copy bcrypt autotune from encrypt(1) and expose via crypt_newhash

ok deraadt miod

src/lib/libc/crypt/bcrypt.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: bcrypt.c,v 1.46 2014/11/24 22:47:01 tedu Exp $	*/
+/*	$OpenBSD: bcrypt.c,v 1.47 2014/12/30 10:27:24 tedu Exp $	*/
 
 /*
  * Copyright (c) 2014 Ted Unangst <tedu@openbsd.org>
@@ -224,6 +224,38 @@ bcrypt_checkpass(const char *pass, const char *goodhash)
 	return 0;
 }
 
+/*
+ * Measure this system's performance by measuring the time for 8 rounds.
+ * We are aiming for something that takes between 0.25 and 0.5 seconds.
+ */
+int
+bcrypt_autorounds(void)
+{
+	clock_t before, after;
+	int r = 8;
+	char buf[_PASSWORD_LEN];
+	int duration;
+
+	before = clock();
+	bcrypt_newhash("testpassword", r, buf, sizeof(buf));
+	after = clock();
+
+	duration = after - before;
+
+	/* too quick? slow it down. */
+	while (r < 16 && duration <= CLOCKS_PER_SEC / 4) {
+		r += 1;
+		duration *= 2;
+	}
+	/* too slow? speed it up. */
+	while (r > 4 && duration > CLOCKS_PER_SEC / 2) {
+		r -= 1;
+		duration /= 2;
+	}
+
+	return r;
+}
+
 /*
  * internal utilities
  */

src/lib/libc/crypt/cryptutil.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: cryptutil.c,v 1.6 2014/12/24 22:10:34 tedu Exp $ */
+/* $OpenBSD: cryptutil.c,v 1.7 2014/12/30 10:27:24 tedu Exp $ */
 /*
  * Copyright (c) 2014 Ted Unangst <tedu@openbsd.org>
  *
@@ -21,6 +21,8 @@
 #include <login_cap.h>
 #include <errno.h>
 
+int bcrypt_autorounds(void);
+
 int
 crypt_checkpass(const char *pass, const char *goodhash)
 {
@@ -64,9 +66,13 @@ crypt_newhash(const char *pass, const char *pref, char *hash, size_t hashlen)
 		errno = EINVAL;
 		goto err;
 	}
-	rounds = strtonum(pref + 9, 4, 31, &errstr);
-	if (errstr)
-		goto err;
+	if (strcmp(pref + 9, "a") == 0) {
+		rounds = bcrypt_autorounds();
+	} else {
+		rounds = strtonum(pref + 9, 4, 31, &errstr);
+		if (errstr)
+			goto err;
+	}
 	rv = bcrypt_newhash(pass, rounds, hash, hashlen);
 
 err: