Index: b/NEWS
===================================================================
--- b/NEWS
+++ b/NEWS
@@ -21,6 +21,8 @@
  GPGME_DATA_TYPE_PGP_ENCRYPTED  NEW.
  GPGME_DATA_TYPE_PGP_SIGNATURE  NEW.
  GPGME_DATA_ENCODING_MIME       NEW.
+ gpgme_data_set_file_size       NEW.
+ gpgme_data_get_file_size       NEW.
 
 
 Noteworthy changes in version 1.6.0 (2015-08-26) [C25/A14/R0]
Index: b/doc/gpgme.texi
===================================================================
--- b/doc/gpgme.texi
+++ b/doc/gpgme.texi
@@ -2077,6 +2077,7 @@
 @subsection Data Buffer Meta-Data
 @cindex data buffer, meta-data
 @cindex data buffer, file name
+@cindex data buffer, file size
 @cindex data buffer, encoding
 
 @deftypefun {char *} gpgme_data_get_file_name (@w{gpgme_data_t @var{dh}})
@@ -2103,6 +2104,26 @@
 @end deftypefun
 
 
+@deftypefun {unsigned long long} gpgme_data_get_file_size (@w{gpgme_data_t @var{dh}})
+The function @code{gpgme_data_get_file_size} returns the manually
+set file size associated with the data object.
+It does not return the actual size of file based data types but
+instead only what has been set with @code{gpgme_data_set_file_size}
+
+If no error occurs, the set file size value is returned.
+Otherwise, or if it was not set @code{0} will be returned.
+@end deftypefun
+
+
+@deftypefun gpgme_error_t gpgme_data_set_file_size (@w{gpgme_data_t @var{dh}}, @w{unsigned long long@var{file_size}})
+The function @code{gpgme_data_set_file_size} sets the file size
+associated with the data object.  The
+file size will be passed to gpg and can be used for progress calculations.
+
+The function returns the error code @code{GPG_ERR_INV_VALUE} if
+@var{dh} is not a valid pointer.
+@end deftypefun
+
 @deftp {Data type} {enum gpgme_data_encoding_t}
 @tindex gpgme_data_encoding_t
 The @code{gpgme_data_encoding_t} type specifies the encoding of a
Index: b/lang/cpp/src/data.cpp
===================================================================
--- b/lang/cpp/src/data.cpp
+++ b/lang/cpp/src/data.cpp
@@ -125,6 +125,11 @@
     if (e) {
         d->data = 0;
     }
+    if (dp->isSupported(DataProvider::Seek)) {
+        off_t size = seek(0, SEEK_END);
+        seek(0, SEEK_SET);
+        gpgme_data_set_file_size(d->data, static_cast<unsigned long long>(size));
+    }
 #ifndef NDEBUG
     //std::cerr << "GpgME::Data(): DataProvider supports: "
     //    << ( d->cbs.read ? "read" : "no read" ) << ", "
Index: b/src/data.h
===================================================================
--- b/src/data.h
+++ b/src/data.h
@@ -89,6 +89,9 @@
   /* File name of the data object.  */
   char *file_name;
 
+  /* File size of the data object.  */
+  unsigned long long file_size;
+
   union
   {
     /* For gpgme_data_new_from_fd.  */
@@ -121,6 +124,7 @@
       int (*cb) (void *, char *, size_t, size_t *);
       void *handle;
     } old_user;
+
   } data;
 };
 
Index: b/src/data.c
===================================================================
--- b/src/data.c
+++ b/src/data.c
@@ -243,6 +243,39 @@
   return dh->file_name;
 }
 
+/* Set the file size associated with the data object with handle DH to
+   FILE_SIZE.  */
+gpgme_error_t
+gpgme_data_set_file_size (gpgme_data_t dh, unsigned long long file_size)
+{
+  TRACE_BEG1 (DEBUG_DATA, "gpgme_data_set_file_size", dh,
+              "file_size=%llu", file_size);
+
+  if (!dh)
+    return TRACE_ERR (gpg_error (GPG_ERR_INV_VALUE));
+
+  dh->file_size = file_size;
+
+  return TRACE_ERR (0);
+}
+
+
+/* Get the file size associated with the data object with handle DH,
+   or 0 if there is none.  */
+unsigned long long
+gpgme_data_get_file_size (gpgme_data_t dh)
+{
+  if (!dh)
+    {
+      TRACE (DEBUG_DATA, "gpgme_data_get_file_size", dh);
+      return 0;
+    }
+
+  TRACE1 (DEBUG_DATA, "gpgme_data_get_file_size", dh,
+          "dh->file_size=%llu", dh->file_size);
+  return dh->file_size;
+}
+
 
