crypto: harden bignum-to-binary conversions

PR-URL: #24719
Refs: #24645
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: bnoordhuis

src/node_crypto.cc

@@ -4186,9 +4186,11 @@ void DiffieHellman::GenerateKeys(const FunctionCallbackInfo<Value>& args) {
 
   const BIGNUM* pub_key;
   DH_get0_key(diffieHellman->dh_.get(), &pub_key, nullptr);
-  size_t size = BN_num_bytes(pub_key);
+  const int size = BN_num_bytes(pub_key);
+  CHECK_GE(size, 0);
   char* data = Malloc(size);
-  BN_bn2bin(pub_key, reinterpret_cast<unsigned char*>(data));
+  CHECK_EQ(size,
+           BN_bn2binpad(pub_key, reinterpret_cast<unsigned char*>(data), size));
   args.GetReturnValue().Set(Buffer::New(env, data, size).ToLocalChecked());
 }
 
@@ -4204,9 +4206,11 @@ void DiffieHellman::GetField(const FunctionCallbackInfo<Value>& args,
   const BIGNUM* num = get_field(dh->dh_.get());
   if (num == nullptr) return env->ThrowError(err_if_null);
 
-  size_t size = BN_num_bytes(num);
+  const int size = BN_num_bytes(num);
+  CHECK_GE(size, 0);
   char* data = Malloc(size);
-  BN_bn2bin(num, reinterpret_cast<unsigned char*>(data));
+  CHECK_EQ(size,
+           BN_bn2binpad(num, reinterpret_cast<unsigned char*>(data), size));
   args.GetReturnValue().Set(Buffer::New(env, data, size).ToLocalChecked());
 }
 
@@ -4542,13 +4546,9 @@ void ECDH::GetPrivateKey(const FunctionCallbackInfo<Value>& args) {
   if (b == nullptr)
     return env->ThrowError("Failed to get ECDH private key");
 
-  int size = BN_num_bytes(b);
+  const int size = BN_num_bytes(b);
   unsigned char* out = node::Malloc<unsigned char>(size);
-
-  if (size != BN_bn2bin(b, out)) {
-    free(out);
-    return env->ThrowError("Failed to convert ECDH private key to Buffer");
-  }
+  CHECK_EQ(size, BN_bn2binpad(b, out, size));
 
   Local<Object> buf =
       Buffer::New(env, reinterpret_cast<char*>(out), size).ToLocalChecked();