STM32L4: fix byte-swapped IV writeback in mbedTLS AES-CBC decrypt

CHANGELOG.md

@@ -87,6 +87,8 @@ A message that notes the main changes in the update.
 - Fixed memory leak with Nanostack memory manager that caused the stack to run of memory when used with zero-copy Ethernet drivers
 - LPC17xx:
   - Fixed I2C single-byte API continuing to send bytes after being NACKed
+- STM32L4:
+  - Fixed multiple bugs causing hardware-accelerated multi-block AES encryption and decryption to produce incorrect results
 - RP2xxx:
   - Fixed issue where reading from an I2C master in slave mode could put the I2C peripheral in a bad state if the master transferred
     more bytes than expected

connectivity/drivers/mbedtls/TARGET_STM/aes_alt_stm32l4.c

@@ -200,6 +200,9 @@ int mbedtls_aes_crypt_cbc(mbedtls_aes_context *ctx,
     if (length % 16) {
         return (MBEDTLS_ERR_AES_INVALID_INPUT_LENGTH);
     }
+    if (length == 0) {
+        return 0;
+    }
     ctx->hcryp_aes.Init.pInitVect = &iv[0];
     if (st_cbc_restore_context(ctx) != 0) {
         return (ST_ERR_AES_BUSY);
@@ -218,15 +221,15 @@ int mbedtls_aes_crypt_cbc(mbedtls_aes_context *ctx,
         }
         ctx->ctx_save_cr = ctx->hcryp_aes.Instance->CR; // save here before overwritten
         ctx->hcryp_aes.Instance->CR &= ~AES_CR_EN;
-        *iv_ptr++ = ctx->hcryp_aes.Instance->IVR3;
-        *iv_ptr++ = ctx->hcryp_aes.Instance->IVR2;
-        *iv_ptr++ = ctx->hcryp_aes.Instance->IVR1;
-        *iv_ptr++ = ctx->hcryp_aes.Instance->IVR0;
+        *iv_ptr++ = __REV(ctx->hcryp_aes.Instance->IVR3);
+        *iv_ptr++ = __REV(ctx->hcryp_aes.Instance->IVR2);
+        *iv_ptr++ = __REV(ctx->hcryp_aes.Instance->IVR1);
+        *iv_ptr++ = __REV(ctx->hcryp_aes.Instance->IVR0);
     } else {
         if (st_hal_cryp_cbc(ctx, CRYP_ALGOMODE_ENCRYPT, length, iv, (uint8_t *)input, (uint8_t *)output) != 0) {
             return ST_ERR_AES_BUSY;
         }
-        memcpy(iv, output, 16);   /* current output is the IV vector for the next call */
+        memcpy(iv, output + length - 16, 16);
         ctx->ctx_save_cr = ctx->hcryp_aes.Instance->CR;
     }
 

connectivity/mbedtls/tests/TESTS/mbedtls/CMakeLists.txt

@@ -1,3 +1,4 @@
+add_subdirectory(aes_cbc)
 add_subdirectory(multi)
 add_subdirectory(sanity)
 add_subdirectory(selftest)

connectivity/mbedtls/tests/TESTS/mbedtls/aes_cbc/CMakeLists.txt

@@ -0,0 +1,11 @@
+# Copyright (c) 2026 Jan Gerrit Gers
+# SPDX-License-Identifier: Apache-2.0
+
+mbed_greentea_add_test(
+    TEST_NAME
+        mbed-connectivity-mbedtls-aes-cbc
+    TEST_SOURCES
+        main.cpp
+    TEST_REQUIRED_LIBS
+        mbed-mbedtls
+)

