As always, there are some functions that just don't fit anywhere.
PQencryptPassword
Prepares the encrypted form of a PostgreSQL password.
char * PQencryptPassword(const char *passwd, const char *user);
This function is intended to be used by client applications that
wish to send commands like ALTER USER joe PASSWORD
'pwd'. It is good practice not to send the original cleartext
password in such a command, because it might be exposed in command
logs, activity displays, and so on. Instead, use this function to
convert the password to encrypted form before it is sent. The
arguments are the cleartext password, and the SQL name of the user
it is for. The return value is a string allocated by
malloc, or NULL if out of
memory. The caller can assume the string doesn't contain any
special characters that would require escaping. Use
PQfreemem to free the result when done with it.