Merge pull request #4979 from gilles-peskine-arm/doc-use-psa-crypto-2.x

Backport 2.x: Document effects of MBEDTLS_USE_PSA_CRYPTO

docs/use-psa-crypto.md

@@ -0,0 +1,204 @@
+This document describes the compile-time configuration option
+`MBEDTLS_USE_PSA_CRYPTO` from a user's perspective, more specifically its
+current effects as well as the parts that aren't covered yet.
+
+Current effects
+===============
+
+General limitations
+-------------------
+
+Compile-time: enabling `MBEDTLS_USE_PSA_CRYPTO` requires
+`MBEDTLS_ECP_RESTARTABLE` and
+`MBEDTLS_PSA_CRYPTO_KEY_ID_ENCODES_OWNER` to be disabled.
+
+Effect: `MBEDTLS_USE_PSA_CRYPTO` currently has no effect on TLS 1.3 (which is
+itself experimental and only partially supported so far): TLS 1.3 always uses
+the legacy APIs even when this option is set.
+
+Stability: any API that's only available when `MBEDTLS_USE_PSA_CRYPTO` is
+defined is considered experimental and may change in incompatible ways at any
+time. Said otherwise, these APIs are explicitly excluded from the usual API
+stability promises.
+
+New APIs / API extensions
+-------------------------
+
+Some of these APIs are meant for the application to use in place of
+pre-existing APIs, in order to get access to the benefits; in the sub-sections
+below these are indicated by "Use in (X.509 and) TLS: opt-in", meaning that
+this requires changes to the application code for the (X.509 and) TLS layers
+to pick up the improvements.
+
+Some of these APIs are mostly meant for internal use by the TLS (and X.509)
+layers; they are indicated below by "Use in (X.509 and) TLS: automatic",
+meaning that no changes to the application code are required for the TLS (and
+X.509) layers to pick up the improvements.
+
+### PSA-held (opaque) keys in the PK layer
+
+There is a new API function `mbedtls_pk_setup_opaque()` that can be used to
+wrap a PSA keypair into a PK context. The key can be used for private-key
+operations and its public part can be exported.
+
+Benefits: isolation of long-term secrets, use of PSA Crypto drivers.
+
+Limitations: only for private keys, only ECC. (That is, only ECDSA signature
+generation. Note: currently this will use randomized ECDSA while Mbed TLS uses
+deterministic ECDSA by default.) The following operations are not supported
+with a context set this way, while they would be available with a normal
+`ECKEY` context: `mbedtls_pk_verify()`, `mbedtls_pk_check_pair()`,
+`mbedtls_pk_debug()`.
+
+Use in X.509 and TLS: opt-in. The application needs to construct the PK context
+using the new API in order to get the benefits; it can then pass the
+resulting context to the following existing APIs:
+
+- `mbedtls_ssl_conf_own_cert()` or `mbedtls_ssl_set_hs_own_cert()` to use the
+  key together with a certificate for ECDSA-based key exchanges (note: while
+this is supported on both sides, it's currently only tested client-side);
+- `mbedtls_x509write_csr_set_key()` to generate a CSR (certificate signature
+  request).
+
+In the TLS and X.509 API, there are two other functions which accept a key or
+keypair as a PK context: `mbedtls_x509write_crt_set_subject_key()` and
+`mbedtls_x509write_crt_set_issuer_key()`. Use of opaque contexts here probably
+works but is so far untested.
+
+### PSA-held (opaque) keys for TLS pre-shared keys (PSK)
+
+There are two new API functions `mbedtls_ssl_conf_psk_opaque()` and
+`mbedtls_ssl_set_hs_psk_opaque()`. Call one of these from an application to
+register a PSA key for use with a PSK key exchange.
+
+Benefits: isolation of long-term secrets.
+
+Limitations: the key can only be used with "pure"
+PSK key exchanges (ciphersuites starting with `TLS_PSK_WITH_`), to the
+exclusion of RSA-PSK, DHE-PSK and ECDHE-PSK key exchanges. It is the responsibility of
+the user to make sure that when provisioning an opaque pre-shared key, the
+only PSK ciphersuites that can be negotiated are "pure" PSK; other XXX-PSK key
+exchanges will result in a handshake failure with the handshake function
+returning `MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE`.
+
+Use in TLS: opt-in. The application needs to register the key using the new
+APIs to get the benefits.
+
+### PSA-based operations in the Cipher layer
+
+There is a new API function `mbedtls_cipher_setup_psa()` to set up a context
+that will call PSA to store the key and perform the operations.
+
+Benefits: use of PSA Crypto drivers; partial isolation of short-term secrets
+(still generated outside of PSA, but then held by PSA).
+
+Limitations: the key is still passed in the clear by the application. The
+multi-part APIs are not supported, only the one-shot APIs. The only modes
+supported are ECB, CBC without padding, GCM and CCM (this excludes stream
+ciphers and ChachaPoly); the only cipher supported is AES (this excludes Aria,
+Camellia, and ChachaPoly). (Note: ECB is currently not tested.) (Note: it is
+possible to perform multiple one-shot operations with the same context;
+however this is not unit-tested, only tested via usage in TLS.)
+
+Use in TLS: automatic. Used when the cipher and mode is supported (with
+gracious fallback to the legacy API otherwise) in all places where a cipher is
+used. There are two such places: in `ssl_tls.c` for record protection, and in
+`ssl_ticket.c` for protecting tickets we issue.
+
+Internal changes
+----------------
+
+All of these internal changes are active as soon as `MBEDTLS_USE_PSA_CRYPTO`
+is enabled, no change required on the application side.
+
+### TLS: cipher operations based on PSA
+
+See "PSA-based operations in the Cipher layer" above.
+
+### PK layer: ECDSA verification based on PSA
+
+Scope: `mbedtls_pk_verify()` will call to PSA for ECDSA signature
+verification.
+
+Benefits: use of PSA Crypto drivers.
+
+Use in TLS and X.509: in all places where an ECDSA signature is verified.
+
+### TLS: ECDHE computation based on PSA
+
+Scope: Client-side, for ECDHE-RSA and ECDHE-ECDSA key exchanges, the
+computation of the ECDHE key exchange is done by PSA.
+
+Limitations: client-side only, ECDHE-PSK not covered
+
+Benefits: use of PSA Crypto drivers.
+
+### TLS: handshake hashes and PRF computed with PSA
+
+Scope: with TLS 1.2, the following are computed with PSA:
+- the running handshake hashes;
+- the hash of the ServerKeyExchange part that is signed;
+- the `verify_data` part of the Finished message;
+- the TLS PRF.
+
+Benefits: use of PSA Crypto drivers.
+
+### X.509: some hashes computed with PSA
+
+Scope: the following hashes are computed with PSA:
+- when verifying a certificate chain, hash of the child for verifying the
+  parent's signature;
+- when writing a CSR, hash of the request for self-signing the request.
+
+Benefits: use of PSA Crypto drivers.
+
+Parts that are not covered yet
+==============================
+
+This is only a high-level overview, grouped by theme
+
+TLS: 1.3 experimental support
+-----------------------------
+
+No part of the experimental support for TLS 1.3 is covered at the moment.
+
+TLS: key exchanges / asymmetric crypto
+--------------------------------------
+
+The following key exchanges are not covered at all:
+
+- RSA
+- DHE-RSA
+- DHE-PSK
+- RSA-PSK
+- ECDHE-PSK
+- ECDH-RSA
+- ECDH-ECDSA
+- ECJPAKE
+
+The following key exchanges are only partially covered:
+
+- ECDHE-RSA: RSA operations are not covered and, server-side, the ECDHE
+  operation isn't either
+- ECDHE-ECDSA: server-side, the ECDHE operation isn't covered. (ECDSA
+  signature generation is only covered if using `mbedtls_pk_setup_opaque()`.)
+
+PSK if covered when the application uses `mbedtls_ssl_conf_psk_opaque()` or
+`mbedtls_ssl_set_hs_psk_opaque()`.
+
+TLS: symmetric crypto
+---------------------
+
+- some ciphers not supported via PSA yet: ARIA, Camellia, ChachaPoly (silent
+  fallback to the legacy APIs)
+- the HMAC part of the CBC and NULL ciphersuites
+- the HMAC computation in `ssl_cookie.c`
+
+X.509
+-----
+
+- most hash operations are still done via the legacy API, except the few that
+  are documented above as using PSA
+- RSA PKCS#1 v1.5 signature generation (from PSA-held keys)
+- RSA PKCS#1 v1.5 signature verification
+- RSA-PSS signature verification

