How are md5.h: pg_md5_hash() function and below functions working?

[Ali Koca <kinetixcicocuk@xxxxxxxxx>]

I can't understand functions in md5.h, these are seemingly little bit 
weird. Such as:

	/* Utilities common to all the MD5 implementations,
	as of md5_common.c */
	extern bool pg_md5_hash(const void *buff, size_t len, char *hexsum);
	extern bool pg_md5_binary(const void *buff, size_t len, void *outbuf);
	extern bool pg_md5_encrypt(const char *passwd, const char *salt,
						   size_t salt_len, char *buf);