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 limitations
-------------------

Compile-time: enabling `MBEDTLS_USE_PSA_CRYPTO` requires
`MBEDTLS_ECP_RESTARTABLE` to be disabled.

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

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: can only wrap a keypair, 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
`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 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).
- `mbedtls_x509write_crt_set_issuer_key()` to generate a certificate.

### 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: only TLS 1.2 for now.

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.

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.

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:

- EC J-PAKE (when `MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED`)
- finite-field (non-EC) Diffie-Hellman (use in key exchanges: DHE-RSA,
  DHE-PSK)

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 exception:

- verification of RSA-PSS signatures with a salt length that is different from
  the hash length.

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

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

Current exception:

- verification of RSA-PSS signatures with a salt length that is different from
  the hash length.

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