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. Use PSA Crypto drivers when available. G2. Allow isolation of long-term secrets (for example, private keys). G3. Allow isolation of short-term secrets (for example, TLS session keys). G4. Have a clean, unified API for Crypto (retire the legacy API). G5. Code size: compile out our implementation when a driver is available.

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 G2-G5 could be done in any order; however they all either depend on G1 or are just much more convenient if G1 is done before (note that this is not a dependency on G1 being complete, it's more like each bit of G2-G5 is helped by some specific bit in 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 reasons why MBEDTLS_USE_PSA_CRYPTO is optional and disabled by default are:

• it's incompatible with MBEDTLS_ECP_RESTARTABLE;
• it does not work well with MBEDTLS_PSA_CRYPTO_CONFIG (could compile with both of them, but then MBEDTLS_PSA_CRYPTO_CONFIG won't have the desired effect)
• to avoid a hard/default dependency of TLS, X.509 and PK on MBEDTLS_PSA_CRYPTO_C, for backward compatibility reasons:
  • When MBEDTLS_PSA_CRYPTO_C is enabled and used, applications need to call psa_crypto_init() before TLS/X.509 uses PSA functions. (This prevents us from even enabling the option by default.)
  • MBEDTLS_PSA_CRYPTO_C has a hard depend on MBEDTLS_ENTROPY_C || MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG but it's currently possible to compilte TLS and X.509 without any of the options. Also, we can't just auto-enable MBEDTLS_ENTROPY_C as it doesn't build out of the box on all platforms, and even less MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG as it requires a user-provided RNG function.

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. The rest of this section explains the reasons for the incompatibilities mentioned above.

At the time of writing (early 2022) it is unclear what could be done about the backward compatibility issues, and in particular if the cost of implementing solutions to these problems would be higher or lower than the cost of maintaining dual code paths until the next major version. (Note: these solutions would probably also solve other problems at the same time.)

MBEDTLS_ECP_RESTARTABLE

Currently this option controls not only the presence of restartable APIs in the crypto library, but also their use in the TLS and X.509 layers. Since PSA Crypto does not support restartable operations, there's a clear conflict: the TLS and X.509 layers can't both use only PSA APIs and get restartable behaviour.

Supporting this in PSA is on our roadmap (it's been requested). But it's way below generalizing support for MBEDTLS_USE_PSA_CRYPTO for “mainstream” use cases on our priority list. So in the medium term MBEDTLS_ECP_RESTARTABLE is incompatible with MBEDTLS_USE_PSA_CRYPTO.

Note: it is possible to make the options compatible at build time simply by deciding that when USE_PSA_CRYPTO is enabled, PSA APIs are used except if restartable behaviour was requested at run-time (in addition to enabling MBEDTLS_ECP_RESTARTABLE in the build). This would require some work to dispatch operations as intended, and test.

Currently (early 2022) the mild consensus seems to be that since we'll need to implement restartable in PSA anyway, it's probably not worth spending time on the compatibility issue while waiting for it to get a more satisfying resolution when PSA starts supporting restartable.

MBEDTLS_PSA_CRYPTO_CONFIG

(This section taken from a comment by Gilles.)

X509 and TLS code use MBEDTLS_xxx macros to decide whether an algorithm is supported. This doesn't make MBEDTLS_USE_PSA_CRYPTO incompatible with MBEDTLS_PSA_CRYPTO_CONFIG per se, but it makes it incompatible with most useful uses of MBEDTLS_PSA_CRYPTO_CONFIG. The point of MBEDTLS_PSA_CRYPTO_CONFIG is to be able to build a library with support for an algorithm through a PSA driver only, without building the software implementation of that algorithm. But then the TLS code would consider the algorithm unavailable.

This is tracked in https://github.com/Mbed-TLS/mbedtls/issues/3674 and https://github.com/Mbed-TLS/mbedtls/issues/3677. But now that I look at it with fresh eyes, I don't think the approach we were planning to use would actually works. This needs more design effort.

This is something we need to support eventually, and several partners want it. I don't know what the priority is for MBEDTLS_USE_PSA_CRYPTO between improving driver support and covering more of the protocol. It seems to me that it'll be less work overall to first implement a good architecture for MBEDTLS_USE_PSA_CRYPTO + MBEDTLS_PSA_CRYPTO_CONFIG and then extend to more protocol features, because implementing that architecture will require changes to the existing code and the less code there is at this point the better, whereas extending to more protocol features will require the same amount of work either way.

Backward compatibility issues with making it always on

1. Existing applications may not be calling psa_crypto_init() before using TLS, X.509 or PK. We can try to work around that by calling (the relevant part of) it ourselves under the hood as needed, but that would likely require splitting init between the parts that can fail and the parts that can't (see https://github.com/ARM-software/psa-crypto-api/pull/536 for that).
2. It's currently not possible to enable MBEDTLS_PSA_CRYPTO_C in configurations that don't have MBEDTLS_ENTROPY_C, and we can't just auto-enable the latter, as it won't build or work out of the box on all platforms. There are two kinds of things we'd need to do if we want to work around that:
   1. Make it possible to enable the parts of PSA Crypto that don't require an RNG (typically, public key operations, symmetric crypto, some key management functions (destroy etc)) in configurations that don't have ENTROPY_C. This requires going through the PSA code base to adjust dependencies. Risk: there may be annoying dependencies, some of which may be surprising.
   2. For operations that require an RNG, provide an alternative function accepting an explicit f_rng parameter (see #5238), that would be available in entropy-less builds. (Then code using those functions still needs to have one version using it, for entropy-less builds, and one version using the standard function, for driver support in build with entropy.)

See https://github.com/Mbed-TLS/mbedtls/issues/5156

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 (early 2022) used for all operations in the PK layer.

This strategy is not very well suited to the Cipher layer, as the PSA implementation is currently done on top of that layer.

This strategy will probably be used for some time for the PK layer, while we figure out what the future of that layer is: parts of it (parse/write, ECDSA signatures in the format that X.509 & TLS want) are not covered by PSA, so they will need to keep existing in some way. (Also, the PK layer is a good place for dispatching to either PSA or mbedtls_xxx_restartable while that part is not covered by PSA yet, if we decide to do that.)

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.
• Upside: opens the door to building TLS/X.509 without that layer, saving some code size.
• Downside: TLS/X.509 code has to be done for each operation.

This strategy is currently (early 2022) used for the MD layer and the Cipher layer.

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.

This strategy is not useful when no context is used, for example with the one-shot function mbedtls_md().

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 outside of PSA most of the time, supporting only G1).

This strategy, with support for key isolation, is currently (early 2022) used for private-key operations 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.

This strategy, without key isolation, was also previously used (until 3.1 included) in the Cipher layer - see mbedtls_cipher_setup_psa(). This allowed use of PSA for cipher operations in TLS with no change to the application code, and a contained change in TLS code. (It only supported a subset of ciphers.)

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.

Summary

Strategies currently (early 2022) used with each abstraction layer:

• PK (for G1): silently call PSA
• PK (for G2): opt-in use of PSA (new key type)
• Cipher (G1): replace calls at each call site
• MD (G1): replace calls at each call site

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, essentially mapping mbedtls_ APIs to their psa_ equivalent.

Unfortunately that's unlikely to 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 maintenance (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 conflicts 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 compatibility 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_family_t, size) but we should probably take this opportunity to move to a identifier independent 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 migrate tests to the PSA Crypto API. Perhaps a combination of both, perhaps evolving gradually over time.