libc: constant time implementation for encode(2) routines, fix base32

the string encoding functions touch secret key material
in a bunch of places (devtls, devcap), so make sure we do
not leak information by cache timing side channels, making
the encoding and decoding routines constant time.

we also expose the alphabets through encXchr()/decXchr()
functions so caller can find the end of a encoded string
before calling decode function (for libmp).

the base32 encoding was broken in several ways. inputs
lengths of len%5 == [2,3,4] had output truncated and
it was using non-standard alphabet. documenting the alphabet
change in the manpage.

6 files changed, 185 insertions, 89 deletions

diff --git a/sys/include/libc.h b/sys/include/libc.h
index 33d0dbaef..98226dc05 100644
--- a/sys/include/libc.h
+++ b/sys/include/libc.h
@@ -369,6 +369,12 @@ extern	int	dec32(uchar*, int, char*, int);
 extern	int	enc32(char*, int, uchar*, int);
 extern	int	dec16(uchar*, int, char*, int);
 extern	int	enc16(char*, int, uchar*, int);
+extern	int	dec64chr(int);
+extern	int	enc64chr(int);
+extern	int	dec32chr(int);
+extern	int	enc32chr(int);
+extern	int	dec16chr(int);
+extern	int	enc16chr(int);
 extern	int	encodefmt(Fmt*);
 extern	void	exits(char*);
 extern	double	frexp(double, int*);
diff --git a/sys/man/2/encode b/sys/man/2/encode
index 3a8ca61e4..db7944945 100644
--- a/sys/man/2/encode
+++ b/sys/man/2/encode
@@ -1,6 +1,8 @@
 .TH ENCODE 2
 .SH NAME
-dec64, enc64, dec32, enc32, dec16, enc16, encodefmt \- encoding byte arrays as strings
+dec64, enc64, dec32, enc32, dec16, enc16, \
+dec64chr, enc64chr, dec32chr, enc32chr, dec16chr, enc16chr, \
+encodefmt \- encoding byte arrays as strings
 .SH SYNOPSIS
 .B #include <u.h>
 .br
@@ -25,8 +27,28 @@ int	dec16(uchar *out, int lim, char *in, int n)
 int	enc16(char *out, int lim, uchar *in, int n)
 .PP
 .B
+int	dec64chr(int c)
+.PP
+.B
+int	enc64chr(int o)
+.PP
+.B
+int	dec32chr(int c)
+.PP
+.B
+int	enc32chr(int o)
+.PP
+.B
+int	dec16chr(int c)
+.PP
+.B
+int	enc16chr(int o)
+.PP
+.B
 int	encodefmt(Fmt*)
 .SH DESCRIPTION
+The functions described here handle encoding and decoding of
+bytes to printable ASCII strings as specified by RFC4648. 
 .PP
 .IR Enc16 ,
 .I enc32
@@ -47,6 +69,20 @@ The decoding fails if the output buffer is not large enough or,
 for base 32, if the input buffer length is not a multiple
 of 8.
 .PP
+.IR Dec16chr ,
+.I dec32chr
+and
+.I dec64chr
+return the value for a symbol of the alphabet or -1 when the
+symbol is not in the alphabet.
+.PP
+.IR Enc16chr ,
+.I enc32chr
+and
+.I enc64chr
+encode a symbol of the alphabet given a value.
+if the value is out of range then zero is returned.
+.PP
 .I Encodefmt
 can be used with
 .IR fmtinstall (2)
@@ -62,7 +98,8 @@ in upper case.  The
 flag forces lower case.
 .TP
 .B <
-base 32
+base 32. The default is upper case, same as
+.BR H .
 .TP
 .B [
 base 64 (same as MIME)
@@ -80,8 +117,19 @@ For example, to display a 15 byte array as hex:
 
 .EE
 .SH SOURCE
+.B /sys/src/libc/port/u16.c
+.br
 .B /sys/src/libc/port/u32.c
 .br
 .B /sys/src/libc/port/u64.c
 .br
 .B /sys/src/libc/port/encodefmt.c
+.SH HISTORY
+In Jan 2018, base 32 encoding was changed from non-standard
+to standard RFC4648 alphabet.
+.TP
+old:
+.B "23456789abcdefghijkmnpqrstuvwxyz"
+.TP
+new:
+.B "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"
diff --git a/sys/src/libc/port/encodefmt.c b/sys/src/libc/port/encodefmt.c
index 721a58c9f..92cc4154a 100644
--- a/sys/src/libc/port/encodefmt.c
+++ b/sys/src/libc/port/encodefmt.c
@@ -48,17 +48,14 @@ encodefmt(Fmt *f)
 	// convert
 	out = buf;
 	switch(f->r){
-	case '<':
-		rv = enc32(out, len, b, ilen);
-		break;
 	case '[':
 		rv = enc64(out, len, b, ilen);
 		break;
+	case '<':
+		rv = enc32(out, len, b, ilen);
+		break;
 	case 'H':
 		rv = enc16(out, len, b, ilen);
-		if(rv >= 0 && (f->flags & FmtLong))
-			for(p = buf; *p; p++)
-				*p = tolower(*p);
 		break;
 	default:
 		rv = -1;
@@ -67,6 +64,10 @@ encodefmt(Fmt *f)
 	if(rv < 0)
 		goto error;
 
