mbedtls/docs/architecture/psa-migration/strategy.md

This document explains the strategy that was used so far in starting the migration to PSA Crypto and mentions future perspectives and open questions.

Goals

Several benefits are expected from migrating to PSA Crypto:

G1. Take advantage of the PSA Crypto driver interface. G2. Allow isolation of long-term secrets (for example, private keys). G3. Allow isolation of short-term secrets (for example, TLS sesssion keys). G4. Have a clean, unified API for Crypto (retire the legacy API).

Currently, some parts of (G1) and (G2) are implemented when MBEDTLS_USE_PSA_CRYPTO is enabled. For (G2) to take effect, the application needs to be changed to use new APIs.

Generally speaking, the numbering above doesn't mean that each goal requires the preceding ones to be completed - for example it would be possible to start or even complete (G4) before (G3) is even started. However, (G2) and (G3) require operations to be done via the PSA Crypto API, which is mostly what (G1) is about. Also, we can't retire the legacy API (G4) until we no longer rely on it, which again is mostly (G1).

So, a solid intermediate goal would be to complete (G1) when MBEDTLS_USA_PSA_CRYPTO is enabled - that is, all crypto operations in X.509 and TLS would be done via the PSA Crypto API.

Compile-time options

We currently have two compile-time options that are relevant to the migration:

• MBEDTLS_PSA_CRYPTO_C - enabled by default, controls the presence of the PSA Crypto APIs.
• MBEDTLS_USE_PSA_CRYPTO - disabled by default (enabled in "full" config), controls usage of PSA Crypto APIs to perform operations in X.509 and TLS (G1 above), as well as the availability of some new APIs (G2 above).

The reason why MBEDTLS_USE_PSA_CRYPTO is optional, and disabled by default, is mostly to avoid introducing a hard (or even default) dependency of X509 and TLS and MBEDTLS_PSA_CRYPTO_C. This is mostly reasons of code size, and historically concerns about the maturity of the PSA code (which we might want to re-evaluate).

The downside of this approach is that until we feel ready to make MBDEDTLS_USE_PSA_CRYPTO non-optional (always enabled), we have to maintain two versions of some parts of the code: one using PSA, the other using the legacy APIs. However, see next section for strategies that can lower that cost.

Taking advantage of the existing abstractions layers - or not

The Crypto library in Mbed TLS currently has 3 abstraction layers that offer algorithm-agnostic APIs for a class of algorithms:

• MD for messages digests aka hashes (including HMAC)
• Cipher for symmetric ciphers (included AEAD)
• PK for asymmetric (aka public-key) cryptography (excluding key exchange)

Note: key exchange (FFDH, ECDH) is not covered by an abstraction layer.

These abstraction layers typically provide, in addition to the API for crypto operations, types and numerical identifiers for algorithms (for example mbedtls_cipher_mode_t and its values). The current strategy is to keep using those identifiers in most of the code, in particular in existing structures and public APIs, even when MBEDTLS_USE_PSA_CRYPTO is enabled. (This is not an issue for G1, G2, G3 above, and is only potentially relevant for G4.)

The are multiple strategies that can be used regarding the place of those layers in the migration to PSA.

Silently call to PSA from the abstraction layer

• Provide a new definition (conditionally on USE_PSA_CRYPTO) of wrapper functions in the abstraction layer, that calls PSA instead of the legacy crypto API.
• Upside: changes contained to a single place, no need to change TLS or X.509 code anywhere.
• Downside: tricky to implement if the PSA implementation is currently done on top of that layer (dependency loop).

This strategy is currently used for ECDSA signature verification in the PK layer, and could be extended to all operations in the PK layer.

This strategy is not very well suited to the Cipher and MD layers, as the PSA implementation is currently done on top of those layers.

Replace calls for each operation

• For every operation that's done through this layer in TLS or X.509, just replace function call with calls to PSA (conditionally on USE_PSA_CRYPTO)
• Upside: conceptually simple, and if the PSA implementation is currently done on top of that layer, avoids concerns about dependency loops.
• Downside: TLS/X.509 code has to be done for each operation.

This strategy is currently used for the MD layer. (Currently only a subset of calling places, but could be extended to all of them.)

Opt-in use of PSA from the abstraction layer

• Provide a new way to set up a context that causes operations on that context to be done via PSA.
• Upside: changes mostly contained in one place, TLS/X.509 code only needs to be changed when setting up the context, but not when using it. In particular, no changes to/duplication of existing public APIs that expect a key to be passed as a context of this layer (eg, mbedtls_pk_context).
• Upside: avoids dependency loop when PSA implemented on top of that layer.
• Downside: when the context is typically set up by the application, requires changes in application code.

There are two variants of this strategy: one where using the new setup function also allows for key isolation (the key is only held by PSA, supporting both G1 and G2 in that area), and one without isolation (the key is still stored outsde of PSA most of the time, supporting only G1).

This strategy, with support for key isolation, is currently used for ECDSA signature generation in the PK layer - see mbedtls_pk_setup_opaque(). This allows use of PSA-held private ECDSA keys in TLS and X.509 with no change to the TLS/X.509 code, but a contained change in the application. If could be extended to other private key operations in the PK layer.

This strategy, without key isolation, is also currently used in the Cipher layer - see mbedtls_cipher_setup_psa(). This allows use of PSA for cipher operations in TLS with no change to the application code, and a contained change in TLS code. (It currently only supports a subset of ciphers, but could easily be extended to all of them.)