include/mbedtls/config.h

@@ -2119,15 +2119,13 @@
  * will still continue to work as usual, so enabling this option should not
  * break backwards compatibility.
  *
- * \warning The PSA Crypto API is in beta stage. While you're welcome to
- * experiment using it, incompatible API changes are still possible, and some
- * parts may not have reached the same quality as the rest of Mbed TLS yet.
+ * \note See docs/use-psa-crypto.md for a complete description of what this
+ * option currently does, and of parts that are not affected by it so far.
  *
- * \warning This option enables new Mbed TLS APIs that are dependent on the
- * PSA Crypto API, so can't come with the same stability guarantees as the
- * rest of the Mbed TLS APIs. You're welcome to experiment with them, but for
- * now, access to these APIs is opt-in (via enabling the present option), in
- * order to clearly differentiate them from the stable Mbed TLS APIs.
+ * \warning This option enables new Mbed TLS APIs which are currently
+ * considered experimental and may change in incompatible ways at any time.
+ * That is, the APIs enabled by this option are not covered by the usual
+ * promises of API stability.
  *
  * Requires: MBEDTLS_PSA_CRYPTO_C.
  *
@@ -3199,10 +3197,6 @@
  *
  * Enable the Platform Security Architecture cryptography API.
  *
- * \warning The PSA Crypto API is still beta status. While you're welcome to
- * experiment using it, incompatible API changes are still possible, and some
- * parts may not have reached the same quality as the rest of Mbed TLS yet.
- *
  * Module:  library/psa_crypto.c
  *
  * Requires: either MBEDTLS_CTR_DRBG_C and MBEDTLS_ENTROPY_C,