allow space-separated list of signature keys

This is necessary when specifying options as environment variables.
Also add documentation for using multiple signature keys.

README.md

@@ -235,7 +235,7 @@ which contains the HMAC key.
 
 Try it out by running:
 
-    imageproxy -signatureKey "secret key"
+    imageproxy -signatureKey "secretkey"
 
 Reload the [codercat URL][], and you should see an error message.  Now load a
 [signed codercat URL][] (which contains the [signature option][]) and verify
@@ -245,7 +245,11 @@ that it loads properly.
 [signature option]: https://godoc.org/willnorris.com/go/imageproxy#hdr-Signature
 
 Some simple code samples for generating signatures in various languages can be
-found in [docs/url-signing.md](/docs/url-signing.md).
+found in [docs/url-signing.md](/docs/url-signing.md).  Multiple valid signature
+keys may be provided to support key rotation by repeating the `signatureKey`
+flag multiple times, or by providing a space-separated list of keys.  To use a
+key with a literal space character, load the key from a file using the "@"
+prefix documented above.
 
 If both a whiltelist and signatureKey are specified, requests can match either.
 In other words, requests that match one of the allowed hosts don't necessarily

cmd/imageproxy/main.go

@@ -109,17 +109,18 @@ func (skl *signatureKeyList) String() string {
 }
 
 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)
+	for _, v := range strings.Fields(value) {
+		key := []byte(v)
+		if strings.HasPrefix(v, "@") {
+			file := strings.TrimPrefix(v, "@")
+			var err error
+			key, err = ioutil.ReadFile(file)
+			if err != nil {
+				log.Fatalf("error reading signature file: %v", err)
+			}
 		}
+		*skl = append(*skl, key)
 	}
-
-	*skl = append(*skl, key)
 	return nil
 }