Clarify the trailer field situation
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/docs/architecture/psa-migration/psa-limitations.md b/docs/architecture/psa-migration/psa-limitations.md
index 45c7806..a7c4afb 100644
--- a/docs/architecture/psa-migration/psa-limitations.md
+++ b/docs/architecture/psa-migration/psa-limitations.md
@@ -83,7 +83,8 @@
- a mask generation function
- most commonly MGF1, which in turn is parametrized by a hash algorithm
- a salt length
-- a trailer field - this is universally 0xBC as far as I've seen
+- a trailer field - the value is fixed to 0xBC by PKCS#1 v2.1, but was left
+ configurable in the original scheme; 0xBC is used everywhere in pratice.
Both the existing `mbedtls_` API and the PSA API support only MGF1 as the
generation function (and only 0xBC as the trailer field), but there are