1 files changed, 60 insertions, 0 deletions
diff --git a/ext/ed25519-amd64-asm/ge25519_unpackneg.c b/ext/ed25519-amd64-asm/ge25519_unpackneg.c
new file mode 100644
index 00000000..ff16fd20
--- /dev/null
+++ b/ext/ed25519-amd64-asm/ge25519_unpackneg.c
@@ -0,0 +1,60 @@
+#include "fe25519.h"
+#include "ge25519.h"
+
+/* d */
+static const fe25519 ecd = {{0x75EB4DCA135978A3, 0x00700A4D4141D8AB, 0x8CC740797779E898, 0x52036CEE2B6FFE73}};
+/* sqrt(-1) */
+static const fe25519 sqrtm1 = {{0xC4EE1B274A0EA0B0, 0x2F431806AD2FE478, 0x2B4D00993DFBD7A7, 0x2B8324804FC1DF0B}};
+
+/* return 0 on success, -1 otherwise */
+int ge25519_unpackneg_vartime(ge25519_p3 *r, const unsigned char p[32])
+{
+  fe25519 t, chk, num, den, den2, den4, den6;
+  unsigned char par = p[31] >> 7;
+
+  fe25519_setint(&r->z,1);
+  fe25519_unpack(&r->y, p); 
+  fe25519_square(&num, &r->y); /* x = y^2 */
+  fe25519_mul(&den, &num, &ecd); /* den = dy^2 */
+  fe25519_sub(&num, &num, &r->z); /* x = y^2-1 */
+  fe25519_add(&den, &r->z, &den); /* den = dy^2+1 */
+
+  /* Computation of sqrt(num/den)
+     1.: computation of num^((p-5)/8)*den^((7p-35)/8) = (num*den^7)^((p-5)/8)
+  */
+  fe25519_square(&den2, &den);
+  fe25519_square(&den4, &den2);
+  fe25519_mul(&den6, &den4, &den2);
+  fe25519_mul(&t, &den6, &num);
+  fe25519_mul(&t, &t, &den);
+
+  fe25519_pow2523(&t, &t);
+  /* 2. computation of r->x = t * num * den^3
+  */
+  fe25519_mul(&t, &t, &num);
+  fe25519_mul(&t, &t, &den);
+  fe25519_mul(&t, &t, &den);
+  fe25519_mul(&r->x, &t, &den);
+
+  /* 3. Check whether sqrt computation gave correct result, multiply by sqrt(-1) if not:
+  */
+  fe25519_square(&chk, &r->x);
+  fe25519_mul(&chk, &chk, &den);
+  if (!fe25519_iseq_vartime(&chk, &num))
+    fe25519_mul(&r->x, &r->x, &sqrtm1);
+
+  /* 4. Now we have one of the two square roots, except if input was not a square
+  */
+  fe25519_square(&chk, &r->x);
+  fe25519_mul(&chk, &chk, &den);
+  if (!fe25519_iseq_vartime(&chk, &num))
+    return -1;
+
+  /* 5. Choose the desired square root according to parity:
+  */
+  if(fe25519_getparity(&r->x) != (1-par))
+    fe25519_neg(&r->x, &r->x);
+
+  fe25519_mul(&r->t, &r->x, &r->y);
+  return 0;
+}