Add README and additional input validation for signer

psiphon/common/protocol/signer/README.md

@@ -0,0 +1,10 @@
+# signer
+
+Example usage:
+
+```
+./signer -server-entry $ENCODED_SERVER_ENTRY -public-key $PUBLIC_KEY -private-key $PRIVATE_KEY sign
+```
+
+* Signer is a tool that adds signatures to encoded server entries (`sign` mode) and generates signing key pairs (`generate` mode).
+* In `sign` mode, the output is an copy of the input encoded server entry with an additional `signature` field.

psiphon/common/protocol/signer/main.go

@@ -61,6 +61,10 @@ func main() {
 	case "generate":
 		err = generate()
 	case "sign":
+		if publicKey == "" || privateKey == "" || encodedServerEntry == "" {
+			flag.Usage()
+			os.Exit(1)
+		}
 		err = sign(publicKey, privateKey, encodedServerEntry)
 	default:
 		flag.Usage()