Add comments to AEAD (non-PSA) examples
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/programs/psa/aead_non_psa.c b/programs/psa/aead_non_psa.c
index 4b48b3a..00c7f0e 100644
--- a/programs/psa/aead_non_psa.c
+++ b/programs/psa/aead_non_psa.c
@@ -1,8 +1,29 @@
-/*
- * This is a companion to aead_psa.c, doing the same operations with the
- * legacy Cipher API. The goal is that comparing the two programs will help people
- * migrating to the PSA Crypto API.
+/**
+ * Cipher API multi-part AEAD demonstration.
*
+ * This program AEAD-encrypts a message, using the algorithm and key size
+ * specified on the command line, using the multi-part API.
+ *
+ * It comes with a companion program aead_psa.c, which does the same
+ * operations with the PSA Crypto API. The goal is that comparing the two
+ * programs will help people migrating to the PSA Crypto API.
+ *
+ * When used with multi-part AEAD operations, the `mbedtls_cipher_context`
+ * serves a triple purpose (1) hold the key, (2) store the algorithm when no
+ * operation is active, and (3) save progress information for the current
+ * operation. With PSA those roles are held by disinct objects: (1) a
+ * psa_key_id_t to hold the key, a (2) psa_algorithm_t to represent the
+ * algorithm, and (3) a psa_operation_t for multi-part progress.
+ *
+ * On the other hand, with PSA, the algorithms encodes the desired tag length;
+ * with Cipher the desired tag length needs to be tracked separately.
+ *
+ * This program and its companion aead_psa.c illustrate this by doing the
+ * same sequence of multi-part AEAD computation with both APIs; looking at the
+ * two side by side should make the differences and similarities clear.
+ */
+
+/*
* Copyright The Mbed TLS Contributors
* SPDX-License-Identifier: Apache-2.0
*
@@ -19,26 +40,18 @@
* limitations under the License.
*/
-/*
- * When used with multi-part AEAD operations, the `mbedtls_cipher_context`
- * serves a triple purpose (1) hold the key, (2) store the algorithm when no
- * operation is active, and (3) save progress information for the current
- * operation. With PSA those roles are held by disinct objects: (1) a
- * psa_key_id_t to hold the key, a (2) psa_algorithm_t to represent the
- * algorithm, and (3) a psa_operation_t for multi-part progress.
- *
- * On the other hand, with PSA, the algorithms encodes the desired tag length;
- * with Cipher the desired tag length needs to be tracked separately.
- *
- * This program and its compation aead_psa.c illustrate this by doing the
- * same sequence of multi-part AEAD computation with both APIs; looking at the
- * two side by side should make the differences and similarities clear.
- */
-
-#include <stdio.h>
-
+/* First include Mbed TLS headers to get the Mbed TLS configuration and
+ * platform definitions that we'll use in this program. Also include
+ * standard C headers for functions we'll use here. */
#include "mbedtls/build_info.h"
+#include "mbedtls/cipher.h"
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+
+/* If the build options we need are not enabled, compile a placeholder. */
#if !defined(MBEDTLS_CIPHER_C) || \
!defined(MBEDTLS_AES_C) || !defined(MBEDTLS_GCM_C) || \
!defined(MBEDTLS_CHACHAPOLY_C)
@@ -51,52 +64,67 @@
}
#else
-#include <string.h>
+/* The real program starts here. */
-#include "mbedtls/cipher.h"
+const char usage[] = "Usage: aead_psa [aes128-gcm|aes256-gcm|aes128-gcm_8|chachapoly]";
-/*
- * Dummy data and helper functions
- */
-const char usage[] = "Usage: aead_non_psa [aes128-gcm|aes256-gcm|aes128-gcm_8|chachapoly]";
-
+/* Dummy data for encryption: IV/nonce, additional data, 2-part message */
const unsigned char iv1[12] = { 0x00 };
const unsigned char add_data1[] = { 0x01, 0x02 };
const unsigned char msg1_part1[] = { 0x03, 0x04 };
const unsigned char msg1_part2[] = { 0x05, 0x06, 0x07 };
+/* Dummy data (2nd message) */
const unsigned char iv2[12] = { 0x10 };
const unsigned char add_data2[] = { 0x11, 0x12 };
const unsigned char msg2_part1[] = { 0x13, 0x14 };
const unsigned char msg2_part2[] = { 0x15, 0x16, 0x17 };
+/* This must at least the sum of the length of the 2 parts for each message.
