ECC: add rationale
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/docs/proposed/psa-conditional-inclusion-c.md b/docs/proposed/psa-conditional-inclusion-c.md
index 39e5146..f54c61f 100644
--- a/docs/proposed/psa-conditional-inclusion-c.md
+++ b/docs/proposed/psa-conditional-inclusion-c.md
@@ -84,10 +84,12 @@
For asymmetric cryptography, `PSA_WANT_KEY_TYPE_xxx_KEY_PAIR` determines whether private-key operations are desired, and `PSA_WANT_KEY_TYPE_xxx_PUBLIC_KEY` determines whether public-key operations are desired. `PSA_WANT_KEY_TYPE_xxx_KEY_PAIR` implicitly enables `PSA_WANT_KEY_TYPE_xxx_PUBLIC_KEY`: there is no way to only include private-key operations (which typically saves little code).
-#### Configuration symbols for curves
+#### Configuration symbols for elliptic curves
For elliptic curve key types, only the specified curves are included. To include a curve, include a symbol of the form **`PSA_WANT_ECC_family_size`**. For example: `PSA_WANT_ECC_SECP_R1_256` for secp256r1, `PSA_WANT_ECC_MONTGOMERY_255` for Curve25519. It is an error to require an ECC key type but no curve, and Mbed TLS will reject this at compile time.
+Rationale: this is a deviation of the general principle that `PSA_ECC_FAMILY_xxx` would have a corresponding symbol `PSA_WANT_ECC_FAMILY_xxx`. This deviation is justified by the fact that it is very common to wish to include only certain curves in a family, and that can lead to a significant gain in code size.
+
#### Configuration symbols for algorithms
For each constant or constructor macro of the form `PSA_ALG_xxx`, the symbol **`PSA_WANT_ALG_xxx`** indicates that support for this algorithm is desired.
@@ -193,7 +195,7 @@
#### Naming of symbols
-The names of [elliptic curve symbols](#configuration-symbols-for-curves) are a bit weird: `SECP_R1_256` instead of `SECP256R1`. Should we make them more classical, but less systematic?
+The names of [elliptic curve symbols](#configuration-symbols-for-elliptic-curves) are a bit weird: `SECP_R1_256` instead of `SECP256R1`, `MONTGOMERY_255` instead of `CURVE25519`. Should we make them more classical, but less systematic?
#### Impossible combinations
