115 lines
4.3 KiB
Markdown
115 lines
4.3 KiB
Markdown
This document describes the compile-time configuration option
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`MBEDTLS_USE_PSA_CRYPTO` from a user's perspective.
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This option makes the X.509 and TLS library use PSA for cryptographic
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operations, and enables new APIs for using keys handled by PSA Crypto.
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General considerations
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----------------------
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**Application code:** when this option is enabled, you need to call
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`psa_crypto_init()` before calling any function from the SSL/TLS, X.509 or PK
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module.
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**Scope:** `MBEDTLS_USE_PSA_CRYPTO` has no effect on the parts of the code that
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are specific to TLS 1.3; those parts always use PSA Crypto. The parts of the
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TLS 1.3 code that are common with TLS 1.2, however, follow this option;
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currently this is the record protection code, computation of the running
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handshake hash, and X.509. You need to enable `MBEDTLS_USE_PSA_CRYPTO` if you
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want TLS 1.3 to use PSA everywhere.
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New APIs / API extensions
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-------------------------
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### PSA-held (opaque) keys in the PK layer
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**New API function:** `mbedtls_pk_setup_opaque()` - can be used to
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wrap a PSA key pair into a PK context. The key can be used for private-key
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operations and its public part can be exported.
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**Benefits:** isolation of long-term secrets, use of PSA Crypto drivers.
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**Limitations:** can only wrap a key pair, can only use it for private key
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operations. (That is, signature generation, and for RSA decryption too.)
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Note: for ECDSA, currently this uses randomized ECDSA while Mbed TLS uses
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deterministic ECDSA by default. The following operations are not supported
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with a context set this way, while they would be available with a normal
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context: `mbedtls_pk_check_pair()`, `mbedtls_pk_debug()`, all public key
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operations.
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**Use in X.509 and TLS:** opt-in. The application needs to construct the PK context
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using the new API in order to get the benefits; it can then pass the
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resulting context to the following existing APIs:
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- `mbedtls_ssl_conf_own_cert()` or `mbedtls_ssl_set_hs_own_cert()` to use the
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key together with a certificate for certificate-based key exchanges;
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- `mbedtls_x509write_csr_set_key()` to generate a CSR (certificate signature
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request);
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- `mbedtls_x509write_crt_set_issuer_key()` to generate a certificate.
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### PSA-held (opaque) keys for TLS pre-shared keys (PSK)
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**New API functions:** `mbedtls_ssl_conf_psk_opaque()` and
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`mbedtls_ssl_set_hs_psk_opaque()`. Call one of these from an application to
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register a PSA key for use with a PSK key exchange.
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**Benefits:** isolation of long-term secrets.
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**Limitations:** none.
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**Use in TLS:** opt-in. The application needs to register the key using one of
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the new APIs to get the benefits.
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### PSA-based operations in the Cipher layer
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There is a new API function `mbedtls_cipher_setup_psa()` to set up a context
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that will call PSA to store the key and perform the operations.
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This function only worked for a small number of ciphers. It is now deprecated
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and it is recommended to use `psa_cipher_xxx()` or `psa_aead_xxx()` functions
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directly instead.
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**Warning:** This function will be removed in a future version of Mbed TLS. If
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you are using it and would like us to keep it, please let us know about your
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use case.
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Internal changes
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----------------
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All of these internal changes are active as soon as `MBEDTLS_USE_PSA_CRYPTO`
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is enabled, no change required on the application side.
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### TLS: most crypto operations based on PSA
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Current exceptions:
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- Finite-field (non-EC) Diffie-Hellman (used in key exchanges: DHE-RSA,
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DHE-PSK).
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- Restartable operations when `MBEDTLS_ECP_RESTARTABLE` is also enabled (see
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the documentation of that option).
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Other than the above exceptions, all crypto operations are based on PSA when
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`MBEDTLS_USE_PSA_CRYPTO` is enabled.
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### X.509: most crypto operations based on PSA
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Current exceptions:
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- Restartable operations when `MBEDTLS_ECP_RESTARTABLE` is also enabled (see
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the documentation of that option).
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Other than the above exception, all crypto operations are based on PSA when
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`MBEDTLS_USE_PSA_CRYPTO` is enabled.
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### PK layer: most crypto operations based on PSA
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Current exceptions:
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- Verification of RSA-PSS signatures with an MGF hash that's different from
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the message hash.
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- Restartable operations when `MBEDTLS_ECP_RESTARTABLE` is also enabled (see
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the documentation of that option).
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Other than the above exceptions, all crypto operations are based on PSA when
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`MBEDTLS_USE_PSA_CRYPTO` is enabled.
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