SRP_Calc_client_key calls BN_mod_exp with private data. However it was
not setting BN_FLG_CONSTTIME and therefore not using the constant time
implementation. This could be exploited in a side channel attack to
recover the password.
Since the attack is local host only this is outside of the current OpenSSL
threat model and therefore no CVE is assigned.
Thanks to Mohammed Sabt and Daniel De Almeida Braga for reporting this
Reviewed-by: Tomas Mraz <email@example.com>
(Merged from https://github.com/openssl/openssl/pull/13888)