From 79b0964612777701bb15952a1ff60db8630c74d7 Mon Sep 17 00:00:00 2001
From: Fabian Albert <fabian.albert@rohde-schwarz.com>
Date: Fri, 8 Mar 2024 11:29:20 +0100
Subject: [PATCH] 32bit fix attempt

---
 .../curve448_utils/curve448_scalar.cpp        | 33 ++++++++++---------
 1 file changed, 18 insertions(+), 15 deletions(-)

diff --git a/src/lib/pubkey/curve448/curve448_utils/curve448_scalar.cpp b/src/lib/pubkey/curve448/curve448_utils/curve448_scalar.cpp
index 49304952320..719b642741d 100644
--- a/src/lib/pubkey/curve448/curve448_utils/curve448_scalar.cpp
+++ b/src/lib/pubkey/curve448/curve448_utils/curve448_scalar.cpp
@@ -17,22 +17,25 @@ constexpr size_t WORDS_C = words_for_bits(28 * 8);
 
 /// @return (q,r) so that x = q*2^446 + r, r < L
 template <size_t S>
-std::pair<std::array<word, S - WORDS_446 + 1>, std::array<word, WORDS_446>> div_mod_2_446(std::span<const word, S> x) {
-   static_assert(S >= WORDS_446, "Input too small");
-   std::array<word, WORDS_446> r;
-   for(size_t i = 0; i < WORDS_446; ++i) {
-      r[i] = x[i];
+auto div_mod_2_446(std::span<const word, S> x) {
+   if constexpr(S < WORDS_446) {
+      std::array<word, WORDS_446> r = {0};
+      copy_mem(std::span(r).template first<S>(), x);
+      return std::make_pair(std::array<word, 1>({0}), r);
+   } else {
+      std::array<word, WORDS_446> r;
+      copy_mem(r, std::span(x).template first<WORDS_446>());
+      // Clear the two most significant bits
+      r[WORDS_446 - 1] &= ~(word(0b11) << (sizeof(word) * 8 - 2));
+
+      constexpr size_t word_shift = 446 / (sizeof(word) * 8);
+      constexpr size_t bit_shift = 446 % (sizeof(word) * 8);
+
+      std::array<word, S - WORDS_446 + 1> q;
+      bigint_shr2(q.data(), x.data(), x.size(), word_shift, bit_shift);
+
+      return std::make_pair(q, r);
    }
-   // Clear the two most significant bits
-   r[WORDS_446 - 1] &= ~(word(0b11) << (sizeof(word) * 8 - 2));
-
-   constexpr size_t word_shift = 446 / (sizeof(word) * 8);
-   constexpr size_t bit_shift = 446 % (sizeof(word) * 8);
-
-   std::array<word, S - WORDS_446 + 1> q;
-   bigint_shr2(q.data(), x.data(), x.size(), word_shift, bit_shift);
-
-   return {q, r};
 }
 
 template <size_t S>