Update of nlopes/slack dependency (#511)

vendor/github.com/nlopes/slack/security.go

@@ -0,0 +1,47 @@
+package slack
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"hash"
+	"net/http"
+)
+
+// SecretsVerifier contains the information needed to verify that the request comes from Slack
+type SecretsVerifier struct {
+	slackSig  string
+	timeStamp string
+	hmac      hash.Hash
+}
+
+// NewSecretsVerifier returns a SecretsVerifier object in exchange for an http.Header object and signing secret
+func NewSecretsVerifier(header http.Header, signingSecret string) (SecretsVerifier, error) {
+	if header["X-Slack-Signature"][0] == "" || header["X-Slack-Request-Timestamp"][0] == "" {
+		return SecretsVerifier{}, errors.New("Headers are empty, cannot create SecretsVerifier")
+	}
+
+	hash := hmac.New(sha256.New, []byte(signingSecret))
+	hash.Write([]byte(fmt.Sprintf("v0:%s:", header["X-Slack-Request-Timestamp"][0])))
+	return SecretsVerifier{
+		slackSig:  header["X-Slack-Signature"][0],
+		timeStamp: header["X-Slack-Request-Timestamp"][0],
+		hmac:      hash,
+	}, nil
+}
+
+func (v *SecretsVerifier) Write(body []byte) (n int, err error) {
+	return v.hmac.Write(body)
+}
+
+// Ensure compares the signature sent from Slack with the actual computed hash to judge validity
+func (v SecretsVerifier) Ensure() error {
+	computed := "v0=" + string(hex.EncodeToString(v.hmac.Sum(nil)))
+	if computed == v.slackSig {
+		return nil
+	}
+
+	return fmt.Errorf("Expected signing signature: %s, but computed: %s", v.slackSig, computed)
+}