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.

1 files changed, 50 insertions, 2 deletions

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"