Zephyr下调用mbedtls函数链接失败问题求助

问题背景 #

为NRF52840开发Zephyr应用时，尝试通过以下函数派生会话密钥：

int derive_session_key(const uint8_t *device_token_key, const uint8_t *nonce1,
                       const uint8_t *nonce2, uint16_t out_key_len,
                       uint8_t *output_key) {

  const uint8_t *ikm = device_token_key;
  const size_t ikm_len = TOKEN_KEY_SIZE;
  uint8_t salt[NONCE_SIZE];
  add_arrays(nonce1, nonce2, salt, NONCE_SIZE);
  const size_t salt_len = NONCE_SIZE;
  const uint8_t *info = (const uint8_t *)INFO_STR;
  const size_t info_len = strlen(INFO_STR);

  int ret =
      mbedtls_hkdf(mbedtls_md_info_from_type(MBEDTLS_MD_SHA256), salt, salt_len,
                   ikm, ikm_len, info, info_len, output_key, out_key_len);

  if (ret != 0) {
    LOG_ERR("Error! Creating session key. Ret : %d", ret);
    return RET_ERROR;
  }

  return RET_OK;
}

proj.conf配置如下：

CONFIG_NRF_SECURITY=y
CONFIG_MBEDTLS=y
CONFIG_MBEDTLS_SHA256_C=y
CONFIG_MBEDTLS_PSA_CRYPTO_C=y
CONFIG_MBEDTLS_HKDF_C=y
CONFIG_MBEDTLS_BUILTIN=y
CONFIG_MBEDTLS_CIPHER_MODE_CBC=y
CONFIG_MBEDTLS_AES_C=y
CONFIG_MBEDTLS_HEAP_SIZE=8192
CONFIG_HEAP_MEM_POOL_SIZE=4096
CONFIG_MBEDTLS_ENABLE_HEAP=y

CONFIG_PSA_WANT_KEY_TYPE_AES=y
CONFIG_PSA_WANT_AES_KEY_SIZE_128=y
CONFIG_PSA_WANT_ALG_CMAC=y
CONFIG_PSA_WANT_ALG_ECB_NO_PADDING=y
CONFIG_PSA_WANT_ALG_CBC_NO_PADDING=y

但链接失败，无法调用mbedtls_hkdf，出现以下警告：

warning: MBEDTLS_AES_C (defined at
/home/user/Belter/top_main/nrf/subsys/nrf_security/Kconfig.legacy:388) was assigned the value 'y'
but got the value 'n'. Check these unsatisfied dependencies: MBEDTLS_LEGACY_CRYPTO_C (=n). See
http://docs.zephyrproject.org/latest/kconfig.html#CONFIG_MBEDTLS_AES_C and/or look up MBEDTLS_AES_C
in the menuconfig/guiconfig interface. The Application Development Primer, Setting Configuration
Values, and Kconfig - Tips and Best Practices sections of the manual might be helpful too.


warning: MBEDTLS_HKDF_C (defined at
/home/user/Belter/top_main/nrf/subsys/nrf_security/Kconfig.legacy:724, modules/mbedtls/Kconfig.tls-
generic:140, modules/mbedtls/Kconfig.tls-generic:140) was assigned the value 'y' but got the value
'n'. Check these unsatisfied dependencies: ((MBEDTLS_LEGACY_CRYPTO_C && NRF_SECURITY) ||
(!(NRF_SECURITY || NORDIC_SECURITY_BACKEND) && MBEDTLS_BUILTIN && MBEDTLS_CFG_FILE = "config-tls-
generic.h" && MBEDTLS) || (!(NRF_SECURITY || NORDIC_SECURITY_BACKEND) && MBEDTLS_BUILTIN &&
MBEDTLS_CFG_FILE = "config-tls-generic.h" && MBEDTLS && 0)) (=n). See
http://docs.zephyrproject.org/latest/kconfig.html#CONFIG_MBEDTLS_HKDF_C and/or look up
MBEDTLS_HKDF_C in the menuconfig/guiconfig interface. The Application Development Primer, Setting
Configuration Values, and Kconfig - Tips and Best Practices sections of the manual might be helpful
too.