+	if((f->flags & FmtLong) != 0 && f->r != '[')
+		for(p = buf; *p; p++)
+			*p = tolower(*p);
+
 	fmtstrcpy(f, buf);
 	if(buf != obuf)
 		free(buf);
diff --git a/sys/src/libc/port/u16.c b/sys/src/libc/port/u16.c
index 4e1637120..60ffa72c8 100644
--- a/sys/src/libc/port/u16.c
+++ b/sys/src/libc/port/u16.c
@@ -1,6 +1,28 @@
 #include <u.h>
 #include <libc.h>
-static char t16e[] = "0123456789ABCDEF";
+
+#define between(x,min,max)	(((min-1-x) & (x-max-1))>>8)
+
+int
+enc16chr(int o)
+{
+	int c;
+
+	c  = between(o,  0,  9) & ('0'+o);
+	c |= between(o, 10, 15) & ('A'+(o-10));
+	return c;
+}
+
+int
+dec16chr(int c)
+{
+	int o;
+
+	o  = between(c, '0', '9') & (1+(c-'0'));
+	o |= between(c, 'A', 'F') & (1+10+(c-'A'));
+	o |= between(c, 'a', 'f') & (1+10+(c-'a'));
+	return o-1;
+}
 
 int
 dec16(uchar *out, int lim, char *in, int n)
@@ -10,14 +32,8 @@ dec16(uchar *out, int lim, char *in, int n)
 	uchar *eout = out + lim;
 
 	while(n-- > 0){
-		c = *in++;
-		if('0' <= c && c <= '9')
-			c = c - '0';
-		else if('a' <= c && c <= 'z')
-			c = c - 'a' + 10;
-		else if('A' <= c && c <= 'Z')
-			c = c - 'A' + 10;
-		else
+		c = dec16chr(*in++);
+		if(c < 0)
 			continue;
 		w = (w<<4) + c;
 		i++;
@@ -44,8 +60,8 @@ enc16(char *out, int lim, uchar *in, int n)
 		c = *in++;
 		if(out + 2 >= eout)
 			goto exhausted;
-		*out++ = t16e[c>>4];
-		*out++ = t16e[c&0xf];
+		*out++ = enc16chr(c>>4);
+		*out++ = enc16chr(c&15);
 	}
 exhausted:
 	*out = 0;
diff --git a/sys/src/libc/port/u32.c b/sys/src/libc/port/u32.c
index 7423984a2..9ad9eb781 100644
--- a/sys/src/libc/port/u32.c
+++ b/sys/src/libc/port/u32.c
@@ -1,21 +1,44 @@
 #include <u.h>
 #include <libc.h>
 
+#define between(x,min,max)	(((min-1-x) & (x-max-1))>>8)
+
+int
+enc32chr(int o)
+{
+	int c;
+
+	c  = between(o,  0, 25) & ('A'+o);
+	c |= between(o, 26, 31) & ('2'+(o-26));
+	return c;
+}
+
+int
+dec32chr(int c)
+{
+	int o;
+
+	o  = between(c, 'A', 'Z') & (1+(c-'A'));
+	o |= between(c, 'a', 'z') & (1+(c-'a'));
+	o |= between(c, '2', '7') & (1+26+(c-'2'));
+	return o-1;
+}
+
 int
 dec32(uchar *dest, int ndest, char *src, int nsrc)
 {
-	char *s, *tab;
 	uchar *start;
-	int i, u[8];
+	int i, j, u[8];
 
