Use libsodium's s < L check, instead checking that libsodium checks that.

Adaptation coming from zcash@2902ac7ce8e754d09a5137cba82d8af10c172977

Author: furszy

src/init.cpp

@@ -1217,6 +1217,11 @@ bool AppInitSanityChecks()
 {
     // ********************************************************* Step 4: sanity checks
 
+    // Initialize libsodium
+    if (init_and_check_sodium() == -1) {
+        return false;
+    }
+
     // Initialize elliptic curve code
     RandomInit();
     ECC_Start();

src/sapling/sodium_sanity.cpp

@@ -10,6 +10,44 @@
 
 #include <sodium.h>
 
+int init_and_check_sodium()
+{
+    if (sodium_init() == -1) {
+        return -1;
+    }
+
+    // What follows is a runtime test that ensures the version of libsodium
+    // we're linked against checks that signatures are canonical (s < L).
+    const unsigned char message[1] = { 0 };
+
+    unsigned char pk[crypto_sign_PUBLICKEYBYTES];
+    unsigned char sk[crypto_sign_SECRETKEYBYTES];
+    unsigned char sig[crypto_sign_BYTES];
+
+    crypto_sign_keypair(pk, sk);
+    crypto_sign_detached(sig, NULL, message, sizeof(message), sk);
+
+    assert(crypto_sign_verify_detached(sig, message, sizeof(message), pk) == 0);
+
+    // Copied from libsodium/crypto_sign/ed25519/ref10/open.c
+    static const unsigned char L[32] =
+            { 0xed, 0xd3, 0xf5, 0x5c, 0x1a, 0x63, 0x12, 0x58,
+              0xd6, 0x9c, 0xf7, 0xa2, 0xde, 0xf9, 0xde, 0x14,
+              0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+              0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x10 };
+
+    // Add L to S, which starts at sig[32].
+    unsigned int s = 0;
+    for (size_t i = 0; i < 32; i++) {
+        s = sig[32 + i] + L[i] + (s >> 8);
+        sig[32 + i] = s & 0xff;
+    }
+
+    assert(crypto_sign_verify_detached(sig, message, sizeof(message), pk) != 0);
+
+    return 0;
+}
+
 void TestLibsodiumEd25519SignatureVerification(
         const std::string &scope,
         const std::string &msg,

src/sapling/sodium_sanity.h

@@ -5,6 +5,7 @@
 #ifndef PIVX_SODIUM_SANITY_H
 #define PIVX_SODIUM_SANITY_H
 
+int init_and_check_sodium();
 void libsodium_sanity_test();
 
 #endif //PIVX_SODIUM_SANITY_H

src/test/test_pivx.cpp

@@ -15,6 +15,7 @@
 #include "net_processing.h"
 #include "rpc/server.h"
 #include "rpc/register.h"
+#include "sapling/sodium_sanity.h"
 #include "script/sigcache.h"
 #include "sporkdb.h"
 #include "txmempool.h"
@@ -40,6 +41,7 @@ std::ostream& operator<<(std::ostream& os, const uint256& num)
 
 BasicTestingSetup::BasicTestingSetup()
 {
+        assert(init_and_check_sodium() != -1);
         ECC_Start();
         SetupEnvironment();
         InitSignatureCache();