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Fix various mistakes in ec_GFp_nistp_recode_scalar_bits comment.

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Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9050)
This commit is contained in:
David Benjamin 2019-05-30 17:01:10 -04:00
parent fdbb3a8655
commit 8be6a4ed02
1 changed files with 12 additions and 12 deletions
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24
crypto/ec/ecp_nistputil.c
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@ -158,13 +158,13 @@ void ec_GFp_nistp_points_make_affine_internal(size_t num, void *point_array,
* 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
* b_k b_(k-1) ... b_2 b_1 b_0
* - b_k ... b_3 b_2 b_1 b_0
* -----------------------------------------
* s_(k+1) s_k ... 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.
* 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
@ -188,7 +188,7 @@ void ec_GFp_nistp_points_make_affine_internal(size_t num, void *point_array,
* (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)
* s_(5j + 4) s_(5j + 3) s_(5j + 2) s_(5j + 1) s_(5j)
*
* The sign-alternating property implies that the resulting digit values are
* integers from -16 to 16.
@ -196,14 +196,14 @@ void ec_GFp_nistp_points_make_affine_internal(size_t num, void *point_array,
* 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).
* six bits b_(5j + 4) ... b_(5j - 1).
*
* This function takes those five bits as an integer (0 .. 63), writing the
* This function takes those six 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.
* value, in the range 0 .. 16). 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(unsigned char *sign,