connectivity/mbedtls/tests/TESTS/mbedtls/aes_cbc/main.cpp

@@ -0,0 +1,334 @@
+/* mbed Microcontroller Library
+ * Copyright (c) 2026 Jan Gerrit Gers
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+/**
+ * AES-CBC chaining tests.
+ *
+ * mbedtls_aes_crypt_cbc() promises a streaming contract: on return, the
+ * callers iv[] holds the last ciphertext block processed. Splitting the
+ * message over many calls should give the same result.
+ *
+ * Targets with MBEDTLS_AES_ALT (like the STM32 AES drivers) may rewrite this,
+ * and may get this wrong.
+ *
+ * Plain MBEDTLS API level. Equally valid on software-only targets.
+ */
+
+#include <stdio.h>
+#include <string.h>
+
+#include "greentea-client/test_env.h"
+#include "unity.h"
+#include "utest.h"
+
+#if !defined(MBEDTLS_CONFIG_FILE)
+#include "mbedtls/config.h"
+#else
+#include MBEDTLS_CONFIG_FILE
+#endif
+
+#if !defined(MBEDTLS_AES_C)
+#error [NOT_SUPPORTED] MBEDTLS_AES_C undefined
+#elif !defined(MBEDTLS_CIPHER_MODE_CBC)
+#error [NOT_SUPPORTED] MBEDTLS_CIPHER_MODE_CBC undefined
+#else
+
+#include "mbedtls/aes.h"
+
+using namespace utest::v1;
+
+#define BLOCK_SIZE  16
+#define MSG_SIZE    64  // 4 AES blocks, so a message can be split several ways
+
+/* The following CBC example vectors were taken from NIST SP 800-38A, Appendix F:
+ * CBC-AES128 from F.2.1 and F.2.2, and CBC-AES256 from F.2.5 and F.2.6,
+ * both share the same plaintext and IV.
+ * https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38a.pdf
+ */
+
+static const unsigned char AES_IV[BLOCK_SIZE] = {
+    0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
+    0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f
+};
+
+static const unsigned char PLAINTEXT[MSG_SIZE] = {
+    0x6b, 0xc1, 0xbe, 0xe2, 0x2e, 0x40, 0x9f, 0x96,
+    0xe9, 0x3d, 0x7e, 0x11, 0x73, 0x93, 0x17, 0x2a,
+    0xae, 0x2d, 0x8a, 0x57, 0x1e, 0x03, 0xac, 0x9c,
+    0x9e, 0xb7, 0x6f, 0xac, 0x45, 0xaf, 0x8e, 0x51,
+    0x30, 0xc8, 0x1c, 0x46, 0xa3, 0x5c, 0xe4, 0x11,
+    0xe5, 0xfb, 0xc1, 0x19, 0x1a, 0x0a, 0x52, 0xef,
+    0xf6, 0x9f, 0x24, 0x45, 0xdf, 0x4f, 0x9b, 0x17,
+    0xad, 0x2b, 0x41, 0x7b, 0xe6, 0x6c, 0x37, 0x10
+};
+
+static const unsigned char AES128_KEY[16] = {
+    0x2b, 0x7e, 0x15, 0x16, 0x28, 0xae, 0xd2, 0xa6,
+    0xab, 0xf7, 0x15, 0x88, 0x09, 0xcf, 0x4f, 0x3c
+};
+
+static const unsigned char AES128_CIPHERTEXT[MSG_SIZE] = {
+    0x76, 0x49, 0xab, 0xac, 0x81, 0x19, 0xb2, 0x46,
+    0xce, 0xe9, 0x8e, 0x9b, 0x12, 0xe9, 0x19, 0x7d,
+    0x50, 0x86, 0xcb, 0x9b, 0x50, 0x72, 0x19, 0xee,
+    0x95, 0xdb, 0x11, 0x3a, 0x91, 0x76, 0x78, 0xb2,
+    0x73, 0xbe, 0xd6, 0xb8, 0xe3, 0xc1, 0x74, 0x3b,
+    0x71, 0x16, 0xe6, 0x9e, 0x22, 0x22, 0x95, 0x16,
+    0x3f, 0xf1, 0xca, 0xa1, 0x68, 0x1f, 0xac, 0x09,
+    0x12, 0x0e, 0xca, 0x30, 0x75, 0x86, 0xe1, 0xa7
+};
+
+static const unsigned char AES256_KEY[32] = {
+    0x60, 0x3d, 0xeb, 0x10, 0x15, 0xca, 0x71, 0xbe,
+    0x2b, 0x73, 0xae, 0xf0, 0x85, 0x7d, 0x77, 0x81,
+    0x1f, 0x35, 0x2c, 0x07, 0x3b, 0x61, 0x08, 0xd7,
+    0x2d, 0x98, 0x10, 0xa3, 0x09, 0x14, 0xdf, 0xf4
+};
+
+static const unsigned char AES256_CIPHERTEXT[MSG_SIZE] = {
+    0xf5, 0x8c, 0x4c, 0x04, 0xd6, 0xe5, 0xf1, 0xba,
