core: vm: implement RAND opcode support

DEXON has a built-in on chain random oracle that allow one to retrieve a
random variable. Add a new opcode `RAND` to load the random variable
onto the stack.

4 files changed, 32 insertions, 0 deletions

diff --git a/core/vm/evm.go b/core/vm/evm.go
index e22acb81c..be2dabcf5 100644
--- a/core/vm/evm.go
+++ b/core/vm/evm.go
@@ -90,6 +90,7 @@ type Context struct {
 	GasLimit    uint64         // Provides information for GASLIMIT
 	BlockNumber *big.Int       // Provides information for NUMBER
 	Time        *big.Int       // Provides information for TIME
+	Randomness  []byte         // Provides information for RAND
 	Difficulty  *big.Int       // Provides information for DIFFICULTY
 }
 
diff --git a/core/vm/instructions.go b/core/vm/instructions.go
index 349f407a2..de0fcaeb6 100644
--- a/core/vm/instructions.go
+++ b/core/vm/instructions.go
@@ -17,6 +17,7 @@
 package vm
 
 import (
+	"encoding/binary"
 	"errors"
 	"fmt"
 	"math/big"
@@ -24,6 +25,7 @@ import (
 	"github.com/dexon-foundation/dexon/common"
 	"github.com/dexon-foundation/dexon/common/math"
 	"github.com/dexon-foundation/dexon/core/types"
+	"github.com/dexon-foundation/dexon/crypto"
 	"github.com/dexon-foundation/dexon/params"
 	"golang.org/x/crypto/sha3"
 )
@@ -404,6 +406,26 @@ func opSha3(pc *uint64, interpreter *EVMInterpreter, contract *Contract, memory
 	return nil, nil
 }
 
+func opRand(pc *uint64, interpreter *EVMInterpreter, contract *Contract, memory *Memory, stack *Stack) ([]byte, error) {
+	evm := interpreter.evm
+
+	nonce := evm.StateDB.GetNonce(contract.Caller())
+	binaryNonce := make([]byte, binary.MaxVarintLen64)
+	binary.PutUvarint(binaryNonce, nonce)
+
+	binaryGas := make([]byte, binary.MaxVarintLen64)
+	binary.PutUvarint(binaryGas, contract.Gas)
+
+	hash := crypto.Keccak256(
+		evm.Randomness,
+		contract.Caller().Bytes(),
+		binaryNonce,
+		binaryGas)
+
+	stack.push(interpreter.intPool.get().SetBytes(hash))
+	return nil, nil
+}
+
 func opAddress(pc *uint64, interpreter *EVMInterpreter, contract *Contract, memory *Memory, stack *Stack) ([]byte, error) {
 	stack.push(contract.Address().Big())
 	return nil, nil
diff --git a/core/vm/jump_table.go b/core/vm/jump_table.go
index da158027a..608e34419 100644
--- a/core/vm/jump_table.go
+++ b/core/vm/jump_table.go
@@ -301,6 +301,12 @@ func newFrontierInstructionSet() [256]operation {
 			memorySize:    memorySha3,
 			valid:         true,
 		},
+		RAND: {
+			execute:       opRand,
+			gasCost:       constGasFunc(params.RandGas),
+			validateStack: makeStackFunc(0, 1),
+			valid:         true,
+		},
 		ADDRESS: {
 			execute:       opAddress,
 			gasCost:       constGasFunc(GasQuickStep),
diff --git a/core/vm/opcodes.go b/core/vm/opcodes.go
index 4349ffd29..8762d4b43 100644
--- a/core/vm/opcodes.go
+++ b/core/vm/opcodes.go
@@ -71,6 +71,7 @@ const (
 	SAR
 
 	SHA3 = 0x20
+	RAND = 0x2f
 )
 
 // 0x30 range - closure state.
@@ -251,6 +252,7 @@ var opCodeToString = map[OpCode]string{
 
 	// 0x20 range - crypto.
 	SHA3: "SHA3",
+	RAND: "RAND",
 
 	// 0x30 range - closure state.
 	ADDRESS:        "ADDRESS",
@@ -420,6 +422,7 @@ var stringToOp = map[string]OpCode{
 	"ADDMOD":         ADDMOD,
 	"MULMOD":         MULMOD,
 	"SHA3":           SHA3,
+	"RAND":           RAND,
 	"ADDRESS":        ADDRESS,
 	"BALANCE":        BALANCE,
 	"ORIGIN":         ORIGIN,