docs/use-psa-crypto.md

This document describes the compile-time configuration option
`MBEDTLS_USE_PSA_CRYPTO` from a user's perspective.

This option makes the X.509 and TLS library use PSA for cryptographic
operations, and enables new APIs for using keys handled by PSA Crypto.

General considerations
----------------------

**Application code:** when this option is enabled, you need to call
`psa_crypto_init()` before calling any function from the SSL/TLS, X.509 or PK
module.

**Scope:** `MBEDTLS_USE_PSA_CRYPTO` has no effect on the parts of the code that
are specific to TLS 1.3; those parts always use PSA Crypto. The parts of the
TLS 1.3 code that are common with TLS 1.2, however, follow this option;
currently this is the record protection code, computation of the running
handshake hash, and X.509. You need to enable `MBEDTLS_USE_PSA_CRYPTO` if you
want TLS 1.3 to use PSA everywhere.

New APIs / API extensions
-------------------------

### PSA-held (opaque) keys in the PK layer

**New API function:** `mbedtls_pk_setup_opaque()` - can be used to
wrap a PSA key pair 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:** can only wrap a key pair, can only use it for private key
operations. (That is, signature generation, and for RSA decryption too.)
Note: for ECDSA, currently this uses 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
context: `mbedtls_pk_check_pair()`, `mbedtls_pk_debug()`, all public key
operations.

**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 certificate-based key exchanges;
- `mbedtls_x509write_csr_set_key()` to generate a CSR (certificate signature
  request);
- `mbedtls_x509write_crt_set_issuer_key()` to generate a certificate.

### PSA-held (opaque) keys for TLS pre-shared keys (PSK)

**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:** none.

**Use in TLS:** opt-in. The application needs to register the key using one of
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.

This function only worked for a small number of ciphers. It is now deprecated
and it is recommended to use `psa_cipher_xxx()` or `psa_aead_xxx()` functions
directly instead.

**Warning:** This function will be removed in a future version of Mbed TLS. If
you are using it and would like us to keep it, please let us know about your
use case.

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: most crypto operations based on PSA

Current exceptions:

- Finite-field (non-EC) Diffie-Hellman (used in key exchanges: DHE-RSA,
  DHE-PSK).
- Restartable operations when `MBEDTLS_ECP_RESTARTABLE` is also enabled (see
  the documentation of that option).

Other than the above exceptions, all crypto operations are based on PSA when
`MBEDTLS_USE_PSA_CRYPTO` is enabled.

### X.509: most crypto operations based on PSA

Current exceptions:

- Verification of RSA-PSS signatures with a salt length that is different from
  the hash length.
- Restartable operations when `MBEDTLS_ECP_RESTARTABLE` is also enabled (see
  the documentation of that option).

Other than the above exception, all crypto operations are based on PSA when
`MBEDTLS_USE_PSA_CRYPTO` is enabled.

### PK layer: most crypto operations based on PSA

Current exceptions:

- Verification of RSA-PSS signatures with a salt length that is different from
  the hash length, or with an MGF hash that's different from the message hash.
- Restartable operations when `MBEDTLS_ECP_RESTARTABLE` is also enabled (see
  the documentation of that option).

Other than the above exception, all crypto operations are based on PSA when
`MBEDTLS_USE_PSA_CRYPTO` is enabled.