Protected Storage service key management

Author: Jamie Fox
Organization: Arm Limited
Contact: Jamie Fox <jamie.fox@arm.com>

Background

The PSA Protected Storage API requires confidentiality for external storage to be provided by:

cryptographic ciphers using device-bound keys, a tamper resistant enclosure, or an inaccessible deployment location, depending on the threat model of the deployed system.

A TBSA-M-compliant device must embed a Hardware Unique Key (HUK), which provides the root of trust (RoT) for confidentiality in the system. It must have at least 128 bits of entropy (and a 128 bit data size), and be accessible only to Trusted code or Trusted hardware that acts on behalf of Trusted code. [TBSA-M]

In the current implementation, the Protected Storage (PS) service reads the HUK directly and imports it into the Crypto partition for further use. This has multiple drawbacks:

If there were a flaw in PS that allowed an attacker to obtain its key, then the HUK would be exposed, and so the attacker would be able to decrypt not just protected storage but also anything else encrypted with the HUK or a key derived from the HUK.
Using the same key for two or more different cryptographic algorithms may reduce the security provided by one or more of them.
It is not possible to re-key if the HUK is used directly, for example in the case of a lost key.
It is incompatible with devices where the HUK is in an enclave and cannot be read directly.

Proposal

Each time the system boots, PS will request that the Crypto service uses a key derivation function (KDF) to derive a storage key from the HUK. The storage key could be kept in on-chip volatile memory private to the Crypto partition, or it could remain inside a secure element. Either way it will not be returned to PS.

For each call to the PSA Protected Storage APIs, PS will make requests to the Crypto service to perform AEAD encryption and/or decryption operations using the storage key (providing a fresh nonce for each encryption).

At no point will PS access the key material itself, only referring to the HUK and storage key by their handles in the Crypto service.

Key derivation

PS will make key derivation requests to the Crypto service with calls to the PSA Crypto APIs. In order to derive the storage key, the following calls will be made:

/* Open a handle to the HUK */
psa_open_key(PSA_KEY_LIFETIME_PERSISTENT,
             TFM_CRYPTO_KEY_ID_HUK,
             &huk_key_handle)

/* Set up a key derivation operation with the HUK as the input key */
psa_key_derivation(&ps_key_generator,
                   huk_key_handle,
                   TFM_CRYPTO_ALG_HUK_DERIVATION,
                   PS_KEY_SALT, PS_KEY_SALT_LEN_BYTES,
                   PS_KEY_LABEL, PS_KEY_LABEL_LEN_BYTES,
                   PS_KEY_LEN_BYTES)

/* Create the storage key from the key generator */
psa_generator_import_key(ps_key_handle,
                         PS_KEY_TYPE,
                         PSA_BYTES_TO_BITS(PS_KEY_LEN_BYTES),
                         &ps_key_generator)

Note

TFM_CRYPTO_KEY_ID_HUK is a PSA Crypto key ID that is assumed in this design to identify the hardware unique key.

ps_key_handle is a PSA Crypto key handle to a volatile key, set up in the normal way. After the call to psa_generator_import_key, it contains the storage key.

PS_KEY_SALT can be NULL, as it is only used in the ‘extract’ step of HKDF, which is redundant when the input key material is a cryptographically strong key. [RFC5869] It must be constant so that the same key can be derived each boot, to decrypt previously-stored data.

PS_KEY_LABEL can be any string that is independent of the input key material and different to the label used in any other derivation from the same input key. It prevents two different contexts from deriving the same output key from the same input key.

In the call to psa_key_derivation(), TFM_CRYPTO_ALG_HUK_DERIVATION is supplied as the key derivation algorithm argument. This indicates that the key derivation should be done from the HUK, and allows it to be implemented in a platform-defined way (e.g. using a crypto accelerator). The system integrator should choose the most optimal algorithm for the platform, or fall back to the software implementation if none is available.

When implemented in software, the key derivation function used by the crypto service to derive the storage key will be HKDF, with SHA-256 as the underlying hash function. HKDF is suitable because:

It is simple and efficient, requiring only two HMAC operations when the length of the output key material is less than or equal to the hash length (as is the case here).
The trade-off is that HKDF is only suitable when the input key material has at least as much entropy as required for the output key material. But this is the case here, as the HUK has 128 bits of entropy, the same as required by PS.
HKDF is standardised in RFC 5869 [RFC5869] and its security has been formally analysed. [HKDF]
It is supported by the TF-M Crypto service.

The choice of underlying hash function is fairly straightforward: it needs to be a modern standardised algorithm, considered to be secure and supported by TF-M Crypto. This narrows it down to just the SHA-2 family. Of the hash functions in the family, SHA-256 is the simplest and provides more than enough output length.

Keeping the storage key private to PS

The salt and label fields are not generally secret, so an Application RoT service could request the Crypto service to derive the same storage key from the HUK, which violates isolation between Application RoT partitions to some extent. This could be fixed in a number of ways:

Only PSA RoT partitions can request Crypto to derive keys from the HUK.
- But then either PS has to be in the PSA RoT or request a service in the PSA RoT to do the derivation on its behalf.
PS has a secret (pseudo)random salt, accessible only to it, that it uses to derive the storage key.
- Where would this salt be stored? It cannot be generated fresh each boot because the storage key must stay the same across reboots.
The Crypto service appends the partition ID to the label, so that no two partitions can derive the same key.
- Still need to make sure only PSA RoT partitions can directly access the HUK or Secure Enclave. The label is not secret, so any actor that can access the HUK could simply perform the derivation itself, rather than making a request to the Crypto service.

The third option would solve the issue with the fewest drawbacks, so this option is the one that is proposed.

Key use

To encrypt and decrypt data, PS will call the PSA Crypto AEAD APIs in the same way as the current implementation, but ps_key_handle will refer to the storage key, rather than the imported HUK. For each encryption operation, the following call is made (and analogously for decryption):

psa_aead_encrypt(ps_key_handle, PS_CRYPTO_ALG,
                 crypto->ref.iv, PS_IV_LEN_BYTES,
                 add, add_len,
                 in, in_len,
                 out, out_size, out_len)

Future changes

In the future, the client’s partition ID and the asset’s UID could be used to derive a key that is unique to that asset, each time the Protected Storage APIs are called (key diversification). To achieve this, the key derivation must use a label parameter that is unique to each client ID, UID pair.

References

TBSA-M: Arm Platform Security Architecture Trusted Base System Architecture for Armv6-M, Armv7-M and Armv8-M, version 1.0
HKDF: Hugo Krawczyk. 2010. Cryptographic extraction and key derivation: the HKDF scheme. In Proceedings of the 30th annual conference on Advances in cryptology (CRYPTO’10)
RFC5869(1,2): IETF RFC 5869: HMAC-based Extract-and-Expand Key Derivation Function (HKDF)