Add constant-flow testing for PKCS7 padding

Signed-off-by: David Horstmann <david.horstmann@arm.com>

library/cipher.c

@@ -14,6 +14,7 @@
 #if defined(MBEDTLS_CIPHER_C)
 
 #include "mbedtls/cipher.h"
+#include "cipher_invasive.h"
 #include "cipher_wrap.h"
 #include "mbedtls/platform_util.h"
 #include "mbedtls/error.h"
@@ -838,8 +839,9 @@ static void add_pkcs_padding(unsigned char *output, size_t output_len,
     }
 }
 
-static int get_pkcs_padding(unsigned char *input, size_t input_len,
+MBEDTLS_STATIC_TESTABLE int get_pkcs_padding(unsigned char *input,
-                            size_t *data_len)
+                                             size_t input_len,
+                                             size_t *data_len)
 {
     size_t i, pad_idx;
     unsigned char padding_len;

library/cipher_invasive.h

@@ -0,0 +1,27 @@
+/**
+ * \file cipher_invasive.h
+ *
+ * \brief Cipher module: interfaces for invasive testing only.
+ *
+ * The interfaces in this file are intended for testing purposes only.
+ * They SHOULD NOT be made available in library integrations except when
+ * building the library for testing.
+ */
+/*
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
+ */
+#ifndef MBEDTLS_CIPHER_INVASIVE_H
+#define MBEDTLS_CIPHER_INVASIVE_H
+
+#include "common.h"
+
+#if defined(MBEDTLS_TEST_HOOKS) && defined(MBEDTLS_CIPHER_C)
+
+MBEDTLS_STATIC_TESTABLE int get_pkcs_padding(unsigned char *input,
+                                             size_t input_len,
+                                             size_t *data_len);
+
+#endif
+
+#endif /* MBEDTLS_CIPHER_INVASIVE_H */

tests/suites/test_suite_cipher.function

@@ -6,6 +6,10 @@
 #include "mbedtls/gcm.h"
 #endif
 
+#include "cipher_invasive.h"
+
+#include "test/constant_flow.h"
+
 #if defined(MBEDTLS_CIPHER_HAVE_SOME_AEAD_VIA_LEGACY_OR_USE_PSA) || defined(MBEDTLS_NIST_KW_C)
 #define MBEDTLS_CIPHER_AUTH_CRYPT
 #endif
@@ -1260,3 +1264,21 @@ exit:
     mbedtls_free(key);
 }
 /* END_CASE */
+
+/* BEGIN_CASE depends_on:MBEDTLS_TEST_HOOKS */
+void get_pkcs_padding(data_t *decrypted_block, int exp_ret, int exp_len)
+{
+    int ret;
+    size_t calculated_len;
+
+    TEST_CF_SECRET(decrypted_block->x, decrypted_block->len);
+    ret = get_pkcs_padding(decrypted_block->x, decrypted_block->len,
+                           &calculated_len);
+    TEST_CF_PUBLIC(decrypted_block->x, decrypted_block->len);
+
+    TEST_EQUAL(ret, exp_ret);
+    if (exp_ret == 0) {
+        TEST_EQUAL(calculated_len, exp_len);
+    }
+}
+/* END_CASE */

tests/suites/test_suite_cipher.padding.data

@@ -217,3 +217,15 @@ check_padding:MBEDTLS_PADDING_NONE:"DABBAD0001":0:5
 
 Check no padding #3 (correct by definition)
 check_padding:MBEDTLS_PADDING_NONE:"":0:0
+
+Constant-time PKCS7 padding, valid #1
+get_pkcs_padding:"00112233445566778899AABBCCDDEE01":0:15
+
+Constant-time PKCS7 padding, valid #2
+get_pkcs_padding:"00112233445566778899AA0505050505":0:11
+
+Constant-time PKCS7 padding, invalid zero
+get_pkcs_padding:"00112233445566778899AABBCCDDEE00":MBEDTLS_ERR_CIPHER_INVALID_PADDING:0
+
+Constant-time PKCS7 padding, invalid > 16
+get_pkcs_padding:"00112233445566778899AABBCCDDEE11":MBEDTLS_ERR_CIPHER_INVALID_PADDING:0