mipsevm,contracts: fix Oracle contract, fix oracle witness data encoding, add...

mipsevm,contracts: fix Oracle contract, fix oracle witness data encoding, add mem-proof for syscall mem read, implement multi-contract source-map tracing

contracts/src/Oracle.sol

@@ -12,9 +12,8 @@ contract Oracle {
         require(preimagePartOk[key][offset], "preimage must exist");
         datLen = 32;
         uint256 length = preimageLengths[key];
-        // TODO: insert length prefix before data
-        if(offset + 32 >= length) {
-            datLen = length - offset;
+        if(offset + 32 >= length + 8) { // add 8 for the length-prefix part
+            datLen = length + 8 - offset;
         }
         dat = preimageParts[key][offset];
     }
@@ -36,18 +35,17 @@ contract Oracle {
         bytes32 key;
         bytes32 part;
         assembly {
-            // calldata layout: 4 (sel) + 0x20 (part offset) + 0x20 (start offset) + 0x20 (size) + preimage payload
-            let startOffset := calldataload(0x24)
-            if not(eq(startOffset, 0x44)) { // must always point to expected location of the size value.
-                revert(0, 0)
-            }
-            size := calldataload(0x44)
-            if iszero(lt(partOffset, size)) { // revert if part offset >= size (i.e. parts must be within bounds)
+            size := calldataload(0x44) // len(sig) + len(partOffset) + len(preimage offset) = 4 + 32 + 32 = 0x64
+            if iszero(lt(partOffset, add(size, 8))) { // revert if part offset >= size+8 (i.e. parts must be within bounds)
                 revert(0, 0)
             }
             let ptr := 0x80 // we leave solidity slots 0x40 and 0x60 untouched, and everything after as scratch-memory.
-            calldatacopy(ptr, 0x64, size) // copy preimage payload into memory so we can hash and read it.
-            part := mload(add(ptr, partOffset))  // this will be zero-padded at the end, since memory at end is clean.
+            mstore(ptr, shl(192, size)) // put size as big-endian uint64 at start of pre-image
+            ptr := add(ptr, 8)
+            calldatacopy(ptr, preimage.offset, size) // copy preimage payload into memory so we can hash and read it.
+            // Note that it includes the 8-byte big-endian uint64 length prefix.
+            // this will be zero-padded at the end, since memory at end is clean.
+            part := mload(add(sub(ptr, 8), partOffset))
             let h := keccak256(ptr, size) // compute preimage keccak256 hash
             key := or(and(h, not(shl(248, 0xFF))), shl(248, 2)) // mask out prefix byte, replace with type 2 byte
         }

mipsevm/evm.go

@@ -20,15 +20,21 @@ import (
 	"github.com/ethereum/go-ethereum/params"
 )
 
-var StepBytes4 = crypto.Keccak256Hash([]byte("Step(bytes32,bytes,bytes)")).Bytes()[:4]
+var (
+	StepBytes4                      = crypto.Keccak256([]byte("Step(bytes32,bytes,bytes)"))[:4]
+	CheatBytes4                     = crypto.Keccak256([]byte("cheat(uint256,bytes32,bytes32,uint256)"))[:4]
+	LoadKeccak256PreimagePartBytes4 = crypto.Keccak256([]byte("loadKeccak256PreimagePart(uint256,bytes)"))[:4]
+)
 
 func LoadContracts() (*Contracts, error) {
 	mips, err := LoadContract("MIPS")
 	if err != nil {
 		return nil, err
 	}
+	oracle, err := LoadContract("Oracle")
 	return &Contracts{
-		MIPS: mips,
+		MIPS:   mips,
+		Oracle: oracle,
 	}, nil
 }
 
@@ -59,17 +65,22 @@ func (c *Contract) SourceMap(sourcePaths []string) (*SourceMap, error) {
 }
 
 type Contracts struct {
-	MIPS *Contract
+	MIPS   *Contract
+	Oracle *Contract
 }
 
 type Addresses struct {
-	MIPS common.Address
+	MIPS         common.Address
+	Oracle       common.Address
+	Sender       common.Address
+	FeeRecipient common.Address
 }
 
 func NewEVMEnv(contracts *Contracts, addrs *Addresses) (*vm.EVM, *state.StateDB) {
 	chainCfg := params.MainnetChainConfig
-	bc := &testChain{}
-	header := bc.GetHeader(common.Hash{}, 100)
+	offsetBlocks := uint64(1000) // blocks after shanghai fork
+	bc := &testChain{startTime: *chainCfg.ShanghaiTime + offsetBlocks*12}
+	header := bc.GetHeader(common.Hash{}, 17034870+offsetBlocks)
 	db := rawdb.NewMemoryDatabase()
 	statedb := state.NewDatabase(db)
 	state, err := state.New(types.EmptyRootHash, statedb, nil)
@@ -81,13 +92,17 @@ func NewEVMEnv(contracts *Contracts, addrs *Addresses) (*vm.EVM, *state.StateDB)
 
