Update key.cpp to use new libsecp256k1

libsecp256k1's API changed, so update key.cpp to use it.

Libsecp256k1 now has explicit context objects, which makes it completely thread-safe.
In turn, keep an explicit context object in key.cpp, which is explicitly initialized
destroyed. This is not really pretty now, but it's more efficient than the static
initialized object in key.cpp (which made for example bitcoin-tx slow, as for most of
its calls, libsecp256k1 wasn't actually needed).

This also brings in the new blinding support in libsecp256k1. By passing in a random
seed, temporary variables during the elliptic curve computations are altered, in such
a way that if an attacker does not know the blind, observing the internal operations
leaks less information about the keys used. This was implemented by Greg Maxwell.

src/key.cpp

@@ -14,21 +14,7 @@
 #include <secp256k1.h>
 #include "ecwrapper.h"
 
-//! anonymous namespace
-namespace {
-
-class CSecp256k1Init {
-public:
-    CSecp256k1Init() {
-        secp256k1_start(SECP256K1_START_SIGN);
-    }
-    ~CSecp256k1Init() {
-        secp256k1_stop();
-    }
-};
-static CSecp256k1Init instance_of_csecp256k1;
-
-} // anon namespace
+static secp256k1_context_t* secp256k1_context = NULL;
 
 bool CKey::Check(const unsigned char *vch) {
     return eccrypto::Check(vch);
@@ -44,7 +30,7 @@ void CKey::MakeNewKey(bool fCompressedIn) {
 }
 
 bool CKey::SetPrivKey(const CPrivKey &privkey, bool fCompressedIn) {
-    if (!secp256k1_ec_privkey_import((unsigned char*)begin(), &privkey[0], privkey.size()))
+    if (!secp256k1_ec_privkey_import(secp256k1_context, (unsigned char*)begin(), &privkey[0], privkey.size()))
         return false;
     fCompressed = fCompressedIn;
     fValid = true;
@@ -57,7 +43,7 @@ CPrivKey CKey::GetPrivKey() const {
     int privkeylen, ret;
     privkey.resize(279);
     privkeylen = 279;
-    ret = secp256k1_ec_privkey_export(begin(), (unsigned char*)&privkey[0], &privkeylen, fCompressed);
+    ret = secp256k1_ec_privkey_export(secp256k1_context, begin(), (unsigned char*)&privkey[0], &privkeylen, fCompressed);
     assert(ret);
     privkey.resize(privkeylen);
     return privkey;
@@ -67,7 +53,7 @@ CPubKey CKey::GetPubKey() const {
     assert(fValid);
     CPubKey result;
     int clen = 65;
-    int ret = secp256k1_ec_pubkey_create((unsigned char*)result.begin(), &clen, begin(), fCompressed);
+    int ret = secp256k1_ec_pubkey_create(secp256k1_context, (unsigned char*)result.begin(), &clen, begin(), fCompressed);
     assert((int)result.size() == clen);
     assert(ret);
     assert(result.IsValid());
@@ -81,7 +67,7 @@ bool CKey::Sign(const uint256 &hash, std::vector<unsigned char>& vchSig, uint32_
     int nSigLen = 72;
     unsigned char extra_entropy[32] = {0};
     WriteLE32(extra_entropy, test_case);
-    int ret = secp256k1_ecdsa_sign(hash.begin(), (unsigned char*)&vchSig[0], &nSigLen, begin(), secp256k1_nonce_function_rfc6979, test_case ? extra_entropy : NULL);
+    int ret = secp256k1_ecdsa_sign(secp256k1_context, hash.begin(), (unsigned char*)&vchSig[0], &nSigLen, begin(), secp256k1_nonce_function_rfc6979, test_case ? extra_entropy : NULL);
     assert(ret);
     vchSig.resize(nSigLen);
     return true;
@@ -106,7 +92,7 @@ bool CKey::SignCompact(const uint256 &hash, std::vector<unsigned char>& vchSig)
         return false;
     vchSig.resize(65);
     int rec = -1;
-    int ret = secp256k1_ecdsa_sign_compact(hash.begin(), &vchSig[1], begin(), secp256k1_nonce_function_rfc6979, NULL, &rec);
+    int ret = secp256k1_ecdsa_sign_compact(secp256k1_context, hash.begin(), &vchSig[1], begin(), secp256k1_nonce_function_rfc6979, NULL, &rec);
     assert(ret);
     assert(rec != -1);
     vchSig[0] = 27 + rec + (fCompressed ? 4 : 0);
@@ -114,7 +100,7 @@ bool CKey::SignCompact(const uint256 &hash, std::vector<unsigned char>& vchSig)
 }
 
 bool CKey::Load(CPrivKey &privkey, CPubKey &vchPubKey, bool fSkipCheck=false) {
-    if (!secp256k1_ec_privkey_import((unsigned char*)begin(), &privkey[0], privkey.size()))
+    if (!secp256k1_ec_privkey_import(secp256k1_context, (unsigned char*)begin(), &privkey[0], privkey.size()))
         return false;
     fCompressed = vchPubKey.IsCompressed();
     fValid = true;
@@ -140,7 +126,7 @@ bool CKey::Derive(CKey& keyChild, unsigned char ccChild[32], unsigned int nChild
     }
     memcpy(ccChild, out+32, 32);
     memcpy((unsigned char*)keyChild.begin(), begin(), 32);
-    bool ret = secp256k1_ec_privkey_tweak_add((unsigned char*)keyChild.begin(), out);
+    bool ret = secp256k1_ec_privkey_tweak_add(secp256k1_context, (unsigned char*)keyChild.begin(), out);
     UnlockObject(out);
     keyChild.fCompressed = true;
     keyChild.fValid = ret;
@@ -206,3 +192,32 @@ bool ECC_InitSanityCheck() {
     CPubKey pubkey = key.GetPubKey();
     return key.VerifyPubKey(pubkey);
 }
+
+
+void ECC_Start() {
+    assert(secp256k1_context == NULL);
+
+    secp256k1_context_t *ctx = secp256k1_context_create(SECP256K1_CONTEXT_SIGN);
+    assert(ctx != NULL);
+
+    {
+        // Pass in a random blinding seed to the secp256k1 context.
+        unsigned char seed[32];
+        LockObject(seed);
+        GetRandBytes(seed, 32);
+        bool ret = secp256k1_context_randomize(ctx, seed);
+        assert(ret);
+        UnlockObject(seed);
+    }
+
+    secp256k1_context = ctx;
+}
+
+void ECC_Stop() {
+    secp256k1_context_t *ctx = secp256k1_context;
+    secp256k1_context = NULL;
+
+    if (ctx) {
+        secp256k1_context_destroy(ctx);
+    }
+}