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cpu: replace custom hex parsing code with dec16() avoding timing side channels

--- a/cpu.c

+++ b/cpu.c

@@ -789,33 +789,6 @@

 }

 static int

-unhex(char c)

-{

-	if('0' <= c && c <= '9')

-		return c-'0';

-	if('a' <= c && c <= 'f')

-		return c-'a'+10;

-	if('A' <= c && c <= 'F')

-		return c-'A'+10;

-	abort();

-	return -1;

-}

-

-static int

-hexparse(char *hex, uchar *dat, int ndat)

-{

-	int i;

-

-	if(strlen(hex) != 2*ndat)

-		return -1;

-	if(hex[strspn(hex, "0123456789abcdefABCDEF")] != '\0')

-		return -1;

-	for(i=0; i<ndat; i++)

-		dat[i] = (unhex(hex[2*i])<<4)|unhex(hex[2*i+1]);

-	return 0;

-}

-

-static int

 findkey(Authkey *key, char *user, char *dom, char *proto)

 {

 	char buf[1024], *f[50], *p, *ep, *nextp, *hex, *pass, *id, *role;

@@ -865,10 +838,10 @@

 		if(hex != nil){

 			memset(key, 0, sizeof(*key));

 			if(strcmp(proto, "dp9ik") == 0) {

-				if(hexparse(hex, key->aes, AESKEYLEN) != 0)

+				if(dec16(key->aes, AESKEYLEN, hex, strlen(hex)) != AESKEYLEN)

 					continue;

 			} else {

-				if(hexparse(hex, (uchar*)key->des, DESKEYLEN) != 0)

+				if(dec16((uchar*)key->des, DESKEYLEN, hex, strlen(hex)) != DESKEYLEN)

 					continue;

 			}

 		} else {