 	env := vm.NewEVM(blockContext, vm.TxContext{}, state, chainCfg, vmCfg)
 	// pre-deploy the contracts
-
 	env.StateDB.SetCode(addrs.MIPS, contracts.MIPS.DeployedBytecode.Object)
-	// TODO: any state to set, or immutables to replace, to link the contracts together?
+	env.StateDB.SetCode(addrs.Oracle, contracts.Oracle.DeployedBytecode.Object)
+	env.StateDB.SetState(addrs.MIPS, common.Hash{}, addrs.Oracle.Hash())
+
+	rules := env.ChainConfig().Rules(header.Number, true, header.Time)
+	env.StateDB.Prepare(rules, addrs.Sender, addrs.FeeRecipient, &addrs.MIPS, vm.ActivePrecompiles(rules), nil)
 	return env, state
 }
 
 type testChain struct {
+	startTime uint64
 }
 
 func (d *testChain) Engine() consensus.Engine {
@@ -109,7 +124,7 @@ func (d *testChain) GetHeader(h common.Hash, n uint64) *types.Header {
 		Number:          new(big.Int).SetUint64(n),
 		GasLimit:        30_000_000,
 		GasUsed:         0,
-		Time:            1337,
+		Time:            d.startTime + n*12,
 		Extra:           nil,
 		MixDigest:       common.Hash{},
 		Nonce:           types.BlockNonce{},

mipsevm/evm_test.go

@@ -17,21 +17,32 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-func TestEVM(t *testing.T) {
-	testFiles, err := os.ReadDir("test/bin")
-	require.NoError(t, err)
-
+func testContractsSetup(t *testing.T) (*Contracts, *Addresses, *SourceMapTracer) {
 	contracts, err := LoadContracts()
 	require.NoError(t, err)
 
-	// the first unlisted source seems to be the ABIDecoderV2 code that the compiler inserts
-	mipsSrcMap, err := contracts.MIPS.SourceMap([]string{"../contracts/src/MIPS.sol", "~compiler?", "../contracts/src/MIPS.sol"})
+	mipsSrcMap, err := contracts.MIPS.SourceMap([]string{"../contracts/src/MIPS.sol"})
+	require.NoError(t, err)
+	oracleSrcMap, err := contracts.Oracle.SourceMap([]string{"../contracts/src/Oracle.sol"})
 	require.NoError(t, err)
 
 	addrs := &Addresses{
-		MIPS: common.Address{0: 0xff, 19: 1},
+		MIPS:         common.Address{0: 0xff, 19: 1},
+		Oracle:       common.Address{0: 0xff, 19: 2},
+		Sender:       common.Address{0x13, 0x37},
+		FeeRecipient: common.Address{0xaa},
 	}
+	tracer := NewSourceMapTracer(map[common.Address]*SourceMap{addrs.MIPS: mipsSrcMap, addrs.Oracle: oracleSrcMap}, os.Stdout)
+	return contracts, addrs, tracer
+}
+
+func TestEVM(t *testing.T) {
+	testFiles, err := os.ReadDir("test/bin")
+	require.NoError(t, err)
+
+	contracts, addrs, tracer := testContractsSetup(t)
 	sender := common.Address{0x13, 0x37}
+	//tracer = logger.NewMarkdownLogger(&logger.Config{}, os.Stdout)
 
 	for _, f := range testFiles {
 		t.Run(f.Name(), func(t *testing.T) {
@@ -41,8 +52,7 @@ func TestEVM(t *testing.T) {
 
 			env, evmState := NewEVMEnv(contracts, addrs)
 			env.Config.Debug = false
-			//env.Config.Tracer = logger.NewMarkdownLogger(&logger.Config{}, os.Stdout)
-			env.Config.Tracer = mipsSrcMap.Tracer(os.Stdout)
+			env.Config.Tracer = tracer
 
 			fn := path.Join("test/bin", f.Name())
 			programMem, err := os.ReadFile(fn)
@@ -108,16 +118,7 @@ func TestEVM(t *testing.T) {
 }
 
 func TestHelloEVM(t *testing.T) {
-	contracts, err := LoadContracts()
-	require.NoError(t, err)
-
-	// the first unlisted source seems to be the ABIDecoderV2 code that the compiler inserts
-	mipsSrcMap, err := contracts.MIPS.SourceMap([]string{"../contracts/src/MIPS.sol", "~compiler?", "../contracts/src/MIPS.sol"})
-	require.NoError(t, err)
-
-	addrs := &Addresses{
-		MIPS: common.Address{0: 0xff, 19: 1},
-	}
+	contracts, addrs, tracer := testContractsSetup(t)
 	sender := common.Address{0x13, 0x37}
 
