Remind the reader that PK doesn't support DH
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/docs/psa-transition.md b/docs/psa-transition.md
index f9ea382..2c0e129 100644
--- a/docs/psa-transition.md
+++ b/docs/psa-transition.md
@@ -906,7 +906,7 @@
#### Exposing a PSA key via PK
-This section discusses how to use a PSA key in a context that requires a PK object, such as PK formatting functions (`mbedtls_pk_write_key_der`, `mbedtls_pk_write_pubkey_der`, `mbedtls_pk_write_pubkey_pem`, `mbedtls_pk_write_key_pem` or `mbedtls_pk_write_pubkey`), Mbed TLS X.509 functions, Mbed TLS SSL functions, or another API that involves `mbedtls_pk_context` objects. Three functions from `pk.h` help with that:
+This section discusses how to use a PSA key in a context that requires a PK object, such as PK formatting functions (`mbedtls_pk_write_key_der`, `mbedtls_pk_write_pubkey_der`, `mbedtls_pk_write_pubkey_pem`, `mbedtls_pk_write_key_pem` or `mbedtls_pk_write_pubkey`), Mbed TLS X.509 functions, Mbed TLS SSL functions, or another API that involves `mbedtls_pk_context` objects. The PSA key must be an RSA or ECC key since the PK module does not support DH keys. Three functions from `pk.h` help with that:
* [`mbedtls_pk_copy_from_psa`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/pk_8h/#pk_8h_1ab8e88836fd9ee344ffe630c40447bd08) copies a PSA key into a PK object. The PSA key must be exportable. The PK object remains valid even if the PSA key is destroyed.
* [`mbedtls_pk_setup_opaque`](https://mbed-tls.readthedocs.io/projects/api/en/development/api/file/pk_8h/#pk_8h_1a4c04ac22ab9c1ae09cc29438c308bf05) sets up a PK object that wraps the PSA key. The PK object can only be used as permitted by the PSA key's policy. The PK object contains a reference to the PSA key identifier, therefore PSA key must not be destroyed as long as the PK object remains alive.