Merge branch 'jc/push-cert' into maint

The "git push --signed" protocol extension did not limit what the
"nonce" that is a server-chosen string can contain or how long it
can be, which was unnecessarily lax.  Limit both the length and the
alphabet to a reasonably small space that can still have enough
entropy.

* jc/push-cert:
  push --signed: tighten what the receiving end can ask to sign

send-pack.c

@@ -281,6 +281,28 @@ static int generate_push_cert(struct strbuf *req_buf,
 	return update_seen;
 }
 
+#define NONCE_LEN_LIMIT 256
+
+static void reject_invalid_nonce(const char *nonce, int len)
+{
+	int i = 0;
+
+	if (NONCE_LEN_LIMIT <= len)
+		die("the receiving end asked to sign an invalid nonce <%.*s>",
+		    len, nonce);
+
+	for (i = 0; i < len; i++) {
+		int ch = nonce[i] & 0xFF;
+		if (isalnum(ch) ||
+		    ch == '-' || ch == '.' ||
+		    ch == '/' || ch == '+' ||
+		    ch == '=' || ch == '_')
+			continue;
+		die("the receiving end asked to sign an invalid nonce <%.*s>",
+		    len, nonce);
+	}
+}
+
 int send_pack(struct send_pack_args *args,
 	      int fd[], struct child_process *conn,
 	      struct ref *remote_refs,
@@ -323,6 +345,7 @@ int send_pack(struct send_pack_args *args,
 		push_cert_nonce = server_feature_value("push-cert", &len);
 		if (!push_cert_nonce)
 			die(_("the receiving end does not support --signed push"));
+		reject_invalid_nonce(push_cert_nonce, len);
 		push_cert_nonce = xmemdupz(push_cert_nonce, len);
 	}