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authsrv: salt the keyseed from /adm/keyseed file

change the keyseed key derivation to hkdf sha256
using the hostowners des key plus 256 bit random
salt from /adm/keyseed.

--- a/sys/src/cmd/auth/authsrv.c

+++ b/sys/src/cmd/auth/authsrv.c

@@ -1007,13 +1007,33 @@

 {

 	static char info[] = "PRF key for generation of dummy user keys";

 	char k[DESKEYLEN], *u;

+	int fd;

+	genrandom(keyseed, sizeof(keyseed));

+

 	u = getuser();

 	if(!finddeskey(KEYDB, u, k)){

-		syslog(0, AUTHLOG, "can't generate keyseed: user %s not in keydb", u);

-		exits(0);

+		syslog(0, AUTHLOG, "initkeyseed: user %s not in keydb", u);

+		return;

 	}

-	hmac_sha2_256((uchar*)info, sizeof(info)-1, (uchar*)k, sizeof(k), keyseed, nil);

+

+	if((fd = create("/adm/keyseed", OWRITE|OEXCL, 0600)) >= 0){

+		write(fd, keyseed, sizeof(keyseed));

+	} else if((fd = open("/adm/keyseed", OREAD)) >= 0){

+		read(fd, keyseed, sizeof(keyseed));

+	} else{

+		syslog(0, AUTHLOG, "initkeyseed: no seed file: %r");

+		memset(k, 0, sizeof(k));

+		return;

+	}

+	close(fd);

+

+	hkdf_x(	keyseed, sizeof(keyseed),

+		(uchar*)info, sizeof(info)-1,

+		(uchar*)k, sizeof(k),

+		keyseed, sizeof(keyseed),

+		hmac_sha2_256, SHA2_256dlen);

+

 	memset(k, 0, sizeof(k));

 }