 	if(ndest+1 < (5*nsrc+7)/8)
 		return -1;
 	start = dest;
-	tab = "23456789abcdefghijkmnpqrstuvwxyz";
 	while(nsrc>=8){
 		for(i=0; i<8; i++){
-			s = strchr(tab,(int)src[i]);
-			u[i] = s ? s-tab : 0;
+			j = dec32chr(src[i]);
+			if(j < 0)
+				j = 0;
+			u[i] = j;
 		}
 		*dest++ = (u[0]<<3) | (0x7 & (u[1]>>2));
 		*dest++ = ((0x3 & u[1])<<6) | (u[2]<<1) | (0x1 & (u[3]>>4));
@@ -29,8 +52,10 @@ dec32(uchar *dest, int ndest, char *src, int nsrc)
 		if(nsrc == 1 || nsrc == 3 || nsrc == 6)
 			return -1;
 		for(i=0; i<nsrc; i++){
-			s = strchr(tab,(int)src[i]);
-			u[i] = s ? s-tab : 0;
+			j = dec32chr(src[i]);
+			if(j < 0)
+				j = 0;
+			u[i] = j;
 		}
 		*dest++ = (u[0]<<3) | (0x7 & (u[1]>>2));
 		if(nsrc == 2)
@@ -50,60 +75,57 @@ out:
 int
 enc32(char *dest, int ndest, uchar *src, int nsrc)
 {
-	char *tab, *start;
+	char *start;
 	int j;
 
-	if(ndest <= (8*nsrc+4)/5 )
+	if(ndest <= (8*nsrc+4)/5)
 		return -1;
 	start = dest;
-	tab = "23456789abcdefghijkmnpqrstuvwxyz";
 	while(nsrc>=5){
 		j = (0x1f & (src[0]>>3));
-		*dest++ = tab[j];
+		*dest++ = enc32chr(j);
 		j = (0x1c & (src[0]<<2)) | (0x03 & (src[1]>>6));
-		*dest++ = tab[j];
+		*dest++ = enc32chr(j);
 		j = (0x1f & (src[1]>>1));
-		*dest++ = tab[j];
+		*dest++ = enc32chr(j);
 		j = (0x10 & (src[1]<<4)) | (0x0f & (src[2]>>4));
-		*dest++ = tab[j];
+		*dest++ = enc32chr(j);
 		j = (0x1e & (src[2]<<1)) | (0x01 & (src[3]>>7));
-		*dest++ = tab[j];
+		*dest++ = enc32chr(j);
 		j = (0x1f & (src[3]>>2));
-		*dest++ = tab[j];
+		*dest++ = enc32chr(j);
 		j = (0x18 & (src[3]<<3)) | (0x07 & (src[4]>>5));
-		*dest++ = tab[j];
+		*dest++ = enc32chr(j);
 		j = (0x1f & (src[4]));
-		*dest++ = tab[j];
+		*dest++ = enc32chr(j);
 		src  += 5;
 		nsrc -= 5;
 	}
 	if(nsrc){
 		j = (0x1f & (src[0]>>3));
-		*dest++ = tab[j];
+		*dest++ = enc32chr(j);
 		j = (0x1c & (src[0]<<2));
 		if(nsrc == 1)
 			goto out;
 		j |= (0x03 & (src[1]>>6));
-		*dest++ = tab[j];
+		*dest++ = enc32chr(j);
 		j = (0x1f & (src[1]>>1));
-		if(nsrc == 2)
-			goto out;
-		*dest++ = tab[j];
+		*dest++ = enc32chr(j);
 		j = (0x10 & (src[1]<<4));
-		if(nsrc == 3)
+		if(nsrc == 2)
 			goto out;
 		j |= (0x0f & (src[2]>>4));
-		*dest++ = tab[j];
+		*dest++ = enc32chr(j);
 		j = (0x1e & (src[2]<<1));
-		if(nsrc == 4)
+		if(nsrc == 3)
 			goto out;
 		j |= (0x01 & (src[3]>>7));
-		*dest++ = tab[j];
+		*dest++ = enc32chr(j);
 		j = (0x1f & (src[3]>>2));
-		*dest++ = tab[j];
+		*dest++ = enc32chr(j);
 		j = (0x18 & (src[3]<<3));
 out:
-		*dest++ = tab[j];
+		*dest++ = enc32chr(j);
 	}
 	*dest = 0;
 	return dest-start;
diff --git a/sys/src/libc/port/u64.c b/sys/src/libc/port/u64.c
index 432cec899..44f716fbe 100644
--- a/sys/src/libc/port/u64.c
+++ b/sys/src/libc/port/u64.c
@@ -1,29 +1,33 @@
 #include <u.h>
 #include <libc.h>
 
-enum {
-	INVAL=	255
-};
+#define between(x,min,max)	(((min-1-x) & (x-max-1))>>8)
 