Note: for private key operations in the PK layer, both the "silent" and the "opt-in" strategy can apply, and can complement each other, as one provides support for key isolation, but at the (unavoidable) code of change in application code, while the other requires no application change to get support for drivers, but fails to provide isolation support.

Migrating away from the legacy API

This section briefly introduces questions and possible plans towards G4, mainly as they relate to choices in previous stages.

The role of the PK/Cipher/MD APIs in user migration

We're currently taking advantage of the existing PK and Cipher layers in order to reduce the number of places where library code needs to be changed. It's only natural to consider using the same strategy (with the PK, MD and Cipher layers) for facilitating migration of application code.

Note: a necessary first step for that would be to make sure PSA is no longer implemented of top of the concerned layers

Zero-cost compatibility layer?

The most favourable case is if we can have a zero-cost abstraction (no runtime, RAM usage or code size penalty), for example just a bunch of #defines, essentialy mapping mbedtls_ APIs to their psa_ equivalent.

Unfortunately that's unlikely fully work. For example, the MD layer uses the same context type for hashes and HMACs, while the PSA API (rightfully) has distinct operation types. Similarly, the Cipher layer uses the same context type for unauthenticated and AEAD ciphers, which again the PSA API distinguishes.

It is unclear how much value, if any, a zero-cost compatibility layer that's incomplete (for example, for MD covering only hashes, or for Cipher covering only AEAD) or differs significantly from the existing API (for example, introducing new context types) would provide to users.

Low-cost compatibility layers?

Another possibility is to keep most or all of the existing API for the PK, MD and Cipher layers, implemented on top of PSA, aiming for the lowest possible cost. For example, mbedtls_md_context_t would be defined as a (tagged) union of psa_hash_operation_t and psa_mac_operation_t, then mbedtls_md_setup() would initialize the correct part, and the rest of the functions be simple wrappers around PSA functions. This would vastly reduce the complexity of the layers compared to the existing (no need to dispatch through function pointers, just call the corresponding PSA API).

Since this would still represent a non-zero cost, not only in terms of code size, but also in terms of maintainance (testing, etc.) this would probably be a temporary solution: for example keep the compatibility layers in 4.0 (and make them optional), but remove them in 5.0.

Again, this provides the most value to users if we can manage to keep the existing API unchanged. Their might be conflcits between this goal and that of reducing the cost, and judgment calls may need to be made.

Note: when it comes to holding public keys in the PK layer, depending on how the rest of the code is structured, it may be worth holding the key data in memory controlled by the PK layer as opposed to a PSA key slot, moving it to a slot only when needed (see current ecdsa_verify_wrap when MBEDTLS_USE_PSA_CRYPTO is defined) For example, when parsing a large number, N, of X.509 certificates (for example the list of trusted roots), it might be undesirable to use N PSA key slots for their public keys as long as the certs are loaded. OTOH, this could also be addressed by merging the "X.509 parsing on-demand" (#2478), and then the public key data would be held as bytes in the X.509 CRT structure, and only moved to a PK context / PSA slot when it's actually used.

Note: the PK layer actually consists of two relatively distinct parts: crypto operations, which will be covered by PSA, and parsing/writing (exporting) from/to various formats, which is currently not fully covered by the PSA Crypto API.

Algorithm identifiers and other identifiers

It should be easy to provide the user with a bunch of #defines for algorithm identifiers, for example #define MBEDTLS_MD_SHA256 PSA_ALG_SHA_256; most of those would be in the MD, Cipher and PK compatibility layers mentioned above, but there might be some in other modules that may be worth considering, for example identifiers for elliptic curves.

Lower layers

Generally speaking, we would retire all of the low-level, non-generic modules, such as AES, SHA-256, RSA, DHM, ECDH, ECP, bignum, etc, without providing compatibility APIs for them. People would be encouraged to switch to the PSA API. (The compatiblity implementation of the existing PK, MD, Cipher APIs would mostly benefit people who already used those generic APis rather than the low-level, alg-specific ones.)

APIs in TLS and X.509

Public APIs in TLS and X.509 may be affected by the migration in at least two ways:

1. APIs that rely on a legacy mbedtls_ crypto type: for example mbedtls_ssl_conf_own_cert() to configure a (certificate and the associated) private key. Currently the private key is passed as a mbedtls_pk_context object, which would probably change to a psa_key_id_t. Since some users would probably still be using the compatibility PK layer, it would need a way to easily extract the PSA key ID from the PK context.

2. APIs the accept list of identifiers: for example mbedtls_ssl_conf_curves() taking a list of mbedtls_ecp_group_ids. This could be changed to accept a list of pairs (psa_ecc_familiy_t, size) but we should probably take this opportunity to move to a identifier independant from the underlying crypto implementation and use TLS-specific identifiers instead (based on IANA values or custom enums), as is currently done in the new mbedtls_ssl_conf_groups() API, see #4859).

Testing

An question that needs careful consideration when we come around to removing the low-level crypto APIs and making PK, MD and Cipher optional compatibility layers is to be sure to preserve testing quality. A lot of the existing test cases use the low level crypto APIs; we would need to either keep using that API for tests, or manually migrated test to the PSA Crypto API. Perhaps a combination of both, perhaps evolving gradually over time.