Merge bitcoin/bitcoin#32436: test: refactor: negate signature-s using libsecp256k1

1ee698f test: refactor: negate signature-s using libsecp256k1 (Sebastian Falbesoner)

Pull request description:

  This small PR gets rid of manual mod-n inversion of the ECDSA signature-s part in unit tests (introduced a long time ago in #5256, triggered by bitcoin-core/secp256k1#69) by using secp256k1 instead. The function wasn't available at that time, but was introduced about three years later, see bitcoin-core/secp256k1#408. Note that as the name suggests, `secp256k1_ec_seckey_negate` is meant to be used for secret keys, but it obviously works in general for scalars modulo the group order.

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Tree-SHA512: dc36ea1572b538d11ae34e1871f310a1cda8083ffb753e93e7ee9d56e91ebd8ec78d35758dfb700254720914b734ef7a071eeef71b6239f19e1e2fb289fb5435

Author: achow101

src/test/script_tests.cpp

@@ -30,6 +30,7 @@
 
 #include <boost/test/unit_test.hpp>
 
+#include <secp256k1.h>
 #include <univalue.h>
 
 // Uncomment if you want to output updated JSON tests.
@@ -144,25 +145,17 @@ void static NegateSignatureS(std::vector<unsigned char>& vchSig) {
     r = std::vector<unsigned char>(vchSig.begin() + 4, vchSig.begin() + 4 + vchSig[3]);
     s = std::vector<unsigned char>(vchSig.begin() + 6 + vchSig[3], vchSig.begin() + 6 + vchSig[3] + vchSig[5 + vchSig[3]]);
 
-    // Really ugly to implement mod-n negation here, but it would be feature creep to expose such functionality from libsecp256k1.
-    static const unsigned char order[33] = {
-        0x00,
-        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
-        0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFE,
-        0xBA, 0xAE, 0xDC, 0xE6, 0xAF, 0x48, 0xA0, 0x3B,
-        0xBF, 0xD2, 0x5E, 0x8C, 0xD0, 0x36, 0x41, 0x41
-    };
     while (s.size() < 33) {
         s.insert(s.begin(), 0x00);
     }
-    int carry = 0;
-    for (int p = 32; p >= 1; p--) {
-        int n = (int)order[p] - s[p] - carry;
-        s[p] = (n + 256) & 0xFF;
-        carry = (n < 0);
-    }
-    assert(carry == 0);
-    if (s.size() > 1 && s[0] == 0 && s[1] < 0x80) {
+    assert(s[0] == 0);
+    // Perform mod-n negation of s by (ab)using libsecp256k1
+    // (note that this function is meant to be used for negating secret keys,
+    //  but it works for any non-zero scalar modulo the group order, i.e. also for s)
+    int ret = secp256k1_ec_seckey_negate(secp256k1_context_static, s.data() + 1);
+    assert(ret);
+
+    if (s[1] < 0x80) {
         s.erase(s.begin());
     }