From e179e651cbb20031905e01f37596e20ec2cb788a Mon Sep 17 00:00:00 2001
From: Pieter Wuille <pieter@wuille.net>
Date: Fri, 28 Jan 2022 18:36:13 -0500
Subject: [PATCH] Abstract out verify logic for fe_add

---
 src/field.h            |  9 ++++++++-
 src/field_10x26_impl.h |  8 +-------
 src/field_5x52_impl.h  |  8 +-------
 src/field_impl.h       | 11 +++++++++++
 4 files changed, 21 insertions(+), 15 deletions(-)

diff --git a/src/field.h b/src/field.h
index 2c1e9d2e1addc..988bc6f39006f 100644
--- a/src/field.h
+++ b/src/field.h
@@ -89,6 +89,7 @@ static const secp256k1_fe secp256k1_const_beta = SECP256K1_FE_CONST(
 #  define secp256k1_fe_get_b32 secp256k1_fe_impl_get_b32
 #  define secp256k1_fe_negate secp256k1_fe_impl_negate
 #  define secp256k1_fe_mul_int secp256k1_fe_impl_mul_int
+#  define secp256k1_fe_add secp256k1_fe_impl_add
 #endif /* !defined(VERIFY) */
 
 /** Normalize a field element.
@@ -215,7 +216,13 @@ static void secp256k1_fe_add_int(secp256k1_fe *r, int a);
  */
 static void secp256k1_fe_mul_int(secp256k1_fe *r, int a);
 
-/** Adds a field element to another. The result has the sum of the inputs' magnitudes as magnitude. */
+/** Increment a field element by another.
+ *
+ * On input, r and a must be valid field elements, not necessarily normalized.
+ * The sum of their magnitudes must not exceed 32.
+ * Performs {r += a}.
+ * On output, r will not be normalized, and will have magnitude incremented by a's.
+ */
 static void secp256k1_fe_add(secp256k1_fe *r, const secp256k1_fe *a);
 
 /** Sets a field element to be the product of two others. Requires the inputs' magnitudes to be at most 8.
diff --git a/src/field_10x26_impl.h b/src/field_10x26_impl.h
index f8c8ddb5b4da0..eefd4da697919 100644
--- a/src/field_10x26_impl.h
+++ b/src/field_10x26_impl.h
@@ -383,8 +383,7 @@ SECP256K1_INLINE static void secp256k1_fe_impl_mul_int(secp256k1_fe *r, int a) {
     r->n[9] *= a;
 }
 
-SECP256K1_INLINE static void secp256k1_fe_add(secp256k1_fe *r, const secp256k1_fe *a) {
-    secp256k1_fe_verify(a);
+SECP256K1_INLINE static void secp256k1_fe_impl_add(secp256k1_fe *r, const secp256k1_fe *a) {
     r->n[0] += a->n[0];
     r->n[1] += a->n[1];
     r->n[2] += a->n[2];
@@ -395,11 +394,6 @@ SECP256K1_INLINE static void secp256k1_fe_add(secp256k1_fe *r, const secp256k1_f
     r->n[7] += a->n[7];
     r->n[8] += a->n[8];
     r->n[9] += a->n[9];
-#ifdef VERIFY
-    r->magnitude += a->magnitude;
-    r->normalized = 0;
-    secp256k1_fe_verify(r);
-#endif
 }
 
 SECP256K1_INLINE static void secp256k1_fe_add_int(secp256k1_fe *r, int a) {
diff --git a/src/field_5x52_impl.h b/src/field_5x52_impl.h
index f5091373405bf..8bd084ff27985 100644
--- a/src/field_5x52_impl.h
+++ b/src/field_5x52_impl.h
@@ -353,18 +353,12 @@ SECP256K1_INLINE static void secp256k1_fe_add_int(secp256k1_fe *r, int a) {
 #endif
 }
 
-SECP256K1_INLINE static void secp256k1_fe_add(secp256k1_fe *r, const secp256k1_fe *a) {
-    secp256k1_fe_verify(a);
+SECP256K1_INLINE static void secp256k1_fe_impl_add(secp256k1_fe *r, const secp256k1_fe *a) {
     r->n[0] += a->n[0];
     r->n[1] += a->n[1];
     r->n[2] += a->n[2];
     r->n[3] += a->n[3];
     r->n[4] += a->n[4];
-#ifdef VERIFY
-    r->magnitude += a->magnitude;
-    r->normalized = 0;
-    secp256k1_fe_verify(r);
-#endif
 }
 
 static void secp256k1_fe_mul(secp256k1_fe *r, const secp256k1_fe *a, const secp256k1_fe * SECP256K1_RESTRICT b) {
diff --git a/src/field_impl.h b/src/field_impl.h
index 8bd18c61ab09a..172b846d89e45 100644
--- a/src/field_impl.h
+++ b/src/field_impl.h
@@ -275,6 +275,17 @@ SECP256K1_INLINE static void secp256k1_fe_mul_int(secp256k1_fe *r, int a) {
     r->normalized = 0;
     secp256k1_fe_verify(r);
 }
+
+static void secp256k1_fe_impl_add(secp256k1_fe *r, const secp256k1_fe *a);
+SECP256K1_INLINE static void secp256k1_fe_add(secp256k1_fe *r, const secp256k1_fe *a) {
+    secp256k1_fe_verify(r);
+    secp256k1_fe_verify(a);
+    VERIFY_CHECK(r->magnitude + a->magnitude <= 32);
+    secp256k1_fe_impl_add(r, a);
+    r->magnitude += a->magnitude;
+    r->normalized = 0;
+    secp256k1_fe_verify(r);
+}
 #endif /* defined(VERIFY) */
 
 #endif /* SECP256K1_FIELD_IMPL_H */