+    0x77, 0x9e, 0xab, 0xfb, 0x5f, 0x7b, 0xfb, 0xd6,
+    0x9c, 0xfc, 0x4e, 0x96, 0x7e, 0xdb, 0x80, 0x8d,
+    0x67, 0x9f, 0x77, 0x7b, 0xc6, 0x70, 0x2c, 0x7d,
+    0x39, 0xf2, 0x33, 0x69, 0xa9, 0xd9, 0xba, 0xcf,
+    0xa5, 0x30, 0xe2, 0x63, 0x04, 0x23, 0x14, 0x61,
+    0xb2, 0xeb, 0x05, 0xe2, 0xc3, 0x9b, 0xe9, 0xfc,
+    0xda, 0x6c, 0x19, 0x07, 0x8c, 0x6a, 0x9d, 0x1b
+};
+
+/* Every way of cutting a 4-block message into calls that is worth testing.
+ * A single-block-only sweep, or one that never puts a multi-block call in front
+ * of another call, is exactly what let the encrypt bug survive. */
+struct split_t {
+    const char *name;
+    size_t      chunk[5];   // It has to fit the zero-terminated as well
+};
+
+static const split_t SPLITS[] = {
+    { "64",          { 64, 0 } },
+    { "16+48",       { 16, 48, 0 } },
+    { "48+16",       { 48, 16, 0 } },
+    { "32+32",       { 32, 32, 0 } },
+    { "16+32+16",    { 16, 32, 16, 0 } },
+    { "16+16+16+16", { 16, 16, 16, 16, 0 } }
+};
+
+static const size_t NUM_SPLITS = sizeof(SPLITS) / sizeof(SPLITS[0]);
+
+static char msg[128];
+
+static void setkey(mbedtls_aes_context *ctx, int mode,
+                   const unsigned char *key, unsigned int keybits)
+{
+    if (mode == MBEDTLS_AES_ENCRYPT) {
+        TEST_ASSERT_EQUAL_INT(0, mbedtls_aes_setkey_enc(ctx, key, keybits));
+    } else {
+        TEST_ASSERT_EQUAL_INT(0, mbedtls_aes_setkey_dec(ctx, key, keybits));
+    }
+}
+
+static const char *mode_name(int mode)
+{
+    return mode == MBEDTLS_AES_ENCRYPT ? "encrypt" : "decrypt";
+}
+
+/* CBC one message in the given chunk sizes, carrying iv[] across calls, then check
+ * against the expected vector. */
+static void run_cbc(int mode,
+                    const unsigned char *key, unsigned int keybits,
+                    const unsigned char *input, const unsigned char *expected,
+                    const split_t *split)
+{
+    mbedtls_aes_context ctx;
+    unsigned char iv[BLOCK_SIZE];
+    unsigned char output[MSG_SIZE];
+    size_t offset = 0;
+
+    memset(output, 0, sizeof(output));
+    memcpy(iv, AES_IV, sizeof(iv));
+
+    mbedtls_aes_init(&ctx);
+    setkey(&ctx, mode, key, keybits);
+
+    for (const size_t *chunk = split->chunk; *chunk != 0; chunk++) {
+        TEST_ASSERT_EQUAL_INT(0, mbedtls_aes_crypt_cbc(&ctx, mode, *chunk, iv,
+                                                       input + offset,
+                                                       output + offset));
+        offset += *chunk;
+    }
+
+    mbedtls_aes_free(&ctx);
+
+    TEST_ASSERT_EQUAL_UINT32(MSG_SIZE, offset);
+    snprintf(msg, sizeof(msg), "AES-%u-CBC %s, split %s",
+             keybits, mode_name(mode), split->name);
+    TEST_ASSERT_EQUAL_UINT8_ARRAY_MESSAGE(expected, output, MSG_SIZE, msg);
+}
+
+static void check_iv_contract(int mode,
+                              const unsigned char *key, unsigned int keybits,
+                              const unsigned char *input,
+                              const unsigned char *ciphertext,
+                              const split_t *split)
+{
+    mbedtls_aes_context ctx;
