Add the -e, --entropy-bits option

Add an option allowing to define how many bits of entropy is
brought in by a random byte.

man/scdrand.1.in

@@ -14,6 +14,8 @@ scdrand \- Extract random numbers from a smartcard
 .IR N ]
 .RB [ \-t | --threshold
 .IR N ]
+.RB [ \-e | --entropy-bits
+.IR N ]
 .RB [ nbytes ]
 .YS
 
@@ -53,6 +55,11 @@ seconds between iterations.
 Feed the kernel entropy pool only if it contains less than
 .N N
 bits of entropy.
+.TP
+.BR -e ", " --entropy-bits " " \fiN\fR
+Define the entropic value of a single random byte.
+.I N
+must be comprised between 1 and 8, inclusive.
 
 .SH EXAMPLES
 .PP

src/scdrand.c

@@ -44,6 +44,7 @@
 #define DEFAULT_RANDOM_BYTES    8
 #define DEFAULT_LOOP_ITERATIONS 1
 #define DEFAULT_LOOP_INTERVAL   10
+#define DEFAULT_ENTROPY_FACTOR  8
 
 static void
 usage(int status)
@@ -71,6 +72,11 @@ specified, the default is %d bytes.\n\n", DEFAULT_RANDOM_BYTES);
                         Set to 0 (default) to always add entropy.\n\
 ");
 
+    printf("\
+  -e, --entropy-bits N  Set the entropic value of a random byte.\n\
+                        Must be between 1 and 8. Default is %d.\n\
+\n", DEFAULT_ENTROPY_FACTOR);
+
     printf("Report bugs to <%s>.\n", PACKAGE_BUGREPORT);
 
     exit(status);
@@ -184,12 +190,14 @@ wait_for_threshold(unsigned threshold, unsigned interval)
  * The function only returns in the parent process; the child process
  * remains in a loop until it terminates.
  *
+ * @param factor The number of entropy bits in a single byte.
+ *
  * @return
  * A file descriptor for the pipe connecting the parent process to
  * its child.
  */
 static int
-start_privileged_process(void)
+start_privileged_process(unsigned factor)
 {
     int fd[2];
     pid_t pid;
@@ -218,7 +226,7 @@ start_privileged_process(void)
             if ( n == 0 )   /* EOF, parent process must have closed its end. */
                 exit(EXIT_SUCCESS);
 
-            rpi->entropy_count = n * 8;
+            rpi->entropy_count = n * factor;
             rpi->buf_size = n;
             memcpy(&(rpi->buf[0]), buffer, n);
 
@@ -259,28 +267,31 @@ int
 main(int argc, char **argv)
 {
     int c, fd, n, loop;
-    unsigned nbytes, interval, threshold;
+    unsigned nbytes, interval, threshold, factor;
     assuan_context_t ctx;
     gpg_error_t e;
     unsigned char random_buffer[MAX_RANDOM_BYTES];
 
     struct option options[] = {
-        { "help",       0, NULL, 'h' },
-        { "version",    0, NULL, 'v' },
-        { "loop",       0, NULL, 'l' },
-        { "max-loop",   1, NULL, 'L' },
-        { "interval",   1, NULL, 'i' },
-        { "threshold",  1, NULL, 't' },
-        { NULL,         0, NULL, 0 }
+        { "help",           0, NULL, 'h' },
+        { "version",        0, NULL, 'v' },
+        { "loop",           0, NULL, 'l' },
+        { "max-loop",       1, NULL, 'L' },
+        { "interval",       1, NULL, 'i' },
+        { "threshold",      1, NULL, 't' },
+        { "entropy-bits",   1, NULL, 'e' },
+        { NULL,             0, NULL, 0 }
     };
 
     setprogname(argv[0]);
     nbytes = DEFAULT_RANDOM_BYTES;
     loop = DEFAULT_LOOP_ITERATIONS;
     interval = DEFAULT_LOOP_INTERVAL;
+    factor = DEFAULT_ENTROPY_FACTOR;
     threshold = 0;
 
-    while ( (c = getopt_long(argc, argv, "hvlL:i:t:", options, NULL)) != -1 ) {
+    while ( (c = getopt_long(argc, argv, "hvlL:i:t:e:",
+                             options, NULL)) != -1 ) {
         switch ( c ) {
         case 'h':
             usage(EXIT_SUCCESS);
@@ -309,10 +320,16 @@ main(int argc, char **argv)
         case 't':
             threshold = get_uinteger_or_die(optarg);
             break;
+
+        case 'e':
+            factor = get_uinteger_or_die(optarg);
+            if ( factor == 0 || factor > 8 )
+                errx(EXIT_FAILURE, "Expected an entropy factor between 1 and 8");
+            break;
         }
     }
 
-    fd = start_privileged_process();
+    fd = start_privileged_process(factor);
 
     if ( optind < argc ) {
         nbytes = get_uinteger_or_die(argv[optind++]);