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Add output parameter to mbedtls_gcm_finish

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Alternative implementations of GCM may delay the output of partial
blocks from mbedtls_gcm_update(). Add an output parameter to
mbedtls_gcm_finish() to allow such implementations to pass the final
partial block back to the caller. With the software implementation,
this final output is always empty.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2021-04-15 16:48:32 +02:00
parent 441907ec30
commit 9461e45a17
5 changed files with 37 additions and 10 deletions
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10
ChangeLog.d/gcm-update.txt
View file

@ -1,3 +1,13 @@
API changes
* The interface of the GCM module has changed to remove restrictions on
how the input to multipart operations is broken down. mbedtls_gcm_finish()
now takes an extra output parameter for the last partial output block.
The software implementation always produces the full output at each
call to mbedtls_gcm_update(), but alternative implementations activated
by MBEDTLS_GCM_ALT may delay partial blocks to the next call to
mbedtls_gcm_update() or mbedtls_gcm_finish().
These changes are backward compatible for users of the cipher API.
Features
* The multi-part GCM interface (mbedtls_gcm_update() or
mbedtls_cipher_update()) no longer requires the size of partial inputs to

14
include/mbedtls/gcm.h
View file

@ -282,13 +282,23 @@ int mbedtls_gcm_update( mbedtls_gcm_context *ctx,
* buffer of at least \p tag_len Bytes.
* \param tag_len The length of the tag to generate. This must be at least
* four.
* \param output The buffer for the final output.
* This must be a writable buffer of at least \p output_len
* bytes.
* With the built-in implementation, there is no final
* output and this can be \p NULL.
* Alternative implementations may return a partial block
* of output.
* \param output_len The size of the \p output buffer in bytes.
* With the built-in implementation, this can be \c 0.
* Alternative implementations may require a 15-byte buffer.
*
* \return \c 0 on success.
* \return #MBEDTLS_ERR_GCM_BAD_INPUT on failure.
*/
int mbedtls_gcm_finish( mbedtls_gcm_context *ctx,
unsigned char *tag,
size_t tag_len );
unsigned char *output, size_t output_len,
unsigned char *tag, size_t tag_len );
/**
* \brief This function clears a GCM context and the underlying

2
library/cipher.c
View file

@ -1101,6 +1101,7 @@ int mbedtls_cipher_write_tag( mbedtls_cipher_context_t *ctx,
#if defined(MBEDTLS_GCM_C)
if( MBEDTLS_MODE_GCM == ctx->cipher_info->mode )
return( mbedtls_gcm_finish( (mbedtls_gcm_context *) ctx->cipher_ctx,
NULL, 0,
tag, tag_len ) );
#endif
@ -1153,6 +1154,7 @@ int mbedtls_cipher_check_tag( mbedtls_cipher_context_t *ctx,
if( 0 != ( ret = mbedtls_gcm_finish(
(mbedtls_gcm_context *) ctx->cipher_ctx,
NULL, 0,
check_tag, tag_len ) ) )
{
return( ret );

15
library/gcm.c
View file

@ -471,8 +471,8 @@ int mbedtls_gcm_update( mbedtls_gcm_context *ctx,
}
int mbedtls_gcm_finish( mbedtls_gcm_context *ctx,
unsigned char *tag,
size_t tag_len )
unsigned char *output, size_t output_len,
unsigned char *tag, size_t tag_len )
{
unsigned char work_buf[16];
size_t i;
@ -482,6 +482,11 @@ int mbedtls_gcm_finish( mbedtls_gcm_context *ctx,
GCM_VALIDATE_RET( ctx != NULL );
GCM_VALIDATE_RET( tag != NULL );
/* We never pass any output in finish(). The output parameter exists only
* for the sake of alternative implementations. */
(void) output;
(void) output_len;
orig_len = ctx->len * 8;
orig_add_len = ctx->add_len * 8;
@ -541,7 +546,7 @@ int mbedtls_gcm_crypt_and_tag( mbedtls_gcm_context *ctx,
if( ( ret = mbedtls_gcm_update( ctx, length, input, output ) ) != 0 )
return( ret );
if( ( ret = mbedtls_gcm_finish( ctx, tag, tag_len ) ) != 0 )
if( ( ret = mbedtls_gcm_finish( ctx, NULL, 0, tag, tag_len ) ) != 0 )
return( ret );
return( 0 );
@ -979,7 +984,7 @@ int mbedtls_gcm_self_test( int verbose )
goto exit;
}
ret = mbedtls_gcm_finish( &ctx, tag_buf, 16 );
ret = mbedtls_gcm_finish( &ctx, NULL, 0, tag_buf, 16 );
if( ret != 0 )
goto exit;
@ -1039,7 +1044,7 @@ int mbedtls_gcm_self_test( int verbose )
goto exit;
}
ret = mbedtls_gcm_finish( &ctx, tag_buf, 16 );
ret = mbedtls_gcm_finish( &ctx, NULL, 0, tag_buf, 16 );
if( ret != 0 )
goto exit;

6
tests/suites/test_suite_gcm.function
View file

@ -41,7 +41,7 @@ static int check_multipart( mbedtls_gcm_context *ctx,
output = NULL;
ASSERT_ALLOC( output, tag->len );
TEST_EQUAL( 0, mbedtls_gcm_finish( ctx, output, tag->len ) );
TEST_EQUAL( 0, mbedtls_gcm_finish( ctx, NULL, 0, output, tag->len ) );
ASSERT_COMPARE( output, tag->len, tag->x, tag->len );
mbedtls_free( output );
output = NULL;
@ -326,10 +326,10 @@ void gcm_invalid_param( )
/* mbedtls_gcm_finish() */
TEST_INVALID_PARAM_RET(
MBEDTLS_ERR_GCM_BAD_INPUT,
mbedtls_gcm_finish( NULL, valid_buffer, valid_len ) );
mbedtls_gcm_finish( NULL, NULL, 0, valid_buffer, valid_len ) );
TEST_INVALID_PARAM_RET(
MBEDTLS_ERR_GCM_BAD_INPUT,
mbedtls_gcm_finish( &ctx, NULL, valid_len ) );
mbedtls_gcm_finish( &ctx, NULL, 0, NULL, valid_len ) );
exit:
mbedtls_gcm_free( &ctx );