-rw-r--r-- 3189 lib25519-20220726/crypto_sign/ed25519/ref10/fe_tobytes.c raw
#include "fe.h" /* Preconditions: |h| bounded by 1.1*2^26,1.1*2^25,1.1*2^26,1.1*2^25,etc. Write p=2^255-19; q=floor(h/p). Basic claim: q = floor(2^(-255)(h + 19 2^(-25)h9 + 2^(-1))). Proof: Have |h|<=p so |q|<=1 so |19^2 2^(-255) q|<1/4. Also have |h-2^230 h9|<2^231 so |19 2^(-255)(h-2^230 h9)|<1/4. Write y=2^(-1)-19^2 2^(-255)q-19 2^(-255)(h-2^230 h9). Then 0<y<1. Write r=h-pq. Have 0<=r<=p-1=2^255-20. Thus 0<=r+19(2^-255)r<r+19(2^-255)2^255<=2^255-1. Write x=r+19(2^-255)r+y. Then 0<x<2^255 so floor(2^(-255)x) = 0 so floor(q+2^(-255)x) = q. Have q+2^(-255)x = 2^(-255)(h + 19 2^(-25) h9 + 2^(-1)) so floor(2^(-255)(h + 19 2^(-25) h9 + 2^(-1))) = q. */ void fe_tobytes(unsigned char *s,const fe h) { crypto_int32 h0 = h[0]; crypto_int32 h1 = h[1]; crypto_int32 h2 = h[2]; crypto_int32 h3 = h[3]; crypto_int32 h4 = h[4]; crypto_int32 h5 = h[5]; crypto_int32 h6 = h[6]; crypto_int32 h7 = h[7]; crypto_int32 h8 = h[8]; crypto_int32 h9 = h[9]; crypto_int32 q; crypto_int32 carry0; crypto_int32 carry1; crypto_int32 carry2; crypto_int32 carry3; crypto_int32 carry4; crypto_int32 carry5; crypto_int32 carry6; crypto_int32 carry7; crypto_int32 carry8; crypto_int32 carry9; q = (19 * h9 + (((crypto_int32) 1) << 24)) >> 25; q = (h0 + q) >> 26; q = (h1 + q) >> 25; q = (h2 + q) >> 26; q = (h3 + q) >> 25; q = (h4 + q) >> 26; q = (h5 + q) >> 25; q = (h6 + q) >> 26; q = (h7 + q) >> 25; q = (h8 + q) >> 26; q = (h9 + q) >> 25; /* Goal: Output h-(2^255-19)q, which is between 0 and 2^255-20. */ h0 += 19 * q; /* Goal: Output h-2^255 q, which is between 0 and 2^255-20. */ carry0 = h0 >> 26; h1 += carry0; h0 -= carry0 << 26; carry1 = h1 >> 25; h2 += carry1; h1 -= carry1 << 25; carry2 = h2 >> 26; h3 += carry2; h2 -= carry2 << 26; carry3 = h3 >> 25; h4 += carry3; h3 -= carry3 << 25; carry4 = h4 >> 26; h5 += carry4; h4 -= carry4 << 26; carry5 = h5 >> 25; h6 += carry5; h5 -= carry5 << 25; carry6 = h6 >> 26; h7 += carry6; h6 -= carry6 << 26; carry7 = h7 >> 25; h8 += carry7; h7 -= carry7 << 25; carry8 = h8 >> 26; h9 += carry8; h8 -= carry8 << 26; carry9 = h9 >> 25; h9 -= carry9 << 25; /* h10 = carry9 */ /* Goal: Output h0+...+2^255 h10-2^255 q, which is between 0 and 2^255-20. Have h0+...+2^230 h9 between 0 and 2^255-1; evidently 2^255 h10-2^255 q = 0. Goal: Output h0+...+2^230 h9. */ s[0] = h0 >> 0; s[1] = h0 >> 8; s[2] = h0 >> 16; s[3] = (h0 >> 24) | (h1 << 2); s[4] = h1 >> 6; s[5] = h1 >> 14; s[6] = (h1 >> 22) | (h2 << 3); s[7] = h2 >> 5; s[8] = h2 >> 13; s[9] = (h2 >> 21) | (h3 << 5); s[10] = h3 >> 3; s[11] = h3 >> 11; s[12] = (h3 >> 19) | (h4 << 6); s[13] = h4 >> 2; s[14] = h4 >> 10; s[15] = h4 >> 18; s[16] = h5 >> 0; s[17] = h5 >> 8; s[18] = h5 >> 16; s[19] = (h5 >> 24) | (h6 << 1); s[20] = h6 >> 7; s[21] = h6 >> 15; s[22] = (h6 >> 23) | (h7 << 3); s[23] = h7 >> 5; s[24] = h7 >> 13; s[25] = (h7 >> 21) | (h8 << 4); s[26] = h8 >> 4; s[27] = h8 >> 12; s[28] = (h8 >> 20) | (h9 << 6); s[29] = h9 >> 2; s[30] = h9 >> 10; s[31] = h9 >> 18; }