diff --git a/include/mbedtls/gcm.h b/include/mbedtls/gcm.h
index f3c3035..06b06b4 100644
--- a/include/mbedtls/gcm.h
+++ b/include/mbedtls/gcm.h
@@ -339,6 +339,10 @@
  *                    then mbedtls_gcm_finish() never produces any output,
  *                    so \p output_size can be \c 0.
  *                  - \p output_size never needs to be more than \c 15.
+ * \param output_length On success, \p *output_length contains the actual
+ *                      length of the output written in \p output.
+ *                      On failure, the content of \p *output_length is
+ *                      unspecified.
  *
  * \return          \c 0 on success.
  * \return          #MBEDTLS_ERR_GCM_BAD_INPUT on failure:
@@ -347,6 +351,7 @@
  */
 int mbedtls_gcm_finish( mbedtls_gcm_context *ctx,
                         unsigned char *output, size_t output_size,
+                        size_t *output_length,
                         unsigned char *tag, size_t tag_len );
 
 /**
diff --git a/library/cipher.c b/library/cipher.c
index 4f56b52..546cace 100644
--- a/library/cipher.c
+++ b/library/cipher.c
@@ -1109,9 +1109,14 @@
 
 #if defined(MBEDTLS_GCM_C)
     if( MBEDTLS_MODE_GCM == ctx->cipher_info->mode )
+    {
+        size_t output_length;
+        /* The code here doesn't yet support alternative implementations
+         * that can delay up to a block of output. */
         return( mbedtls_gcm_finish( (mbedtls_gcm_context *) ctx->cipher_ctx,
-                                    NULL, 0,
+                                    NULL, 0, &output_length,
                                     tag, tag_len ) );
+    }
 #endif
 
 #if defined(MBEDTLS_CHACHAPOLY_C)
@@ -1158,12 +1163,16 @@
 #if defined(MBEDTLS_GCM_C)
     if( MBEDTLS_MODE_GCM == ctx->cipher_info->mode )
     {
+        size_t output_length;
+        /* The code here doesn't yet support alternative implementations
+         * that can delay up to a block of output. */
+
         if( tag_len > sizeof( check_tag ) )
             return( MBEDTLS_ERR_CIPHER_BAD_INPUT_DATA );
 
         if( 0 != ( ret = mbedtls_gcm_finish(
                        (mbedtls_gcm_context *) ctx->cipher_ctx,
-                       NULL, 0,
+                       NULL, 0, &output_length,
                        check_tag, tag_len ) ) )
         {
             return( ret );
diff --git a/library/gcm.c b/library/gcm.c
index 8fa4ee7..835b1b2 100644
--- a/library/gcm.c
+++ b/library/gcm.c
@@ -532,6 +532,7 @@
 
 int mbedtls_gcm_finish( mbedtls_gcm_context *ctx,
                         unsigned char *output, size_t output_size,
+                        size_t *output_length,
                         unsigned char *tag, size_t tag_len )
 {
     unsigned char work_buf[16];
@@ -546,6 +547,7 @@
      * for the sake of alternative implementations. */
     (void) output;
     (void) output_size;
+    *output_length = 0;
 
     orig_len = ctx->len * 8;
     orig_add_len = ctx->add_len * 8;
@@ -616,7 +618,7 @@
                                     output, length, &olen ) ) != 0 )
         return( ret );
 
-    if( ( ret = mbedtls_gcm_finish( ctx, NULL, 0, tag, tag_len ) ) != 0 )
+    if( ( ret = mbedtls_gcm_finish( ctx, NULL, 0, &olen, tag, tag_len ) ) != 0 )
         return( ret );
 
     return( 0 );
@@ -1068,7 +1070,7 @@
                     goto exit;
             }
 
-            ret = mbedtls_gcm_finish( &ctx, NULL, 0, tag_buf, 16 );
+            ret = mbedtls_gcm_finish( &ctx, NULL, 0, &olen, tag_buf, 16 );
             if( ret != 0 )
                 goto exit;
 
@@ -1140,7 +1142,7 @@
                     goto exit;
             }
 
-            ret = mbedtls_gcm_finish( &ctx, NULL, 0, tag_buf, 16 );
+            ret = mbedtls_gcm_finish( &ctx, NULL, 0, &olen, tag_buf, 16 );
             if( ret != 0 )
                 goto exit;
 
diff --git a/tests/suites/test_suite_gcm.function b/tests/suites/test_suite_gcm.function
index 49859dd..c530e6b 100644
--- a/tests/suites/test_suite_gcm.function
+++ b/tests/suites/test_suite_gcm.function
@@ -50,7 +50,8 @@
     output = NULL;
 
     ASSERT_ALLOC( output, tag->len );
-    TEST_EQUAL( 0, mbedtls_gcm_finish( ctx, NULL, 0, output, tag->len ) );
+    TEST_EQUAL( 0, mbedtls_gcm_finish( ctx, NULL, 0, &olen, output, tag->len ) );
+    TEST_EQUAL( 0, olen );
     ASSERT_COMPARE( output, tag->len, tag->x, tag->len );
     mbedtls_free( output );
     output = NULL;
@@ -96,7 +97,8 @@
     output = NULL;
 
     ASSERT_ALLOC( output, tag->len );
-    TEST_EQUAL( 0, mbedtls_gcm_finish( ctx, NULL, 0, output, tag->len ) );
+    TEST_EQUAL( 0, mbedtls_gcm_finish( ctx, NULL, 0, &olen, output, tag->len ) );
+    TEST_EQUAL( 0, olen );
     ASSERT_COMPARE( output, tag->len, tag->x, tag->len );
 
 exit:
@@ -125,7 +127,9 @@
     }
 
     ASSERT_ALLOC( output_tag, tag->len );
-    TEST_EQUAL( 0, mbedtls_gcm_finish( ctx, NULL, 0, output_tag, tag->len ) );
+    TEST_EQUAL( 0, mbedtls_gcm_finish( ctx, NULL, 0, &olen,
+                                       output_tag, tag->len ) );
+    TEST_EQUAL( 0, olen );
     ASSERT_COMPARE( output_tag, tag->len, tag->x, tag->len );
 
 exit:
@@ -138,11 +142,13 @@
                                    const data_t *tag )
 {
     uint8_t *output = NULL;
+    size_t olen = 0;
 
     TEST_EQUAL( 0, mbedtls_gcm_starts( ctx, mode,
                                        iv->x, iv->len ) );
     ASSERT_ALLOC( output, tag->len );
-    TEST_EQUAL( 0, mbedtls_gcm_finish( ctx, NULL, 0, output, tag->len ) );
+    TEST_EQUAL( 0, mbedtls_gcm_finish( ctx, NULL, 0, &olen, output, tag->len ) );
+    TEST_EQUAL( 0, olen );
     ASSERT_COMPARE( output, tag->len, tag->x, tag->len );
 
 exit: