cmdgen: option to specify the random number device to use.

E.g. you might pass '--random-device=/dev/urandom'.

Mostly because I got sick of waiting for /dev/random to finish
blocking while I was trying to generate throwaway keys for testing bug
fixes in cmdgen itself. But it might also be useful on systems that
call their random device by a different name that we haven't
encountered.

(Since cmdgen also reads the saved PuTTY random seed file, setting
this option to /dev/zero will not render key generation deterministic.
It's tempting to provide _some_ way to do that, for testing purposes
and clearly marked as dangerous of course, but I think it would take
more faff than this.)

cmdgen.c

@@ -37,7 +37,7 @@
  *    run tests.
  */
 #define get_random_data get_random_data_diagnostic
-char *get_random_data(int len)
+char *get_random_data(int len, const char *device)
 {
     char *buf = snewn(len, char);
     memset(buf, 'x', len);
@@ -177,6 +177,8 @@ void help(void)
 	    "        specify file containing old key passphrase\n"
 	    "  --new-passphrase file\n"
 	    "        specify file containing new key passphrase\n"
+	    "  --random-device device\n"
+	    "        specify device to read entropy from (e.g. /dev/urandom)\n"
 	    );
 }
 
@@ -245,6 +247,7 @@ int main(int argc, char **argv)
     char *old_passphrase = NULL, *new_passphrase = NULL;
     int load_encrypted;
     progfn_t progressfn = is_interactive() ? progress_update : no_progress;
+    const char *random_device = NULL;
 
     /* ------------------------------------------------------------------
      * Parse the command line to figure out what we've been asked to do.
@@ -338,6 +341,16 @@ int main(int argc, char **argv)
 				if (!new_passphrase)
 				    errs = TRUE;
 			    }
+			} else if (!strcmp(opt, "-random-device")) {
+			    if (!val && argc > 1)
+				--argc, val = *++argv;
+			    if (!val) {
+				errs = TRUE;
+				fprintf(stderr, "puttygen: option `-%s'"
+					" expects an argument\n", opt);
+			    } else {
+                                random_device = val;
+			    }
 			} else {
 			    errs = TRUE;
 			    fprintf(stderr,
@@ -677,7 +690,7 @@ int main(int argc, char **argv)
 	    strftime(default_comment, 30, "rsa-key-%Y%m%d", &tm);
 
 	random_ref();
-	entropy = get_random_data(bits / 8);
+	entropy = get_random_data(bits / 8, random_device);
 	if (!entropy) {
 	    fprintf(stderr, "puttygen: failed to collect entropy, "
 		    "could not generate key\n");

putty.h

@@ -1352,7 +1352,7 @@ void filename_free(Filename *fn);
 int filename_serialise(const Filename *f, void *data);
 Filename *filename_deserialise(void *data, int maxsize, int *used);
 char *get_username(void);	       /* return value needs freeing */
-char *get_random_data(int bytes);      /* used in cmdgen.c */
+char *get_random_data(int bytes, const char *device); /* used in cmdgen.c */
 char filename_char_sanitise(char c);   /* rewrite special pathname chars */
 
 /*

unix/uxgen.c

@@ -3,21 +3,27 @@
  */
 
 #include <stdio.h>
+#include <errno.h>
+
 #include <fcntl.h>
 #include <unistd.h>
 
 #include "putty.h"
 
-char *get_random_data(int len)
+char *get_random_data(int len, const char *device)
 {
     char *buf = snewn(len, char);
     int fd;
     int ngot, ret;
 
-    fd = open("/dev/random", O_RDONLY);
+    if (!device)
+        device = "/dev/random";
+
+    fd = open(device, O_RDONLY);
     if (fd < 0) {
 	sfree(buf);
-	perror("puttygen: unable to open /dev/random");
+	fprintf(stderr, "puttygen: %s: open: %s\n",
+                device, strerror(errno));
 	return NULL;
     }
 
@@ -27,7 +33,8 @@ char *get_random_data(int len)
 	if (ret < 0) {
 	    close(fd);
             sfree(buf);
-	    perror("puttygen: unable to read /dev/random");
+            fprintf(stderr, "puttygen: %s: read: %s\n",
+                    device, strerror(errno));
 	    return NULL;
 	}
 	ngot += ret;