Added command to derive the public key for a specified secret key

Makefile

@@ -4,7 +4,7 @@ TWEETNACLC=randombytes.c tools.c tweetnacl.c
 TWEETNACL=$(TWEETNACLC) randombytes.h tools.h tweetnacl.h
 
 all: tweetnacl-decrypt tweetnacl-encrypt tweetnacl-keypair \
-     tweetnacl-sigpair tweetnacl-sign tweetnacl-verify
+     tweetnacl-sigpair tweetnacl-sign tweetnacl-verify tweetnacl-derivepubkey
 
 bin: ;
 	mkdir bin
@@ -36,6 +36,10 @@ tweetnacl-verify: bin $(TWEETNACL) tweetnacl-verify.c
 	$(CC) $(CFLAGS) $(TWEETNACLC) tweetnacl-verify.c \
 		-o bin/tweetnacl-verify
 
+tweetnacl-derivepubkey: bin $(TWEETNACL) tweetnacl-derivepubkey.c
+	$(CC) $(CFLAGS) $(TWEETNACLC) tweetnacl-derivepubkey.c \
+		-o bin/tweetnacl-derivepubkey
+
 test: ;
 	mkdir tmp
 	bin/tweetnacl-keypair tmp/a.pub tmp/a.sec
@@ -47,4 +51,5 @@ test: ;
 	echo 'Verified message!' > tmp/msg02
 	bin/tweetnacl-sign tmp/s.sec tmp/msg02 tmp/signed
 	bin/tweetnacl-verify tmp/s.pub tmp/signed -
+	bin/tweetnacl-derivepubkey tmp/a.sec tmp/a.pub2
 	rm -rf tmp

README.md

@@ -80,6 +80,17 @@ $ tweetnacl-verify sign.pub message.signed message.txt
 
 The `sign.pub` public signature key is used to verify the signed message in `message.signed`. If the signature is verified, the message is placed into `message.txt` and the program exits with successful status 0. If the signature is not verified, an error message is printed to stderr and the program exists with the unsuccessful status 0. If the signature is verified and the message output file is `-` then the message is printed to stdout.
 
+### tweetnacl-derivepubkey
+
+Derives the public key for the specified secret key
+
+Usage:
+
+```shell
+$ tweetnacl-derivepubkey key.sec key.pub
+```
+
+
 ## Example
 
 Encrypting and decrypting:

tweetnacl-derivepubkey.c

@@ -0,0 +1,35 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "tools.h"
+#include "tweetnacl.h"
+
+void output_key(char filename[], unsigned char key[], int key_size) {
+    if (strcmp(filename, "-") != 0) {
+        FILE *out = create_file(filename);
+        fwrite(key, key_size, 1, out);
+        fclose(out);
+     } else {
+        fwrite(bytes_to_hex(key, key_size), key_size * 2, 1, stdout);
+        fputs("\n", stdout);
+     }
+}
+
+// derives the public key from a given secret key
+int main(int argc, char *argv[]) {
+    if (argc != 3) error(2,
+        "Usage: tweetnacl-derivepubkey key.sec key.pub");
+
+    unsigned char public_key[crypto_box_PUBLICKEYBYTES];
+    unsigned char secret_key[crypto_box_SECRETKEYBYTES];
+
+    read_key(argv[1], secret_key, crypto_box_SECRETKEYBYTES);
+
+    crypto_scalarmult_base(public_key, secret_key);
+
+    output_key(argv[2], public_key, crypto_box_SECRETKEYBYTES);
+
+    return 0;
+}
+