 	elfProgram, err := elf.Open("../example/bin/hello.elf")
@@ -140,8 +141,7 @@ func TestHelloEVM(t *testing.T) {
 
 	env, evmState := NewEVMEnv(contracts, addrs)
 	env.Config.Debug = false
-	//env.Config.Tracer = logger.NewMarkdownLogger(&logger.Config{}, os.Stdout)
-	env.Config.Tracer = mipsSrcMap.Tracer(os.Stdout)
+	env.Config.Tracer = tracer
 
 	start := time.Now()
 	for i := 0; i < 400_000; i++ {
@@ -188,3 +188,75 @@ func TestHelloEVM(t *testing.T) {
 	require.Equal(t, "hello world!", stdOutBuf.String(), "stdout says hello")
 	require.Equal(t, "", stdErrBuf.String(), "stderr silent")
 }
+
+func TestClaimEVM(t *testing.T) {
+	contracts, addrs, tracer := testContractsSetup(t)
+
+	elfProgram, err := elf.Open("../example/bin/claim.elf")
+	require.NoError(t, err, "open ELF file")
+
+	state, err := LoadELF(elfProgram)
+	require.NoError(t, err, "load ELF into state")
+
+	err = patchVM(elfProgram, state)
+	require.NoError(t, err, "apply Go runtime patches")
+
+	mu, err := NewUnicorn()
+	require.NoError(t, err, "load unicorn")
+	defer mu.Close()
+	err = LoadUnicorn(state, mu)
+	require.NoError(t, err, "load state into unicorn")
+
+	oracle, expectedStdOut, expectedStdErr := claimTestOracle(t)
+
+	var stdOutBuf, stdErrBuf bytes.Buffer
+	us, err := NewUnicornState(mu, state, oracle, io.MultiWriter(&stdOutBuf, os.Stdout), io.MultiWriter(&stdErrBuf, os.Stderr))
+	require.NoError(t, err, "hook unicorn to state")
+
+	env, evmState := NewEVMEnv(contracts, addrs)
+	env.Config.Debug = false
+	env.Config.Tracer = tracer
+
+	for i := 0; i < 2000_000; i++ {
+		if us.state.Exited {
+			break
+		}
+
+		insn := state.Memory.GetMemory(state.PC)
+		if i%1000 == 0 { // avoid spamming test logs, we are executing many steps
+			t.Logf("step: %4d pc: 0x%08x insn: 0x%08x", state.Step, state.PC, insn)
+		}
+
+		stepWitness := us.Step(true)
+		input := stepWitness.EncodeStepInput()
+		startingGas := uint64(30_000_000)
+
+		// we take a snapshot so we can clean up the state, and isolate the logs of this instruction run.
+		snap := env.StateDB.Snapshot()
+
+		// prepare pre-image oracle data, if any
+		if stepWitness.HasPreimage() {
+			poInput, err := stepWitness.EncodePreimageOracleInput()
+			require.NoError(t, err, "encode preimage oracle input")
+			_, leftOverGas, err := env.Call(vm.AccountRef(addrs.Sender), addrs.Oracle, poInput, startingGas, big.NewInt(0))
+			require.NoErrorf(t, err, "evm should not fail, took %d gas", startingGas-leftOverGas)
+		}
+
+		ret, leftOverGas, err := env.Call(vm.AccountRef(addrs.Sender), addrs.MIPS, input, startingGas, big.NewInt(0))
+		require.NoErrorf(t, err, "evm should not fail, took %d gas", startingGas-leftOverGas)
+		require.Len(t, ret, 32, "expecting 32-byte state hash")
+		// remember state hash, to check it against state
+		postHash := common.Hash(*(*[32]byte)(ret))
+		logs := evmState.Logs()
+		require.Equal(t, 1, len(logs), "expecting a log with post-state")
+		evmPost := logs[0].Data
+		require.Equal(t, crypto.Keccak256Hash(evmPost), postHash, "logged state must be accurate")
+		env.StateDB.RevertToSnapshot(snap)
+	}
+
+	require.True(t, state.Exited, "must complete program")
+	require.Equal(t, uint8(0), state.ExitCode, "exit with 0")
+
+	require.Equal(t, expectedStdOut, stdOutBuf.String(), "stdout")
+	require.Equal(t, expectedStdErr, stdErrBuf.String(), "stderr")
+}

mipsevm/solutil.go

@@ -175,13 +175,13 @@ func ParseSourceMap(sources []string, bytecode []byte, sourceMap string) (*Sourc
 	return srcMap, nil
 }
 
