X-Git-Url: https://git.distorted.org.uk/~mdw/catacomb/blobdiff_plain/0f00dc4c8eb47e67bc0f148c2dd109f73a451e0a..318c3c31be343fdba362cb60f33aab3e88798d8a:/math/ec-prime.c

diff --git a/math/ec-prime.c b/math/ec-prime.c
index f6a7f960..46b98856 100644
--- a/math/ec-prime.c
+++ b/math/ec-prime.c
@@ -267,7 +267,7 @@ static ec *ecprojadd(ec_curve *c, ec *d, const ec *a, const ec *b)
       }
     }
     u = F_ADD(f, u, u, uu);		/* %$t = uu + u$% */
-    s = F_ADD(f, s, s, ss);		/* %$m = ss + r$% */
+    s = F_ADD(f, s, s, ss);		/* %$m = ss + s$% */
 
     uu = F_MUL(f, uu, a->z, w);		/* %$z_0 w$% */
     dz = F_MUL(f, ss, uu, b->z);	/* %$z' = z_0 z_1 w$% */
@@ -328,6 +328,17 @@ static int ecprojcheck(ec_curve *c, const ec *p)
   return (rc);
 }
 
+static int eccompr(ec_curve *c, const ec *p)
+{
+  /* --- Just take the LSB of %$y$% ---
+   *
+   * Since @p@ is odd, either %$y$% or %$-y = p - y$% must be odd, so this
+   * disambiguates.
+   */
+
+  return (MP_ODDP(p->y));
+}
+
 static void ecdestroy(ec_curve *c)
 {
   MP_DROP(c->a);
@@ -378,19 +389,19 @@ extern ec_curve *ec_primeproj(field *f, mp *a, mp *b)
 static const ec_ops ec_primeops = {
   "prime",
   ecdestroy, ec_stdsamep, ec_idin, ec_idout, ec_idfix,
-  ecfind, ecneg, ecadd, ec_stdsub, ecdbl, eccheck
+  ecfind, ecneg, ecadd, ec_stdsub, ecdbl, eccheck, eccompr
 };
 
 static const ec_ops ec_primeprojops = {
   "primeproj",
   ecdestroy, ec_stdsamep, ec_projin, ec_projout, ec_projfix,
-  ecfind, ecneg, ecprojadd, ec_stdsub, ecprojdbl, ecprojcheck
+  ecfind, ecneg, ecprojadd, ec_stdsub, ecprojdbl, ecprojcheck, eccompr
 };
 
 static const ec_ops ec_primeprojxops = {
   "primeproj",
   ecdestroy, ec_stdsamep, ec_projin, ec_projout, ec_projfix,
-  ecfind, ecneg, ecprojadd, ec_stdsub, ecprojxdbl, ecprojcheck
+  ecfind, ecneg, ecprojadd, ec_stdsub, ecprojxdbl, ecprojcheck, eccompr
 };
 
 /*----- Test rig ----------------------------------------------------------*/