warning: MBEDTLS_SHA256_C (defined at
/home/user/Belter/top_main/nrf/subsys/nrf_security/Kconfig.legacy:798) was assigned the value 'y'
but got the value 'n'. Check these unsatisfied dependencies: MBEDTLS_LEGACY_CRYPTO_C (=n). See
http://docs.zephyrproject.org/latest/kconfig.html#CONFIG_MBEDTLS_SHA256_C and/or look up
MBEDTLS_SHA256_C in the menuconfig/guiconfig interface. The Application Development Primer, Setting
Configuration Values, and Kconfig - Tips and Best Practices sections of the manual might be helpful
too.

手动定义MBEDTLS_HKDF_C也无效，求解决办法。

解决办法 #

方案1：启用mbedtls Legacy Crypto API #

根据警告信息，核心问题是**MBEDTLS_LEGACY_CRYPTO_C未启用**。当启用CONFIG_NRF_SECURITY=y时，mbedtls的传统加密API（如mbedtls_hkdf）依赖该配置项才能激活。

在proj.conf中添加以下配置：

CONFIG_MBEDTLS_LEGACY_CRYPTO_C=y

添加后清理并重新构建项目：

west build -t clean
west build

方案2：切换到PSA Crypto API（推荐） #

Zephyr推荐使用PSA Crypto API替代mbedtls的传统API，无需依赖legacy配置，同时更符合现代加密标准。

1. 首先在proj.conf中添加PSA HKDF支持：

CONFIG_PSA_WANT_ALG_HKDF=y
CONFIG_PSA_WANT_ALG_SHA_256=y

2. 修改代码使用PSA API实现HKDF：

#include <psa/crypto.h>

int derive_session_key(const uint8_t *device_token_key, const uint8_t *nonce1,
                       const uint8_t *nonce2, uint16_t out_key_len,
                       uint8_t *output_key) {
    psa_status_t status;
    psa_key_attributes_t attr = PSA_KEY_ATTRIBUTES_INIT;
    psa_key_handle_t key_handle;
    uint8_t salt[NONCE_SIZE];
    add_arrays(nonce1, nonce2, salt, NONCE_SIZE);

    // 配置临时密钥属性：仅用于派生，算法为HKDF-SHA256
    psa_set_key_usage_flags(&attr, PSA_KEY_USAGE_DERIVE);
    psa_set_key_algorithm(&attr, PSA_ALG_HKDF(PSA_ALG_SHA_256));
    psa_set_key_type(&attr, PSA_KEY_TYPE_RAW_DATA);
    psa_set_key_bits(&attr, TOKEN_KEY_SIZE * 8);

    // 导入初始密钥材料（IKM）
    status = psa_import_key(&attr, device_token_key, TOKEN_KEY_SIZE, &key_handle);
    if (status != PSA_SUCCESS) {
        LOG_ERR("Import IKM key failed: %d", status);
        return RET_ERROR;
    }

    // 执行HKDF密钥派生
    status = psa_key_derive(key_handle,
                            PSA_ALG_HKDF(PSA_ALG_SHA_256),
                            salt, NONCE_SIZE,
                            (const uint8_t *)INFO_STR, strlen(INFO_STR),
                            output_key, out_key_len, NULL);
    
    // 销毁临时密钥句柄
    psa_destroy_key(key_handle);

    if (status != PSA_SUCCESS) {
        LOG_ERR("HKDF derive failed: %d", status);
        return RET_ERROR;
    }

    return RET_OK;
}

3. 清理并重新构建项目：

west build -t clean
west build

原因说明 #

当启用CONFIG_NRF_SECURITY=y时，Zephyr会使用Nordic提供的安全后端，此时mbedtls的传统API（如mbedtls_hkdf、mbedtls_sha256）默认被禁用，必须通过CONFIG_MBEDTLS_LEGACY_CRYPTO_C=y显式启用。

而PSA Crypto API是Zephyr官方推荐的加密接口，与Nordic安全后端兼容良好，无需依赖legacy配置，同时具备更好的安全性和可移植性。

内容的提问来源于stack exchange，提问作者Engineer999