+    unsigned char iv[BLOCK_SIZE];
+    unsigned char output[MSG_SIZE];
+    size_t offset = 0;
+
+    memset(output, 0, sizeof(output));
+    memcpy(iv, AES_IV, sizeof(iv));
+
+    mbedtls_aes_init(&ctx);
+    setkey(&ctx, mode, key, keybits);
+
+    for (const size_t *chunk = split->chunk; *chunk != 0; chunk++) {
+        TEST_ASSERT_EQUAL_INT(0, mbedtls_aes_crypt_cbc(&ctx, mode, *chunk, iv,
+                                                       input + offset,
+                                                       output + offset));
+        offset += *chunk;
+
+        snprintf(msg, sizeof(msg), "AES-%u-CBC %s, split %s: IV after %u bytes",
+                 keybits, mode_name(mode), split->name, (unsigned)offset);
+        TEST_ASSERT_EQUAL_UINT8_ARRAY_MESSAGE(ciphertext + offset - BLOCK_SIZE,
+                                              iv, BLOCK_SIZE, msg);
+    }
+
+    mbedtls_aes_free(&ctx);
+}
+
+
+template<int mode, const unsigned char *key, unsigned int keybits,
+         const unsigned char *input, const unsigned char *expected>
+static void run_single_call()
+{
+    run_cbc(mode, key, keybits, input, expected, &SPLITS[0]);
+}
+
+/* The regression proper: every split, both directions, both key sizes. */
+template<int mode, const unsigned char *key, unsigned int keybits,
+         const unsigned char *input, const unsigned char *expected>
+static void run_all_splits()
+{
+    for (size_t i = 0; i < NUM_SPLITS; i++) {
+        run_cbc(mode, key, keybits, input, expected, &SPLITS[i]);
+    }
+}
+
+template<int mode, const unsigned char *key, unsigned int keybits,
+         const unsigned char *input, const unsigned char *ciphertext>
+static void check_iv_contract_all_splits()
+{
+    for (size_t i = 0; i < NUM_SPLITS; i++) {
+        check_iv_contract(mode, key, keybits, input, ciphertext, &SPLITS[i]);
+    }
+}
+
+/* A zero-length call must be a no-op */
+static void test_zero_length_call_is_a_noop()
+{
+    mbedtls_aes_context ctx;
+    unsigned char iv[BLOCK_SIZE];
+    unsigned char output[MSG_SIZE];
+
+    memcpy(iv, AES_IV, sizeof(iv));
+    mbedtls_aes_init(&ctx);
+    setkey(&ctx, MBEDTLS_AES_ENCRYPT, AES128_KEY, 128);
+
+    TEST_ASSERT_EQUAL_INT(0, mbedtls_aes_crypt_cbc(&ctx, MBEDTLS_AES_ENCRYPT, 0,
+                                                   iv, PLAINTEXT, output));
+    TEST_ASSERT_EQUAL_UINT8_ARRAY(AES_IV, iv, BLOCK_SIZE);
+
+    TEST_ASSERT_EQUAL_INT(0, mbedtls_aes_crypt_cbc(&ctx, MBEDTLS_AES_ENCRYPT,
+                                                   MSG_SIZE, iv, PLAINTEXT,
+                                                   output));
+    mbedtls_aes_free(&ctx);
+
+    TEST_ASSERT_EQUAL_UINT8_ARRAY(AES128_CIPHERTEXT, output, MSG_SIZE);
+}
+
+/* Two live contexts share one AES peripheral e.g. alternating block by block. */
+static void test_decrypt_interleaved_contexts()
+{
+    mbedtls_aes_context ctx_a, ctx_b;
+    unsigned char iv_a[BLOCK_SIZE], iv_b[BLOCK_SIZE];
+    unsigned char out_a[MSG_SIZE], out_b[MSG_SIZE];
+
+    memcpy(iv_a, AES_IV, sizeof(iv_a));
+    memcpy(iv_b, AES_IV, sizeof(iv_b));
+
+    mbedtls_aes_init(&ctx_a);
+    mbedtls_aes_init(&ctx_b);
