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authorMoxie Marlinspike <moxie@thoughtcrime.org>2014-11-24 12:54:30 -0800
committerMoxie Marlinspike <moxie@thoughtcrime.org>2014-11-24 12:54:30 -0800
commit60800e155612bea797eed93c67046a23d26054cc (patch)
treed88368c1c26162e27e790195133ca2b526597afe /jni/ed25519/fe_tobytes.c
Break out into separate repo.
Diffstat (limited to 'jni/ed25519/fe_tobytes.c')
-rw-r--r--jni/ed25519/fe_tobytes.c119
1 files changed, 119 insertions, 0 deletions
diff --git a/jni/ed25519/fe_tobytes.c b/jni/ed25519/fe_tobytes.c
new file mode 100644
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+++ b/jni/ed25519/fe_tobytes.c
@@ -0,0 +1,119 @@
+#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;
+}