wallet: refactor: introduce CMasterKey::DEFAULT_DERIVE_ITERATIONS constant

This gets rid of the magic number used in both the `CMasterKey` ctor
and the wallet encryption / passphrase change methods.

src/wallet/crypter.h

@@ -42,6 +42,11 @@ public:
     //! Use this for more parameters to key derivation (currently unused)
     std::vector<unsigned char> vchOtherDerivationParameters;
 
+    //! Default/minimum number of key derivation rounds
+    // 25000 rounds is just under 0.1 seconds on a 1.86 GHz Pentium M
+    // ie slightly lower than the lowest hardware we need bother supporting
+    static constexpr unsigned int DEFAULT_DERIVE_ITERATIONS = 25000;
+
     SERIALIZE_METHODS(CMasterKey, obj)
     {
         READWRITE(obj.vchCryptedKey, obj.vchSalt, obj.nDerivationMethod, obj.nDeriveIterations, obj.vchOtherDerivationParameters);
@@ -49,9 +54,7 @@ public:
 
     CMasterKey()
     {
-        // 25000 rounds is just under 0.1 seconds on a 1.86 GHz Pentium M
+        nDeriveIterations = DEFAULT_DERIVE_ITERATIONS;
-        // ie slightly lower than the lowest hardware we need bother supporting
-        nDeriveIterations = 25000;
         nDerivationMethod = 0;
         vchOtherDerivationParameters = std::vector<unsigned char>(0);
     }

src/wallet/test/fuzz/crypter.cpp

@@ -38,7 +38,7 @@ FUZZ_TARGET(crypter, .init = initialize_crypter)
         // Limiting the value of rounds since it is otherwise uselessly expensive and causes a timeout when fuzzing.
         crypt.SetKeyFromPassphrase(/*key_data=*/secure_string,
                                    /*salt=*/ConsumeFixedLengthByteVector(fuzzed_data_provider, WALLET_CRYPTO_SALT_SIZE),
-                                   /*rounds=*/fuzzed_data_provider.ConsumeIntegralInRange<unsigned int>(0, 25000),
+                                   /*rounds=*/fuzzed_data_provider.ConsumeIntegralInRange<unsigned int>(0, CMasterKey::DEFAULT_DERIVE_ITERATIONS),
                                    /*derivation_method=*/derivation_method);
     }
 

src/wallet/test/wallet_crypto_tests.cpp

@@ -83,7 +83,7 @@ static void TestEncrypt(const CCrypter& crypt, const std::span<const unsigned ch
 BOOST_AUTO_TEST_CASE(passphrase) {
     // These are expensive.
 
-    TestCrypter::TestPassphrase("0000deadbeef0000"_hex_u8, "test", 25000,
+    TestCrypter::TestPassphrase("0000deadbeef0000"_hex_u8, "test", CMasterKey::DEFAULT_DERIVE_ITERATIONS,
                                 "fc7aba077ad5f4c3a0988d8daa4810d0d4a0e3bcb53af662998898f33df0556a"_hex_u8,
                                 "cf2f2691526dd1aa220896fb8bf7c369"_hex_u8);
 
@@ -99,7 +99,7 @@ BOOST_AUTO_TEST_CASE(passphrase) {
 BOOST_AUTO_TEST_CASE(encrypt) {
     constexpr std::array<uint8_t, WALLET_CRYPTO_SALT_SIZE> salt{"0000deadbeef0000"_hex_u8};
     CCrypter crypt;
-    crypt.SetKeyFromPassphrase("passphrase", salt, 25000, 0);
+    crypt.SetKeyFromPassphrase("passphrase", salt, CMasterKey::DEFAULT_DERIVE_ITERATIONS, 0);
     TestCrypter::TestEncrypt(crypt, "22bcade09ac03ff6386914359cfe885cfeb5f77ff0d670f102f619687453b29d"_hex_u8);
 
     for (int i = 0; i != 100; i++)
@@ -113,7 +113,7 @@ BOOST_AUTO_TEST_CASE(encrypt) {
 BOOST_AUTO_TEST_CASE(decrypt) {
     constexpr std::array<uint8_t, WALLET_CRYPTO_SALT_SIZE> salt{"0000deadbeef0000"_hex_u8};
     CCrypter crypt;
-    crypt.SetKeyFromPassphrase("passphrase", salt, 25000, 0);
+    crypt.SetKeyFromPassphrase("passphrase", salt, CMasterKey::DEFAULT_DERIVE_ITERATIONS, 0);
 
     // Some corner cases the came up while testing
     TestCrypter::TestDecrypt(crypt,"795643ce39d736088367822cdc50535ec6f103715e3e48f4f3b1a60a08ef59ca"_hex_u8);

src/wallet/wallet.cpp

@@ -628,8 +628,8 @@ bool CWallet::ChangeWalletPassphrase(const SecureString& strOldWalletPassphrase,
                 crypter.SetKeyFromPassphrase(strNewWalletPassphrase, pMasterKey.second.vchSalt, pMasterKey.second.nDeriveIterations, pMasterKey.second.nDerivationMethod);
                 pMasterKey.second.nDeriveIterations = (pMasterKey.second.nDeriveIterations + static_cast<unsigned int>(pMasterKey.second.nDeriveIterations * target / (SteadyClock::now() - start))) / 2;
 
-                if (pMasterKey.second.nDeriveIterations < 25000)
+                if (pMasterKey.second.nDeriveIterations < CMasterKey::DEFAULT_DERIVE_ITERATIONS)
-                    pMasterKey.second.nDeriveIterations = 25000;
+                    pMasterKey.second.nDeriveIterations = CMasterKey::DEFAULT_DERIVE_ITERATIONS;
 
                 WalletLogPrintf("Wallet passphrase changed to an nDeriveIterations of %i\n", pMasterKey.second.nDeriveIterations);
 
@@ -825,15 +825,15 @@ bool CWallet::EncryptWallet(const SecureString& strWalletPassphrase)
     CCrypter crypter;
     constexpr MillisecondsDouble target{100};
     auto start{SteadyClock::now()};
-    crypter.SetKeyFromPassphrase(strWalletPassphrase, kMasterKey.vchSalt, 25000, kMasterKey.nDerivationMethod);
+    crypter.SetKeyFromPassphrase(strWalletPassphrase, kMasterKey.vchSalt, CMasterKey::DEFAULT_DERIVE_ITERATIONS, kMasterKey.nDerivationMethod);
-    kMasterKey.nDeriveIterations = static_cast<unsigned int>(25000 * target / (SteadyClock::now() - start));
+    kMasterKey.nDeriveIterations = static_cast<unsigned int>(CMasterKey::DEFAULT_DERIVE_ITERATIONS * target / (SteadyClock::now() - start));
 
     start = SteadyClock::now();
     crypter.SetKeyFromPassphrase(strWalletPassphrase, kMasterKey.vchSalt, kMasterKey.nDeriveIterations, kMasterKey.nDerivationMethod);
     kMasterKey.nDeriveIterations = (kMasterKey.nDeriveIterations + static_cast<unsigned int>(kMasterKey.nDeriveIterations * target / (SteadyClock::now() - start))) / 2;
 
-    if (kMasterKey.nDeriveIterations < 25000)
+    if (kMasterKey.nDeriveIterations < CMasterKey::DEFAULT_DERIVE_ITERATIONS)
-        kMasterKey.nDeriveIterations = 25000;
+        kMasterKey.nDeriveIterations = CMasterKey::DEFAULT_DERIVE_ITERATIONS;
 
     WalletLogPrintf("Encrypting Wallet with an nDeriveIterations of %i\n", kMasterKey.nDeriveIterations);