Update Docs for EVP_MAC

For GMAC/CMAC, its not possible to re-init the algorithm without
explicitly passing an OSSL_MAC_PARAM_IV to each init call, as it is
not possible to extract the IV value from the prior init call (be it
explicitly passed or auto generated).  As such, document the fact that
re-initalization requires passing an IV parameter

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23235)

(cherry picked from commit 7c1d533a512181b13de3bc0b7fa2fd8c481032d3)

diff --git a/doc/man3/EVP_MAC.pod b/doc/man3/EVP_MAC.pod
index 56ac92a486728ef9661720dd031b75d02aa73dd7..762c0175ff131aa71a2e56025b3696801d0684ab 100644 (file)
--- a/doc/man3/EVP_MAC.pod
+++ b/doc/man3/EVP_MAC.pod
@@ -145,6 +145,9 @@ the key.  If I<key> is NULL, the key must be set via I<params> either
 as part of this call or separately using EVP_MAC_CTX_set_params().
 Providing non-NULL I<params> to this function is equivalent to calling
 EVP_MAC_CTX_set_params() with those I<params> for the same I<ctx> beforehand.
+Note: There are additional requirements for some MAC algorithms during
+re-initalization (i.e. calling EVP_MAC_init() on an EVP_MAC after EVP_MAC_final()
+has been called on the same object).  See the NOTES section below.
 
 EVP_MAC_init() should be called before EVP_MAC_update() and EVP_MAC_final().
 
@@ -342,6 +345,13 @@ not be considered a breaking change to the API.
 The usage of the parameter names "custom", "iv" and "salt" correspond to
 the names used in the standard where the algorithm was defined.
 
+Some MAC algorithms store internal state that cannot be extracted during
+re-initalization.  For example GMAC cannot extract an B<IV> from the
+underlying CIPHER context, and so calling EVP_MAC_init() on an EVP_MAC object
+after EVP_MAC_final() has been called cannot reset its cipher state to what it
+was when the B<IV> was initially generated.  For such instances, an
+B<OSSL_MAC_PARAM_IV> parameter must be passed with each call to EVP_MAC_init().
+
 =head1 RETURN VALUES
 
 EVP_MAC_fetch() returns a pointer to a newly fetched B<EVP_MAC>, or