diff --git a/cmd/imageproxy/main.go b/cmd/imageproxy/main.go
index b2e622f..bbdad53 100644
--- a/cmd/imageproxy/main.go
+++ b/cmd/imageproxy/main.go
@@ -48,7 +48,7 @@ var denyHosts = flag.String("denyHosts", "", "comma separated list of denied rem
 var referrers = flag.String("referrers", "", "comma separated list of allowed referring hosts")
 var baseURL = flag.String("baseURL", "", "default base URL for relative remote URLs")
 var cache tieredCache
-var signatureKey = flag.String("signatureKey", "", "HMAC key used in calculating request signatures")
+var signatureKeyList SignatureKeyList
 var scaleUp = flag.Bool("scaleUp", false, "allow images to scale beyond their original dimensions")
 var timeout = flag.Duration("timeout", 0, "time limit for requests served by this proxy")
 var verbose = flag.Bool("verbose", false, "print verbose logging messages")
@@ -58,6 +58,7 @@ var userAgent = flag.String("userAgent", "willnorris/imageproxy", "specify the u
 
 func init() {
 	flag.Var(&cache, "cache", "location to cache images (see https://github.com/willnorris/imageproxy#cache)")
+	flag.Var(&signatureKeyList, "signatureKey", "HMAC key used in calculating request signatures")
 }
 
 func main() {
@@ -77,18 +78,7 @@ func main() {
 	if *contentTypes != "" {
 		p.ContentTypes = strings.Split(*contentTypes, ",")
 	}
-	if *signatureKey != "" {
-		key := []byte(*signatureKey)
-		if strings.HasPrefix(*signatureKey, "@") {
-			file := strings.TrimPrefix(*signatureKey, "@")
-			var err error
-			key, err = ioutil.ReadFile(file)
-			if err != nil {
-				log.Fatalf("error reading signature file: %v", err)
-			}
-		}
-		p.SignatureKey = key
-	}
+	p.SignatureKeys = signatureKeyList
 	if *baseURL != "" {
 		var err error
 		p.DefaultBaseURL, err = url.Parse(*baseURL)
@@ -112,6 +102,27 @@ func main() {
 	log.Fatal(http.ListenAndServe(*addr, nil))
 }
 
+type SignatureKeyList [][]byte
+
+func (skl *SignatureKeyList) String() string {
+	return fmt.Sprint(*skl)
+}
+
+func (skl *SignatureKeyList) Set(value string) error {
+	key := []byte(value)
+	if strings.HasPrefix(value, "@") {
+		file := strings.TrimPrefix(value, "@")
+		var err error
+		key, err = ioutil.ReadFile(file)
+		if err != nil {
+			log.Fatalf("error reading signature file: %v", err)
+		}
+	}
+
+	*skl = append(*skl, key)
+	return nil
+}
+
 // tieredCache allows specifying multiple caches via flags, which will create
 // tiered caches using the twotier package.
 type tieredCache struct {
diff --git a/imageproxy.go b/imageproxy.go
index 4bf4ca5..e8bb9c8 100644
--- a/imageproxy.go
+++ b/imageproxy.go
@@ -64,8 +64,9 @@ type Proxy struct {
 	// The Logger used by the image proxy
 	Logger *log.Logger
 
-	// SignatureKey is the HMAC key used to verify signed requests.
-	SignatureKey []byte
+	// SignatureKeys is a list of HMAC keys used to verify signed requests.
+	// Any of them can be used to verify signed requests.
+	SignatureKeys [][]byte
 
 	// Allow images to scale beyond their original dimensions.
 	ScaleUp bool
@@ -258,7 +259,7 @@ func (p *Proxy) allowed(r *Request) error {
 		return errDeniedHost
 	}
 
-	if len(p.AllowHosts) == 0 && len(p.SignatureKey) == 0 {
+	if len(p.AllowHosts) == 0 && len(p.SignatureKeys) == 0 {
 		return nil // no allowed hosts or signature key, all requests accepted
 	}
 
@@ -266,8 +267,10 @@ func (p *Proxy) allowed(r *Request) error {
 		return nil
 	}
 
-	if len(p.SignatureKey) > 0 && validSignature(p.SignatureKey, r) {
-		return nil
+	for _, signatureKey := range p.SignatureKeys {
+		if len(signatureKey) > 0 && validSignature(signatureKey, r) {
+			return nil
+		}
 	}
 
 	return errNotAllowed
diff --git a/imageproxy_test.go b/imageproxy_test.go
index 2779573..33267f8 100644
--- a/imageproxy_test.go
+++ b/imageproxy_test.go
@@ -116,7 +116,13 @@ func TestCopyHeader(t *testing.T) {
 
 func TestAllowed(t *testing.T) {
 	allowHosts := []string{"good"}
-	key := []byte("c0ffee")
+	key := [][]byte{
+		[]byte("c0ffee"),
+	}
+	multipleKey := [][]byte{
+		[]byte("c0ffee"),
+		[]byte("beer"),
+	}
 
 	genRequest := func(headers map[string]string) *http.Request {
 		req := &http.Request{Header: make(http.Header)}
@@ -132,7 +138,7 @@ func TestAllowed(t *testing.T) {
 		allowHosts []string
 		denyHosts  []string
 		referrers  []string
-		key        []byte
+		keys       [][]byte
 		request    *http.Request
 		allowed    bool
 	}{
@@ -151,7 +157,10 @@ func TestAllowed(t *testing.T) {
 
 		// signature key
 		{"http://test/image", Options{Signature: "NDx5zZHx7QfE8E-ijowRreq6CJJBZjwiRfOVk_mkfQQ="}, nil, nil, nil, key, nil, true},
+		{"http://test/image", Options{Signature: "NDx5zZHx7QfE8E-ijowRreq6CJJBZjwiRfOVk_mkfQQ="}, nil, nil, nil, multipleKey, nil, true}, // signed with key "c0ffee"
+		{"http://test/image", Options{Signature: "FWIawYV4SEyI4zKJMeGugM-eJM1eI_jXPEQ20ZgRe4A="}, nil, nil, nil, multipleKey, nil, true}, // signed with key "beer"
 		{"http://test/image", Options{Signature: "deadbeef"}, nil, nil, nil, key, nil, false},
+		{"http://test/image", Options{Signature: "deadbeef"}, nil, nil, nil, multipleKey, nil, false},
 		{"http://test/image", emptyOptions, nil, nil, nil, key, nil, false},
 
 		// allowHosts and signature
@@ -169,7 +178,7 @@ func TestAllowed(t *testing.T) {
 		p := NewProxy(nil, nil)
 		p.AllowHosts = tt.allowHosts
 		p.DenyHosts = tt.denyHosts
-		p.SignatureKey = tt.key
+		p.SignatureKeys = tt.keys
 		p.Referrers = tt.referrers
 
 		u, err := url.Parse(tt.url)