 /* Functions to support the wait interface.  */
 
Index: b/src/engine-gpg.c
===================================================================
--- b/src/engine-gpg.c
+++ b/src/engine-gpg.c
@@ -1713,6 +1713,27 @@
 
 
 static gpgme_error_t
+append_size_arg (engine_gpg_t gpg, gpgme_data_t data)
+{
+  gpgme_error_t err = 0;
+  if (gpgme_data_get_file_size (data))
+    {
+      err = add_arg (gpg, "--set-filesize");
+      if (!err)
+        {
+          char *size_str;
+          if (asprintf (&size_str, "%llu",
+                        gpgme_data_get_file_size (data)) < 0)
+            return gpg_error_from_syserror ();
+          err = add_arg (gpg, size_str);
+          free (size_str);
+        }
+    }
+  return err;
+}
+
+
+static gpgme_error_t
 gpg_encrypt (void *engine, gpgme_key_t recp[], gpgme_encrypt_flags_t flags,
 	     gpgme_data_t plain, gpgme_data_t ciph, int use_armor)
 {
@@ -1761,6 +1782,8 @@
 	err = add_arg (gpg, gpgme_data_get_file_name (plain));
     }
   if (!err)
+    err = append_size_arg (gpg, plain);
+  if (!err)
     err = add_arg (gpg, "--");
   if (!err)
     err = add_data (gpg, plain, -1, 0);
@@ -1831,6 +1854,8 @@
 	err = add_arg (gpg, gpgme_data_get_file_name (plain));
     }
   if (!err)
+    err = append_size_arg (gpg, plain);
+  if (!err)
     err = add_arg (gpg, "--");
   if (!err)
     err = add_data (gpg, plain, -1, 0);
@@ -2420,6 +2445,8 @@
       if (!err)
 	err = add_arg (gpg, gpgme_data_get_file_name (in));
     }
+  if (!err)
+    err = append_size_arg (gpg, in);
 
   /* Tell the gpg object about the data.  */
   if (!err)
Index: b/src/gpgme.h.in
===================================================================
--- b/src/gpgme.h.in
+++ b/src/gpgme.h.in
@@ -1279,6 +1279,19 @@
 gpgme_error_t gpgme_data_set_file_name (gpgme_data_t dh,
 					const char *file_name);
 
+/* Get the file size associated with the data object with handle DH, or
+   0 if there is none set. This only returns file_size explicitly set by
+   gpgme_data_set_file_size and not the actual file size for file based
+   data types. */
+unsigned long long gpgme_data_get_file_size (gpgme_data_t dh);
+
+/* Set the file size associated with the data object with handle DH to
+   FILE_SIZE. The file size is mainly useful for callback based data
+   types where it can be used to give a filesize indication that will
+   be used e.g. for progress calculation. */
+gpgme_error_t gpgme_data_set_file_size (gpgme_data_t dh,
+					unsigned long long file_size);
+
 /* Try to identify the type of the data in DH.  */
 gpgme_data_type_t gpgme_data_identify (gpgme_data_t dh, int reserved);
 
Index: b/src/gpgme.def
===================================================================
--- b/src/gpgme.def
+++ b/src/gpgme.def
@@ -226,5 +226,8 @@
 
     gpgme_pubkey_algo_string              @169
     gpgme_set_ctx_flag                    @170
+
+    gpgme_data_set_file_size              @171
+    gpgme_data_get_file_size              @172
 ; END
 
Index: b/src/libgpgme.vers
===================================================================
--- b/src/libgpgme.vers
+++ b/src/libgpgme.vers
@@ -29,6 +29,8 @@
 
     gpgme_data_set_file_name;
     gpgme_data_get_file_name;
+    gpgme_data_set_file_size;
+    gpgme_data_get_file_size;
     gpgme_data_identify;
 
     gpgme_sig_notation_clear;