Document the general strategy for PSA migration

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

docs/architecture/psa-migration/strategy.md

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+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
+`#define`s, 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 `#define`s 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_id`s. 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.