+    TEST_ASSERT_EQUAL_INT(0, mbedtls_aes_setkey_dec(&ctx_a, AES128_KEY, 128));
+    TEST_ASSERT_EQUAL_INT(0, mbedtls_aes_setkey_dec(&ctx_b, AES256_KEY, 256));
+
+    for (size_t offset = 0; offset < MSG_SIZE; offset += BLOCK_SIZE) {
+        TEST_ASSERT_EQUAL_INT(0, mbedtls_aes_crypt_cbc(&ctx_a, MBEDTLS_AES_DECRYPT,
+                                                       BLOCK_SIZE, iv_a,
+                                                       AES128_CIPHERTEXT + offset,
+                                                       out_a + offset));
+        TEST_ASSERT_EQUAL_INT(0, mbedtls_aes_crypt_cbc(&ctx_b, MBEDTLS_AES_DECRYPT,
+                                                       BLOCK_SIZE, iv_b,
+                                                       AES256_CIPHERTEXT + offset,
+                                                       out_b + offset));
+    }
+
+    mbedtls_aes_free(&ctx_a);
+    mbedtls_aes_free(&ctx_b);
+
+    TEST_ASSERT_EQUAL_UINT8_ARRAY(PLAINTEXT, out_a, MSG_SIZE);
+    TEST_ASSERT_EQUAL_UINT8_ARRAY(PLAINTEXT, out_b, MSG_SIZE);
+}
+
+static utest::v1::status_t greentea_setup(const size_t number_of_cases)
+{
+    GREENTEA_SETUP(30, "default_auto");
+    return verbose_test_setup_handler(number_of_cases);
+}
+
+static Case cases[] = {
+    Case("AES-128-CBC (enc) single call",
+         run_single_call<MBEDTLS_AES_ENCRYPT, AES128_KEY, 128, PLAINTEXT, AES128_CIPHERTEXT>),
+    Case("AES-128-CBC (dec) single call",
+         run_single_call<MBEDTLS_AES_DECRYPT, AES128_KEY, 128, AES128_CIPHERTEXT, PLAINTEXT>),
+    Case("AES-128-CBC (enc) call all splits",
+         run_all_splits<MBEDTLS_AES_ENCRYPT, AES128_KEY, 128, PLAINTEXT, AES128_CIPHERTEXT>),
+    Case("AES-128-CBC (dec) call all splits",
+         run_all_splits<MBEDTLS_AES_DECRYPT, AES128_KEY, 128, AES128_CIPHERTEXT, PLAINTEXT>),
+    Case("AES-256-CBC (enc) call all splits",
+         run_all_splits<MBEDTLS_AES_ENCRYPT, AES256_KEY, 256, PLAINTEXT, AES256_CIPHERTEXT>),
+    Case("AES-256-CBC (dec) call all splits",
+         run_all_splits<MBEDTLS_AES_DECRYPT, AES256_KEY, 256, AES256_CIPHERTEXT, PLAINTEXT>),
+    Case("AES-128-CBC (enc) leaves last cipher block in IV",
+         check_iv_contract_all_splits<MBEDTLS_AES_ENCRYPT, AES128_KEY, 128, PLAINTEXT, AES128_CIPHERTEXT>),
+    Case("AES-128-CBC (dec) leaves last cipher block in IV",
+         check_iv_contract_all_splits<MBEDTLS_AES_DECRYPT, AES128_KEY, 128, AES128_CIPHERTEXT, AES128_CIPHERTEXT>),
+    Case("AES-256-CBC (enc) leaves last cipher block in IV",
+         check_iv_contract_all_splits<MBEDTLS_AES_ENCRYPT, AES256_KEY, 256, PLAINTEXT, AES256_CIPHERTEXT>),
+    Case("AES-256-CBC (dec) leaves last cipher block in IV",
+         check_iv_contract_all_splits<MBEDTLS_AES_DECRYPT, AES256_KEY, 256, AES256_CIPHERTEXT, AES256_CIPHERTEXT>),
+    Case("AES-CBC zero-length call is a no-op", test_zero_length_call_is_a_noop),
+    Case("AES-CBC decrypt with interleaved contexts", test_decrypt_interleaved_contexts)
+};
+
+static Specification specification(greentea_setup, cases);
+
+int main()
+{
+    return !Harness::run(specification);
+}
+
+#endif // MBEDTLS_AES_C && MBEDTLS_CIPHER_MODE_CBC