libsec: add rfc5869 hmac-based key derivation function hkdf_x()

sys/include/libsec.h

@@ -464,3 +464,8 @@ void curve25519_dh_finish(uchar x[32], uchar y[32], uchar z[32]);
 /* password-based key derivation function 2 (rfc2898) */
 void pbkdf2_x(uchar *p, ulong plen, uchar *s, ulong slen, ulong rounds, uchar *d, ulong dlen,
 	DigestState* (*x)(uchar*, ulong, uchar*, ulong, uchar*, DigestState*), int xlen);
+
+/* hmac-based key derivation function (rfc5869) */
+void hkdf_x(uchar *salt, ulong nsalt, uchar *info, ulong ninfo, uchar *key, ulong nkey, uchar *d, ulong dlen,
+	DigestState* (*x)(uchar*, ulong, uchar*, ulong, uchar*, DigestState*), int xlen);
+

sys/src/libsec/port/hkdf.c

@@ -0,0 +1,39 @@
+#include "os.h"
+#include <mp.h>
+#include <libsec.h>
+
+/* rfc5869 */
+void
+hkdf_x(salt, nsalt, info, ninfo, key, nkey, d, dlen, x, xlen)
+	uchar *salt, *info, *key, *d;
+	ulong nsalt, ninfo, nkey, dlen;
+	DigestState* (*x)(uchar*, ulong, uchar*, ulong, uchar*, DigestState*);
+	int xlen;
+{
+	uchar prk[256], tmp[256], cnt;
+	DigestState *ds;
+
+	assert(xlen <= sizeof(tmp));
+
+	memset(tmp, 0, xlen);
+	if(nsalt == 0){
+		salt = tmp;
+		nsalt = xlen;
+	}
+	/* note that salt and key are swapped in this case */
+	(*x)(key, nkey, salt, nsalt, prk, nil);
+	ds = nil;
+	for(cnt=1;; cnt++) {
+		if(ninfo > 0)
+			ds = (*x)(info, ninfo, prk, xlen, nil, ds);
+		(*x)(&cnt, 1, prk, xlen, tmp, ds);
+		if(dlen <= xlen){
+			memmove(d, tmp, dlen);
+			break;
+		}
+		memmove(d, tmp, xlen);
+		dlen -= xlen;
+		d += xlen;
+		ds = (*x)(tmp, xlen, prk, xlen, nil, nil);
+	}
+}

sys/src/libsec/port/mkfile

@@ -21,9 +21,10 @@ CFILES = des.c desmodes.c desECB.c desCBC.c des3ECB.c des3CBC.c\
 	ecc.c\
 	ripemd.c\
 	dh.c\
-	pbkdf2.c\
 	curve25519.c\
 	curve25519_dh.c\
+	pbkdf2.c\
+	hkdf.c\
 
 ALLOFILES=${CFILES:%.c=%.$O}