-static uchar t64d[256] = {
-   INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,
-   INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,
-   INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,   62,INVAL,INVAL,INVAL,   63,
-      52,   53,   54,   55,   56,   57,   58,   59,   60,   61,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,
-   INVAL,    0,    1,    2,    3,    4,    5,    6,    7,    8,    9,   10,   11,   12,   13,   14,
-      15,   16,   17,   18,   19,   20,   21,   22,   23,   24,   25,INVAL,INVAL,INVAL,INVAL,INVAL,
-   INVAL,   26,   27,   28,   29,   30,   31,   32,   33,   34,   35,   36,   37,   38,   39,   40,
-      41,   42,   43,   44,   45,   46,   47,   48,   49,   50,   51,INVAL,INVAL,INVAL,INVAL,INVAL,
-   INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,
-   INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,
-   INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,
-   INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,
-   INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,
-   INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,
-   INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,
-   INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL,INVAL
-};
-static char t64e[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+int
+enc64chr(int o)
+{
+	int c;
+
+	c  = between(o,  0, 25) & ('A'+o);
+	c |= between(o, 26, 51) & ('a'+(o-26));
+	c |= between(o, 52, 61) & ('0'+(o-52));
+	c |= between(o, 62, 62) & ('+');
+	c |= between(o, 63, 63) & ('/');
+	return c;
+}
+
+int
+dec64chr(int c)
+{
+	int o;
+
+	o  = between(c, 'A', 'Z') & (1+(c-'A'));
+	o |= between(c, 'a', 'z') & (1+26+(c-'a'));
+	o |= between(c, '0', '9') & (1+52+(c-'0'));
+	o |= between(c, '+', '+') & (1+62);
+	o |= between(c, '/', '/') & (1+63);
+	return o-1;
+}
 
 int
 dec64(uchar *out, int lim, char *in, int n)
@@ -36,9 +40,8 @@ dec64(uchar *out, int lim, char *in, int n)
 	b24 = 0;
 	i = 0;
 	while(n-- > 0){
-
-		c = t64d[*(uchar*)in++];
-		if(c == INVAL)
+		c = dec64chr(*in++);
+		if(c < 0)
 			continue;
 		switch(i){
 		case 0:
@@ -58,8 +61,8 @@ dec64(uchar *out, int lim, char *in, int n)
 			*out++ = b24>>16;
 			*out++ = b24>>8;
 			*out++ = b24;
-			i = -1;
-			break;
+			i = 0;
+			continue;
 		}
 		i++;
 	}
@@ -89,34 +92,34 @@ enc64(char *out, int lim, uchar *in, int n)
 	char *e = out + lim;
 
 	for(i = n/3; i > 0; i--){
-		b24 = (*in++)<<16;
-		b24 |= (*in++)<<8;
+		b24 = *in++<<16;
+		b24 |= *in++<<8;
 		b24 |= *in++;
 		if(out + 4 >= e)
 			goto exhausted;
-		*out++ = t64e[(b24>>18)];
-		*out++ = t64e[(b24>>12)&0x3f];
-		*out++ = t64e[(b24>>6)&0x3f];
-		*out++ = t64e[(b24)&0x3f];
+		*out++ = enc64chr(b24>>18);
+		*out++ = enc64chr((b24>>12)&0x3f);
+		*out++ = enc64chr((b24>>6)&0x3f);
+		*out++ = enc64chr(b24&0x3f);
 	}
 
 	switch(n%3){
 	case 2:
-		b24 = (*in++)<<16;
-		b24 |= (*in)<<8;
+		b24 = *in++<<16;
+		b24 |= *in<<8;
 		if(out + 4 >= e)
 			goto exhausted;
-		*out++ = t64e[(b24>>18)];
-		*out++ = t64e[(b24>>12)&0x3f];
-		*out++ = t64e[(b24>>6)&0x3f];
+		*out++ = enc64chr(b24>>18);
+		*out++ = enc64chr((b24>>12)&0x3f);
+		*out++ = enc64chr((b24>>6)&0x3f);
 		*out++ = '=';
 		break;
 	case 1:
-		b24 = (*in)<<16;
+		b24 = *in<<16;
 		if(out + 4 >= e)
 			goto exhausted;
-		*out++ = t64e[(b24>>18)];
-		*out++ = t64e[(b24>>12)&0x3f];
+		*out++ = enc64chr(b24>>18);
+		*out++ = enc64chr((b24>>12)&0x3f);
 		*out++ = '=';
 		*out++ = '=';
 		break;