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/* Copyright (c) 2015, Google Inc.
*
* Permission to use, copy, modify, and/or distribute this software for any
* purpose with or without fee is hereby granted, provided that the above
* copyright notice and this permission notice appear in all copies.
*
* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
* SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
* OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
* CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
#include <openssl/base.h>
#if defined(OPENSSL_64_BIT) && !defined(OPENSSL_WINDOWS)
#include <openssl/ec.h>
#include "internal.h"
/* This function looks at 5+1 scalar bits (5 current, 1 adjacent less
* significant bit), and recodes them into a signed digit for use in fast point
* multiplication: the use of signed rather than unsigned digits means that
* fewer points need to be precomputed, given that point inversion is easy (a
* precomputed point dP makes -dP available as well).
*
* BACKGROUND:
*
* Signed digits for multiplication were introduced by Booth ("A signed binary
* multiplication technique", Quart. Journ. Mech. and Applied Math., vol. IV,
* pt. 2 (1951), pp. 236-240), in that case for multiplication of integers.
* Booth's original encoding did not generally improve the density of nonzero
* digits over the binary representation, and was merely meant to simplify the
* handling of signed factors given in two's complement; but it has since been
* shown to be the basis of various signed-digit representations that do have
* further advantages, including the wNAF, using the following general
* approach:
*
* (1) Given a binary representation
*
* b_k ... b_2 b_1 b_0,
*
* of a nonnegative integer (b_k in {0, 1}), rewrite it in digits 0, 1, -1
* by using bit-wise subtraction as follows:
*
* b_k b_(k-1) ... b_2 b_1 b_0
* - b_k ... b_3 b_2 b_1 b_0
* -------------------------------------
* s_k b_(k-1) ... s_3 s_2 s_1 s_0
*
* A left-shift followed by subtraction of the original value yields a new
* representation of the same value, using signed bits s_i = b_(i+1) - b_i.
* This representation from Booth's paper has since appeared in the
* literature under a variety of different names including "reversed binary
* form", "alternating greedy expansion", "mutual opposite form", and
* "sign-alternating {+-1}-representation".
*
* An interesting property is that among the nonzero bits, values 1 and -1
* strictly alternate.
*
* (2) Various window schemes can be applied to the Booth representation of
* integers: for example, right-to-left sliding windows yield the wNAF
* (a signed-digit encoding independently discovered by various researchers
* in the 1990s), and left-to-right sliding windows yield a left-to-right
* equivalent of the wNAF (independently discovered by various researchers
* around 2004).
*
* To prevent leaking information through side channels in point multiplication,
* we need to recode the given integer into a regular pattern: sliding windows
* as in wNAFs won't do, we need their fixed-window equivalent -- which is a few
* decades older: we'll be using the so-called "modified Booth encoding" due to
* MacSorley ("High-speed arithmetic in binary computers", Proc. IRE, vol. 49
* (1961), pp. 67-91), in a radix-2^5 setting. That is, we always combine five
* signed bits into a signed digit:
*
* s_(4j + 4) s_(4j + 3) s_(4j + 2) s_(4j + 1) s_(4j)
*
* The sign-alternating property implies that the resulting digit values are
* integers from -16 to 16.
*
* Of course, we don't actually need to compute the signed digits s_i as an
* intermediate step (that's just a nice way to see how this scheme relates
* to the wNAF): a direct computation obtains the recoded digit from the
* six bits b_(4j + 4) ... b_(4j - 1).
*
* This function takes those five bits as an integer (0 .. 63), writing the
* recoded digit to *sign (0 for positive, 1 for negative) and *digit (absolute
* value, in the range 0 .. 8). Note that this integer essentially provides the
* input bits "shifted to the left" by one position: for example, the input to
* compute the least significant recoded digit, given that there's no bit b_-1,
* has to be b_4 b_3 b_2 b_1 b_0 0. */
void ec_GFp_nistp_recode_scalar_bits(uint8_t *sign, uint8_t *digit,
uint8_t in) {
uint8_t s, d;
s = ~((in >> 5) - 1); /* sets all bits to MSB(in), 'in' seen as
* 6-bit value */
d = (1 << 6) - in - 1;
d = (d & s) | (in & ~s);
d = (d >> 1) + (d & 1);
*sign = s & 1;
*digit = d;
}
#endif /* 64_BIT && !WINDOWS */
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