"os"

	"io/ioutil"
	"mime/multipart"

	"github.com/k0kubun/pp"
	"github.com/pborman/uuid"

	"bytes"
	"io"
	"net/http"
)

func postFile(filename string, url string, formData map[string]string) error {
	bodyBuf := &bytes.Buffer{}
	bodyWriter := multipart.NewWriter(bodyBuf)

	for k, v := range formData {
		bodyWriter.WriteField(k, v)
	}

	fileWriter, err := bodyWriter.CreateFormFile("file", filename)
	if err != nil {
		return err
	}

	fh, err := os.Open(filename)
	if err != nil {
		return err
	}
	defer fh.Close()

	if _, err = io.Copy(fileWriter, fh); err != nil {
		return err
	}

	contentType := bodyWriter.FormDataContentType()
	bodyWriter.Close()

	resp, err := http.Post(url, contentType, bodyBuf)
	if err != nil {
		return err
	}
	defer resp.Body.Close()
	resp_body, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		return err
	}
	fmt.Println(resp.Status)
	fmt.Println(string(resp_body))
	return nil
}

func main() {
	awsAccessKeyID := flag.String("aid", "", "AWS_ACCESS_KEY_ID")
	awsSecretKeyID := flag.String("sid", "", "AWS_SECRET_KEY_ID")
	s3BucketName := flag.String("b", "", "S3_BUCKET_NAME")
	filePath := flag.String("f", "", "Upload file path")
	flag.Parse()

	f, err := os.Open(filePath)
	if err != nil {
		log.Fatalln(err)
	}

	fileID := uuid.New()
	userID := 111111 // TODO:
	// input data
	fi, err := f.Stat()
	if err != nil {
		log.Fatalln(err)
	}
	uploadObjectKey := fmt.Sprintf("files/%d/%s.%s", userID, fileID, "png")

	policies, err := CreatePolicies(AWSCredentials{
		AWSAccessKeyID: awsAccessKeyID,
		AWSSecretKeyID: awsSecretKeyID,
	}, UploadConfig{
		BucketName:  s3BucketName,
		FileSize:    fi.Size(),
		ContentType: "image/png", // TODO:
		ObjectKey:   uploadObjectKey,
	})
	if err != nil {
		log.Fatalln(err)
	}

	pp.Print(policies)

	if err := postFile(filePath, policies.URL, policies.Form); err != nil {
		log.Fatalln(err)
	}
}

s3policy.go

package main

import (
	"encoding/base64"
	"crypto/hmac"
	"crypto/sha1"
	"fmt"
	"time"
	"encoding/json"
	"strings"
)

// AWSCredentials Amazon Credentials
type AWSCredentials struct {
	AWSSecretKeyID string
	AWSAccessKeyID string
}

// UploadConfig generate policies from config
type UploadConfig struct {
	BucketName  string
	ObjectKey   string
	ContentType string
	FileSize    int64
}

// UploadPolicies Amazon s3 upload policies
type UploadPolicies struct {
	URL  string
	Form map[string]string
}

// PolicyJSON is policy rule
type PolicyJSON struct {
	Expiration string        `json:"expiration"`
	Conditions []interface{} `json:"conditions"`
}

const expirationTimeFormat = "2006-01-02T15:04:05ZZ07:00"
const expirationHour = 1 * time.Hour
const uploadURLFormat = "http://%s.s3.amazonaws.com/" // <bucketName>

// CreatePolicies create amazon s3 to upload policies return
func CreatePolicies(awsCredentials AWSCredentials, fileInfo UploadConfig) (UploadPolicies, error) {
	data, err := json.Marshal(&PolicyJSON{
		Expiration: time.Now().Add(expirationHour).Format(expirationTimeFormat),
		Conditions: []interface{}{
			map[string]string{"bucket": fileInfo.BucketName},
			map[string]string{"key": fileInfo.ObjectKey},
			map[string]string{"Content-Type": fileInfo.ContentType},
			[]interface{}{"content-length-range", fileInfo.FileSize, fileInfo.FileSize},
		},
	})
	if err != nil {
		return UploadPolicies{}, err
	}

	policy := strings.Replace(base64.StdEncoding.EncodeToString(data), "\n", "", -1)
	mac := hmac.New(sha1.New, []byte(awsCredentials.AWSSecretKeyID))
	mac.Write([]byte(policy))
	expectedMAC := mac.Sum(nil)
	signature := strings.Replace(base64.StdEncoding.EncodeToString(expectedMAC), "\n", "", -1)

	uploadURL := fmt.Sprintf(uploadURLFormat, fileInfo.BucketName)
	return UploadPolicies{
		URL: uploadURL,
		Form: map[string]string{
			"AWSAccessKeyId": awsCredentials.AWSAccessKeyID,
			"key":            fileInfo.ObjectKey,
			"Content-Type":   fileInfo.ContentType,
			"signature":      signature,
			"policy":         policy,
		},
	}, nil
}

v4も対応したので、このライブラリを使った方が良い
https://github.com/kyokomi/s3gopolicy