Submitted by: @XaBbl4

--- Set parameters to firebird.conf:
AuthServer = Srp
AuthClient = Srp, Legacy_Auth
UserManager = Srp, Legacy_UserManager

--- Add aliases to databases.conf:
test.db = $(root)/test.fdb
{
AuthServer = Srp, Legacy_Auth
AuthClient = Legacy_Auth
WireCrypt = disabled
}
localhost:test.db
{
AuthClient = Legacy_Auth
}

--- Add users:
create user srp password 'srp';
create user legacy password 'legacy' using plugin Legacy_UserManager;

--- Create new database:
create database 'localhost:test.db' user 'sysdba' password 'masterkey';

--- Execute query:
set term ^;
execute block RETURNS(tmp integer)
as
begin
execute statement 'select 1 from rdb$database'
on external 'localhost:test.db'
as user 'legacy' password 'legacy'
into tmp;
suspend;
end^

--- Attach success and output:
TMP

       1

--- Execute query:
set term ^;
execute block RETURNS(tmp integer)
as
begin
execute statement 'select 2 from rdb$database'
on external 'localhost:test.db'
as user 'srp' password 'srp'
into tmp;
suspend;
end^

--- Attach to success, and output:
TMP

       2

--- But must be except (like in Linux):
Statement failed, SQLSTATE = 42000
Execute statement error at attach :
335545106 : Error occurred during login, please check server firebird.log for details
Data source : Firebird::localhost:test.db
-At block line: 4, col: 3