Add API gpgme_op_random_value.

* src/genrandom.c (getrandom_size_t): New.
(gpgme_op_random_value): New.
* src/gpgme.def: Add new function.
* src/libgpgme.vers: Ditto.
* src/gpgme.h.in: Add prototype.

* tests/run-genrandom.c: Add an option to use the new function.
--

The implementation is not optimized but sufficient for our use case.
Possible improvements for this and gpgme_op_random_bytes are a cache
for random bytes in the context so that we do not need to get out to
gpgme for just a few random bytes.

GnuPG-bug-id: 6694

4 files changed, 91 insertions, 1 deletions

diff --git a/src/genrandom.c b/src/genrandom.c
index a4baa0b2..0f787cf4 100644
--- a/src/genrandom.c
+++ b/src/genrandom.c
@@ -58,6 +58,46 @@ do_genrandom (gpgme_ctx_t ctx, gpgme_data_t dataout, size_t length, int zbase)
 }
 
 
+/* Store a random value into RETVAL or return an error.  */
+static gpgme_error_t
+getrandom_size_t (gpgme_ctx_t ctx, size_t *retval)
+{
+  gpgme_error_t err = 0;
+  gpgme_data_t data = NULL;
+  char *datap = NULL;
+  size_t datalen;
+
+  err = gpgme_data_new (&data);
+  if (err)
+    goto leave;
+
+  err = do_genrandom (ctx, data, sizeof (size_t), 0);
+  if (!err)
+    err = _gpgme_wait_one (ctx);
+  if (err)
+    goto leave;
+
+  datap = gpgme_data_release_and_get_mem (data, &datalen);
+  data = NULL;
+  if (!datap)
+    {
+      err = gpg_error_from_syserror ();
+      goto leave;
+    }
+  if (datalen != sizeof (size_t))
+    {
+      err = gpg_error (GPG_ERR_INTERNAL);
+      goto leave;
+    }
+  memcpy (retval, datap, datalen);
+
+ leave:
+  free (datap);
+  gpgme_data_release (data);
+  return err;
+}
+
+
 /* Fill BUFFER of size BUFSIZE with random bytes retrieved from gpg.
  * If GPGME_RANDOM_MODE_ZBASE32 is used BUFSIZE needs to be at least
  * 31 and will be filled with a string of 30 ascii characters followed
@@ -147,3 +187,46 @@ gpgme_op_random_bytes (gpgme_ctx_t ctx, gpgme_random_mode_t mode,
   gpgme_data_release (data);
   return TRACE_ERR (err);
 }
+
+
+
+/* On success the function stores an unbiased random value in the
+ * range [0,limit) at RETVAL.  On error the function returns an error
+ * code and the value at RETVAL is undefined.  A context must be
+ * provided so that the function can get raw random bytes from the gpg
+ * enine (i.e. from Libgcrypt). */
+gpgme_error_t
+gpgme_op_random_value (gpgme_ctx_t ctx, size_t limit, size_t *retval)
+{
+  gpgme_error_t err;
+  size_t t, x;
+
+  TRACE_BEG  (DEBUG_CTX, "gpgme_op_random_value", ctx, "limit=%zu", limit);
+
+  if (!ctx || limit < 2 || !retval)
+    {
+      err = gpg_error (GPG_ERR_INV_VALUE);
+      goto leave;
+    }
+
+  /* This is the OpenBSD algorithm as used by arc4random_uniform and
+   * described in "Fast Random Integer Generation in an Interval" by
+   * Daniel Lemire (arXiv:1805.10941v4, 2018).  */
+  t = (-limit) % limit;
+  x = 0;  /* Avoid (false) compiler warning.  */
+  do
+    {
+      err = getrandom_size_t (ctx, &x);
+      if (err)
+        goto leave;
+      /* fprintf (stderr, "getrandom returned %zu (limit=%zu, t=%zu)\n", */
+      /*          x, limit, t); */
+    }
+  while (x < t);
+  x %= limit;
+
+  *retval = x;
+
+ leave:
+  return TRACE_ERR (err);
+}
diff --git a/src/gpgme.def b/src/gpgme.def
index d203901f..3bdaac4e 100644
--- a/src/gpgme.def
+++ b/src/gpgme.def
@@ -276,4 +276,5 @@ EXPORTS
     gpgme_op_setownertrust_start          @214
 
     gpgme_op_random_bytes                 @215
+    gpgme_op_random_value                 @216
 ; END
diff --git a/src/gpgme.h.in b/src/gpgme.h.in
index 9c0c7977..705d9741 100644
--- a/src/gpgme.h.in
+++ b/src/gpgme.h.in
@@ -2481,10 +2481,15 @@ typedef enum
   }
 gpgme_random_mode_t;
 
-/* Fill BUFFER with BUFSIZE random bytes from gpg.  */
+/* Fill BUFFER with BUFSIZE random bytes from gpg or return an error.  */
 gpgme_error_t gpgme_op_random_bytes (gpgme_ctx_t ctx, gpgme_random_mode_t mode,
                                      char *buffer, size_t bufsize);
 
+/* Store an unbiased random value in the range [0,LIMIT) at RETVAL or
+ * return an error.  */
+gpgme_error_t gpgme_op_random_value (gpgme_ctx_t ctx, size_t limit,
+                                     size_t *retval);
+
 
 
 /*
diff --git a/src/libgpgme.vers b/src/libgpgme.vers
index 83f8c87b..555f9fc0 100644
--- a/src/libgpgme.vers
+++ b/src/libgpgme.vers
@@ -274,6 +274,7 @@ GPGME_1.0 {
     gpgme_op_setownertrust_start;
 
     gpgme_op_random_bytes;
+    gpgme_op_random_value;
 
   local:
     *;