commit 5a7be1041936e9dbdea16eff5051fafbcfde021b
author Gilles Peskine <Gilles.Peskine@arm.com> Wed Jun 23 21:51:32 2021 +0200
committer Gilles Peskine <Gilles.Peskine@arm.com> Wed Jun 23 21:51:32 2021 +0200
tree 620a5b77578ae885caddd028be62803605247f34
parent f06b92d7241ce5c0b51922863713ab0127042665

Add output_length parameter to mbedtls_gcm_finish

Without this parameter, it would be hard for callers to know how many bytes
of output the function wrote into the output buffer. It would be possible,
since the cumulated output must have the same length as the cumulated input,
but it would be cumbersome for the caller to keep track.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

library/cipher.c

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 );

library/gcm.c

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;