+ * This is a macro for the sake of compilers with insufficient C99 support. */
#define MSG_MAX_SIZE 5
+/* Dummy key material - never do this in production!
+ * 32-byte is enough to all the key size supported by this program. */
const unsigned char key_bytes[32] = { 0x2a };
-void print_out( const char *title, unsigned char *out, size_t len )
+/* Print the contents of a buffer in hex */
+void print_buf( const char *title, unsigned char *buf, size_t len )
{
printf( "%s:", title );
for( size_t i = 0; i < len; i++ )
- printf( " %02x", out[i] );
+ printf( " %02x", buf[i] );
printf( "\n" );
}
-#define CHK( code ) \
- do { \
- ret = code; \
- if( ret != 0 ) { \
- printf( "%03d: ret = -0x%04x\n", __LINE__, (unsigned) -ret ); \
- goto exit; \
- } \
+/* Run an Mbed TLS function and bail out if it fails. */
+#define CHK( expr ) \
+ do \
+ { \
+ ret = ( expr ); \
+ if( ret != 0 ) \
+ { \
+ printf( "Error %d at line %d: %s\n", \
+ ret, \
+ __LINE__, \
+ #expr ); \
+ goto exit; \
+ } \
} while( 0 )
+/*
+ * Prepare encryption material:
+ * - interpret command-line argument
+ * - set up key
+ * - outputs: context and tag length, which together hold all the information
+ */
static int aead_prepare( const char *info,
mbedtls_cipher_context_t *ctx,
size_t *tag_len )
{
int ret;
+ /* Convert arg to type + tag_len */
mbedtls_cipher_type_t type;
if( strcmp( info, "aes128-gcm" ) == 0 ) {
type = MBEDTLS_CIPHER_AES_128_GCM;
@@ -115,9 +143,11 @@
return( MBEDTLS_ERR_CIPHER_BAD_INPUT_DATA );
}
+ /* Prepare context for the given type */
CHK( mbedtls_cipher_setup( ctx,
mbedtls_cipher_info_from_type( type ) ) );
+ /* Import key */
int key_len = mbedtls_cipher_get_key_bitlen( ctx );
CHK( mbedtls_cipher_setkey( ctx, key_bytes, key_len, MBEDTLS_ENCRYPT ) );
@@ -125,9 +155,15 @@
return( ret );
}
+/*
+ * Print out some information.
+ *
+ * All of this information was present in the command line argument, but his
+ * function demonstrates how each piece can be recovered from (ctx, tag_len).
+ */
static void aead_info( const mbedtls_cipher_context_t *ctx, size_t tag_len )
{
- // no convenient way to get the cipher type (for example, AES)
+ // no convenient way to get the just cipher type (for example, AES)
const char *ciph = "???";
int key_bits = mbedtls_cipher_get_key_bitlen( ctx );
mbedtls_cipher_mode_t mode = mbedtls_cipher_get_cipher_mode( ctx );
@@ -139,6 +175,9 @@
printf( "cipher: %s, %d, %s, %u\n", ciph, key_bits, mode_str, (unsigned) tag_len );
}
+/*
+ * Encrypt a 2-part message.
+ */
static int aead_encrypt( mbedtls_cipher_context_t *ctx, size_t tag_len,
const unsigned char *iv, size_t iv_len,
const unsigned char *ad, size_t ad_len,
@@ -163,12 +202,15 @@
p += tag_len;
olen = p - out;
- print_out( "cipher", out, olen );
+ print_buf( "cipher", out, olen );
exit:
return( ret );
}
+/*
+ * AEAD demo: set up key/alg, print out info, encrypt messages.
+ */
static int aead_demo( const char *info )
{
int ret = 0;
@@ -203,13 +245,20 @@
*/
int main( int argc, char **argv )
{
+ /* Check usage */
if( argc != 2 )
{
puts( usage );
return( 1 );
}
- aead_demo( argv[1] );
+ int ret;
+
+ /* Run the demo */
+ CHK( aead_demo( argv[1] ) );
+
+exit:
+ return( ret == 0 ? EXIT_SUCCESS : EXIT_FAILURE );
}
#endif