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.

As of Mbed TLS 3.2, most of (G1) and all of (G2) is implemented when MBEDTLS_USE_PSA_CRYPTO is enabled. For (G2) to take effect, the application needs to be changed to use new APIs. For a more detailled account of what's implemented, see docs/use-psa-crypto.md, where new APIs are about (G2), and internal changes implement (G1).

Generally speaking, the numbering above doesn't mean that each goal requires the preceding ones to be completed.

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).
• PSA_CRYPTO_CONFIG - disabled by default, supports builds with drivers and without the corresponding software implementation (G5 above).

The reasons why MBEDTLS_USE_PSA_CRYPTO is optional and disabled by default are:

• it's incompatible with MBEDTLS_ECP_RESTARTABLE;
• 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 are able 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 and currently planned for end of 2022, see https://github.com/orgs/Mbed-TLS/projects/1#column-18883250.

It will then require follow-up work to make use of the new PSA API in PK/X.509/TLS in all places where we currently allow restartable operations.

Backward compatibility issues with making MBEDTLS_USE_PSA_CRYPTO 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

Supporting builds with drivers without the software implementation

This section presents a plan towards G5: save code size by compiling out our software implementation when a driver is available.

Additionally, we want to save code sive by compiling out the abstractions layers that we are not using when MBEDTLS_USE_PSA_CRYPTO is enabled (see previous section): MD and Cipher.

Let's expand a bit on the definition of the goal: in such a configuration (driver used, software implementation and abstraction layer compiled out), we want:

a. the library to build in a reasonably-complete configuration, b. with all tests passing, c. and no more tests skipped than the same configuration with software implementation.

Criterion (c) ensures not only test coverage, but that driver-based builds are at feature parity with software-based builds.

We can roughly divide the work needed to get there in the following steps:

0. Have a working driver interface for the algorithms we want to replace.
1. Have users of these algorithms call to PSA, not the legacy API, for all operations. (This is G1, and for PK, X.509 and TLS this is controlled by MBEDTLS_USE_PSA_CRYPTO.) This needs to be done in the library and tests.
2. Have users of these algorithms not depend on the legacy API for information management (getting a size for a given algorithm, etc.)
3. Adapt compile-time guards used to query availability of a given algorithm; this needs to be done in the library (for crypto operations and data) and tests.

Note: the first two steps enable use of drivers, but not by themselves removal of the software implementation.

Note: the fact that step 1 is not achieved for all of libmbedcrypto (see below) is the reason why criterion (a) has "a reasonably-complete configuration", to allow working around internal crypto dependencies when working on other parts such as X.509 and TLS - for example, a configuration without RSA PKCS#1 v2.1 still allows reasonable use of X.509 and TLS.

Status as of Mbed TLS 3.2:

• Step 0 is achieved for most algorithms, with only a few gaps remaining.
• Step 1 is achieved for most of PK, X.509, and TLS when MBEDTLS_USE_PSA_CRYPTO is enabled with only a few gaps remaining (see docs/use-psa-crypto.md).
• Step 1 is not achieved for a lot of the crypto library including the PSA core. For example, entropy.c calls the legacy API mbedtls_sha256 (or mbedtls_sha512 optionally); hmac_drbg.c calls the legacy API mbedtls_md and ctr_drbg.c calls the legacy API mbedtls_aes; the PSA core depends on the entropy module and at least one of the DRBG modules (unless MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG is used). Further, several crypto modules have similar issues, for example RSA PKCS#1 v2.1 calls mbedtls_md directly.
• Step 2 is achieved for most of X.509 and TLS (same gaps as step 1) when MBEDTLS_USE_PSA_CRYPTO is enabled - this was tasks like #5795, #5796, #5797. It is being done in PK and RSA PKCS#1 v1.5 by PR #6065.
• Step 3 was mostly not started at all before 3.2; it is being done for PK by PR #6065.

Strategy for step 1:

Regarding PK, X.509, and TLS, this is mostly achieved with only a few gaps. (The strategy was outline in the previous section.)

Regarding libmbedcrypto, including the PSA Crypto core, this has not been studied yet. For dependencies outside the PSA Crypto code (such as RSA PKCS#1 v2.1 depending on MD), it should be checked whether this can be achieved without backwards compatibility issues (currently applications can call mbedtls_rsa_xxx() functions without calling psa_crypto_init() first), otherwise a new compile-time option might be needed. For dependencies in the PSA Crypto core, splitting psa_crypto_init() might be a topic (which might also help for dependencies outside the core), with likely questions about ordering (can we initialize drivers before the RNG or do some divers expect a working RNG?) and trying to avoid circular dependencies.

Strategy for step 2:

The most satisfying situation here is when we can just use the PSA Crypto API for information management as well. However sometimes it may not be convenient, for example in parts of the code that accept old-style identifier (such as mbedtls_md_type_t) in their API and can't assume PSA to be compiled in (such as rsa.c).

It is suggested that, as a temporary solution until we clean this up later when removing the legacy API including its identifiers (G4), we may occasionally use ad-hoc internal functions, such as the one introduced by PR 6065 in library/md_internal.h.

An alternative would be to have two different code paths depending on whether MBEDTLS_PSA_CRYPTO_C is defined or not. However this is not great for readability or testability.

Strategy for step 3:

There are currently two (complementary) ways for crypto-using code to check if a particular algorithm is supported: using MBEDTLS_xxx macros, and using PSA_WANT_xxx macros. For example, PSA-based code that want to use SHA-256 will check for PSA_WANT_ALG_SHA_256, while legacy-based code that wants to use SHA-256 will check for MBEDTLS_SHA256_C if using the mbedtls_sha256 API, or for MBEDTLS_MD_C && MBEDTLS_SHA256_C if suing the mbedtls_md API.

It is suggested to introduce a new set of macros, MBEDTLS_USE_PSA_WANT_xxx, for use in the parts of the code that use either API depending of whether MBEDTLS_USE_PSA_CRYPTO is enabled (that is, PK, X.509 and TLS 1.2). This is done for hash algorithms, as an example, by PR 6065. These macros can be used in library code (most useful when algorithm availability is check far from the code that will be using it, such as in TLS negotiation) as well as test dependencies.

It should also be noted that there is a fourth case: utility functions / information tables that are not tied to a particular crypto API, and may be used by functions that are either purely PSA-based, purely legacy-based, or hybrid governed by MBEDTL_USE_PSA_CRYPTO should use MBEDTLS_xxx || PSA_WANT_xxx - for example, oid_md_alg from oid.c, used by both X.509 and RSA. A new family of macros MBEDTLS_OR_PSA_WANT_xxx is defined for this.

To sum up, there are 4 categories:

• legacy-based code depends on MBEDTLS_xxx;
• PSA-based code depends on PSA_WANT_xxx;
• hybrid code governed by MBEDTLS_USE_PSA_CRYPTO can use MBEDTLS_USE_PSA_WANT_xxx to express dependencies in common parts;
• data and crypto-agnostic helpers that can be used by code from at least two of the above categories should depend on MBEDTLS_OR_PSA_WANT_xxx.

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 layer 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.