Commit 9517295b authored by David Benjamin's avatar David Benjamin
Browse files

Fix various mistakes in ec_GFp_nistp_recode_scalar_bits comment. 



Reviewed-by: default avatarNicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: default avatarMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9050)

(cherry picked from commit 8be6a4ed02297f380bbea269f2e1c08a592444bc)
parent 3268087d

crypto/ec/ecp_nistputil.c

+12 −12
Original line number Diff line number Diff line
@@ -160,11 +160,11 @@ void ec_GFp_nistp_points_make_affine_internal(size_t num, void *point_array, 
 *

 *        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

 *       -----------------------------------------

 *        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,