Add signature file output

It's optional, whee!

README.md

@@ -4,6 +4,11 @@ Tool for producing identities
 
 Anagram of "Gorgon".
 
+## Actions
+
+ * `cargo run -- create eli.key`: makes a new key.
+ * `cargo run -- sign eli.key message.txt [message.sig]`: signs a message, optionally writes the signature to a file.
+
 ## TODO
 
  * Fix up the aegis generation script to ensure that I either have, or don't need, the extensions from Step CA:

gongor.rs

@@ -12,6 +12,8 @@ fn main() ->  std::io::Result<()>{
     if action == "sign" {
         let keyfilepath = std::env::args().nth(2).expect("no keyfilepath given");
         let messagefilepath = std::env::args().nth(3).expect("no message given");
+        let maybe_signature_filepath = std::env::args().nth(4);
+
 
         let mut keyfile = File::open(&keyfilepath).expect("no file found");
         let mut keybuffer: [u8; 32] = [0; 32];
@@ -28,7 +30,14 @@ fn main() ->  std::io::Result<()>{
         println!("Using key {keyfilepath}");
 
         let signature: Signature = signing_key.sign(&messagebuf);
+        if maybe_signature_filepath.is_some() {
+            let filename = maybe_signature_filepath.expect("Not possible");
+            let mut file = File::create(filename.clone())?;
+            println!("Writing signature to {filename}");
+            file.write_all(&signature.to_bytes())?;
+        } else {
             println!("Signture: {signature}");
+        }
         return Ok(())
     } else if action == "create" {
         let keyfilepath = std::env::args().nth(2).expect("no keyfilepath given");