From: Mark Wooding <mdw@distorted.org.uk>
Date: Mon, 2 Oct 2017 12:25:39 +0000 (+0100)
Subject: pub/ed{25519,448}.c: Double by adding self, rather than multiplying by two.
X-Git-Tag: 2.4.2~19
X-Git-Url: http://www.chiark.greenend.org.uk/ucgi/~mdw/git/catacomb/commitdiff_plain/6baf2beaba0a9e7df4a3781be9dcc70671fdc550

pub/ed{25519,448}.c: Double by adding self, rather than multiplying by two.

Saves a reduction pass.
---

diff --git a/pub/ed25519.c b/pub/ed25519.c
index 7fddf987..676fe8ca 100644
--- a/pub/ed25519.c
+++ b/pub/ed25519.c
@@ -232,7 +232,7 @@ static void ptdbl(f25519 *X, f25519 *Y, f25519 *Z,
 					/* (E = a C = -C) */
   f25519_sub(&t0, &t2, &t1);		/* t0 = F = D - C = E + D */
   f25519_sqr(&t1, Z0);			/* t1 = H = Z0^2 */
-  f25519_mulconst(&t1, &t1, 2);		/* t1 = 2 H */
+  f25519_add(&t1, &t1, &t1);		/* t1 = 2 H */
   f25519_sub(&t1, &t0, &t1);		/* t1 = J = F - 2 H */
   f25519_mul(X, X, &t1);		/* X = (B - C - D) J */
   f25519_mul(Y, Y, &t0);		/* Y = -F (E - D) */
diff --git a/pub/ed448.c b/pub/ed448.c
index ae565566..60328021 100644
--- a/pub/ed448.c
+++ b/pub/ed448.c
@@ -237,7 +237,7 @@ static void ptdbl(fgoldi *X, fgoldi *Y, fgoldi *Z,
   fgoldi_sub(&t1, &t1, &t2);		/* t1 = C - D */
   fgoldi_sub(X, &t0, Y);		/* X = c (B - E) */
   fgoldi_sqr(&t0, Z0);			/* t0 = H = (c Z0)^2 */
-  fgoldi_mulconst(&t0, &t0, 2);		/* t0 = 2 H */
+  fgoldi_add(&t0, &t0, &t0);		/* t0 = 2 H */
   fgoldi_sub(&t0, Y, &t0);		/* t0 = J = E - 2 H */
   fgoldi_mul(X, X, &t0);		/* X = c (B - E) J */
   fgoldi_mul(Z, Y, &t0);		/* Z = E J */