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Security Implications

The following are security implications to be addressed before using the reference solution as is, because of the nature of the reference platform.

AES Counter with CBC-MAC (CCM) mode trade-offs

The Nonce length 'N' for CCM mode is dependent on the maximum message length 'L' value and should be equal to '15-L' octets. Refer to RFC3610 for more information on trade-offs between 'L' and 'N' value. The current implementation supports L=8 with the following configurations:

  • AES-CCM-64-128-128 (L=64 (8 octets, maximum 2^64 bytes message length), Tag = 128 bits, Key = 128 bits)
  • AES-CCM-64-128-256 (L=64 (8 octets, maximum 2^64 bytes message length), Tag = 128 bits, Key = 256 bits)

The Nonce length 'N' is thus fixed at 7 octets (i.e 15-8).

As per FDO and COSE RFC8152 specifications, L=2 could also be used. N=13 MUST be used in this case. Affected file(s):

  • crypto/openssl/openssl_AES_routines.c
  • lib/fdotypes.h

Linux* OS (OpenSSL* toolkit as the cryptography library)

  1. The random number needs to be seeded with an entropy source. Affected file(s):

    • crypto/openssl/openssl_crypto_support.c
    • crypto/mbedtls/mbedtls_cryptoSupport.c (Not supported)
    • crypto/se/se_crypto_support.c (Not supported)

  2. In the reference implementation, the device key and the keys that encrypt or protect the integrity of the FIDO Device Onboard (FDO) data are stored in clear text on the file system. In production systems, these keys must be stored in a secure storage such as a Secure Engine that can be a third-party application, library, or hardware.

    • PLATFORM_HMAC_KEY: The key is stored in the data/platform_hmac_key.bin file in the reference implementation.
    • PLATFORM_AES_KEY: The key is stored in the data/platform_aes_key.bin file in the reference implementation.
    • ECDSA_PRIVKEY: The key is stored in the data/ecdsa256privkey.dat file or data/ecdsa256privkey.pem file in the reference implementation.
      Affected file(s):
    • base.mk

NOTE: The configurations mentioned for below platforms are not supported yet. They will be updated in a future release.

NUCLEO-F429ZI board: Arm Cortex* -M4/Arm Mbed* OS (mbedTLS as the cryptography library)

  1. The mbedTLS library must use the True Random Number Generator (TRNG) hardware for the entropy source. Refer to mbedTLS Hardware Entropy Source for more information.

  2. In the reference implementation, the device key and the keys that encrypt or protect the integrity of the FDO data are stored in clear text on the file system. In production systems, these keys must be stored in a secure storage such as the Secure Engine.

    • PLATFORM_HMAC_KEY: The key is stored in the data/platform_hmac_key.bin file in the reference implementation.
    • PLATFORM_AES_KEY: The key is stored in the data/platform_aes_key.bin file in the reference implementation.
    • ECDSA_PRIVKEY: The key is stored in the data/ecdsa256privkey.dat file or the data/ecdsa256privkey.pem file in the reference implementation.
      Affected file(s):
    • base.mk

  3. FDO recommends to switch on the following compilation options for mbedTLS:

    MBEDTLS_SSL_ENCRYPT_THEN_MAC
MBEDTLS_SSL_EXTENDED_MASTER_SECRET

NUCLEO-F767ZI board: Arm Cortex-M7/Arm Mbed OS (mbedTLS as the cryptography library)

  1. The mbedTLS library must use the TRNG hardware for the entropy source. Refer to mbedTLS Hardware Entropy Source for more information.

  2. In the reference implementation, the device key and the keys that encrypt or protect the integrity of the FDO data are stored in clear text on the file system. In production systems, these keys must be stored in a secure storage such as the Secure Engine.

    • PLATFORM_HMAC_KEY: The key is stored in the data/platform_hmac_key.bin file in the reference implementation.

    • PLATFORM_AES_KEY: The key is stored in the data/platform_aes_key.bin file in the reference implementation.

    • ECDSA_PRIVKEY: The key is stored in the data/ecdsa256privkey.dat file or the data/ecdsa256privkey.pem file in the reference implementation.
      Affected file(s):

    • cmake/blob_path.cmake

  3. FDO recommends to switch on the following compilation options for mbedTLS:

    MBEDTLS_SSL_ENCRYPT_THEN_MAC
MBEDTLS_SSL_EXTENDED_MASTER_SECRET

WaRP7 board: Arm Cortex-A7/Linux OS (mbedTLS as the cryptography library)

  1. The mbedTLS library must use the True Random Number Generator (TRNG) hardware for the entropy source. Refer to mbedTLS Hardware Entropy Source for more information.

  2. In the reference implementation, the device key and the keys that encrypt or protect the integrity of the FDO data are stored in clear text on the file system. In production systems, these keys must be stored in a secure storage such as the Secure Engine.

    • PLATFORM_HMAC_KEY: The key is stored in the data/platform_hmac_key.bin file in the reference implementation.
    • PLATFORM_AES_KEY: The key is stored in the data/platform_aes_key.bin file in the reference implementation.
    • ECDSA_PRIVKEY: The key is stored in the data/ecdsa256privkey.dat file or thedata/ecdsa256privkey.pem file in the reference implementation.
      Affected file(s):
    • cmake/blob_path.cmake

  3. FDO data must be protected by appropriate file system permissions as a defense in depth. Read/write permissions must be provided only to the user running the FDO application. The list of files is as follows:

  • data/Normal.blob: The integrity of this file is protected using PLATFORM_HMAC_KEY, with read/write permissions provided only to the FDO user.
  • data/Secure.blob: This file is encrypted using PLATFORM_AES_KEY, with read/write permissions provided only to the FDO user.
  • data/raw.blob: This file is not protected using cryptography but must have read/write permissions provided only to the FDO user.
  • data: This directory must be read or written only by the FDO user.
    Affected file(s):
  • cmake/blob_path.cmake
  1. FDO recommends to switch on the following compilation options for mbedTLS: 
    MBEDTLS_SSL_ENCRYPT_THEN_MAC
MBEDTLS_SSL_EXTENDED_MASTER_SECRET

mbedTLS Hardware Entropy Source

To enable the TRNG hardware as the entropy source, the MBEDTLS_ENTROPY_HARDWARE_ALT macro must be uncommented in include/mbedtls/config.h in the mbedTLS source code. The mbedtls_hardware_poll() function, with prototype declared in entropy_poll.h, must be implemented to collect entropy from the hardware source.

In addition, the Arm Mbed OS must support the TRNG hardware using the appropriate drivers. For this, the device_has array for the target platform in the targets/targets.json file must have TRNG as one of the attributes. The functions declared in hal/trng_api.h from the Arm Mbed OS source code must be implemented to access the hardware entropy source.