vk_apps_auth.go

import (
	"bytes"
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
	"net/url"
	"strings"

	"github.com/pkg/errors"
)

func DoQueryAuthAndReturnUserId(launchQuery string, vkSecret string) (*string, error) {
	// parse
	var vkUserId string
	var vkSign []byte

	query, err := url.ParseQuery(launchQuery)
	if err != nil {
		return nil, errors.Wrap(err, "cannot parse launch query string")
	}

	for key, vals := range query {
		val := vals[0]

		switch key {
		case "vk_user_id":
			vkUserId = val
		case "sign":
			vkSign = []byte(val)
		}

		if !strings.HasPrefix(key, "vk_") {
			delete(query, key)
		}
	}

	// hash
	mac := hmac.New(sha256.New, []byte(vkSecret))
	mac.Write([]byte(query.Encode()))
	rawOurSignHmac := mac.Sum(nil)

	encoding := base64.RawURLEncoding
	rawOurSignBase64 := make([]byte, encoding.EncodedLen(len(rawOurSignHmac)))
	encoding.Encode(rawOurSignBase64, rawOurSignHmac[:])

	ourSign := rawOurSignBase64

	// check
	ok := bytes.Equal(ourSign, vkSign)
	if ok {
		return &vkUserId, nil
	} else {
		return nil, nil
	}
}