-func (s *SourceMap) Tracer(out io.Writer) *SourceMapTracer {
-	return &SourceMapTracer{s, out}
+func NewSourceMapTracer(srcMaps map[common.Address]*SourceMap, out io.Writer) *SourceMapTracer {
+	return &SourceMapTracer{srcMaps, out}
 }
 
 type SourceMapTracer struct {
-	srcMap *SourceMap
-	out    io.Writer
+	srcMaps map[common.Address]*SourceMap
+	out     io.Writer
 }
 
 func (s *SourceMapTracer) CaptureTxStart(gasLimit uint64) {}
@@ -198,12 +198,25 @@ func (s *SourceMapTracer) CaptureEnter(typ vm.OpCode, from common.Address, to co
 
 func (s *SourceMapTracer) CaptureExit(output []byte, gasUsed uint64, err error) {}
 
+func (s *SourceMapTracer) info(codeAddr *common.Address, pc uint64) string {
+	info := "non-contract"
+	if codeAddr != nil {
+		srcMap, ok := s.srcMaps[*codeAddr]
+		if ok {
+			info = srcMap.FormattedInfo(pc)
+		} else {
+			info = "unknown-contract"
+		}
+	}
+	return info
+}
+
 func (s *SourceMapTracer) CaptureState(pc uint64, op vm.OpCode, gas, cost uint64, scope *vm.ScopeContext, rData []byte, depth int, err error) {
-	fmt.Fprintf(s.out, "%-40s : pc %x opcode %s\n", s.srcMap.FormattedInfo(pc), pc, op.String())
+	fmt.Fprintf(s.out, "%-40s : pc %x opcode %s\n", s.info(scope.Contract.CodeAddr, pc), pc, op.String())
 }
 
 func (s *SourceMapTracer) CaptureFault(pc uint64, op vm.OpCode, gas, cost uint64, scope *vm.ScopeContext, depth int, err error) {
-	fmt.Fprintf(s.out, "%-40s: pc %x opcode %s FAULT %v\n", s.srcMap.FormattedInfo(pc), pc, op.String(), err)
+	fmt.Fprintf(s.out, "%-40s: pc %x opcode %s FAULT %v\n", s.info(scope.Contract.CodeAddr, pc), pc, op.String(), err)
 	fmt.Println("----")
 	fmt.Fprintf(s.out, "calldata: %x\n", scope.Contract.Input)
 	fmt.Println("----")

mipsevm/state.go

@@ -11,7 +11,7 @@ type State struct {
 	Memory *Memory `json:"memory"`
 
 	PreimageKey    common.Hash `json:"preimageKey"`
-	PreimageOffset uint32      `json:"preimageOffset"`
+	PreimageOffset uint32      `json:"preimageOffset"` // note that the offset includes the 8-byte length prefix
 
 	PC     uint32 `json:"pc"`
 	NextPC uint32 `json:"nextPC"`

mipsevm/state_test.go

@@ -132,22 +132,7 @@ func (t *testOracle) GetPreimage(k [32]byte) []byte {
 
 var _ PreimageOracle = (*testOracle)(nil)
 
-func TestClaim(t *testing.T) {
-	elfProgram, err := elf.Open("../example/bin/claim.elf")
-	require.NoError(t, err, "open ELF file")
-
-	state, err := LoadELF(elfProgram)
-	require.NoError(t, err, "load ELF into state")
-
-	err = patchVM(elfProgram, state)
-	require.NoError(t, err, "apply Go runtime patches")
-
-	mu, err := NewUnicorn()
-	require.NoError(t, err, "load unicorn")
-	defer mu.Close()
-	err = LoadUnicorn(state, mu)
-	require.NoError(t, err, "load state into unicorn")
-
+func claimTestOracle(t *testing.T) (po PreimageOracle, stdOut string, stdErr string) {
 	s := uint64(1000)
 	a := uint64(3)
 	b := uint64(4)
@@ -198,6 +183,27 @@ func TestClaim(t *testing.T) {
 		},
 	}
 
+	return oracle, fmt.Sprintf("computing %d * %d + %d\nclaim %d is good!\n", s, a, b, s*a+b), "started!"
+}
+
+func TestClaim(t *testing.T) {
+	elfProgram, err := elf.Open("../example/bin/claim.elf")
+	require.NoError(t, err, "open ELF file")
+
+	state, err := LoadELF(elfProgram)
+	require.NoError(t, err, "load ELF into state")
+
+	err = patchVM(elfProgram, state)
+	require.NoError(t, err, "apply Go runtime patches")
+
+	mu, err := NewUnicorn()
+	require.NoError(t, err, "load unicorn")
+	defer mu.Close()
+	err = LoadUnicorn(state, mu)
+	require.NoError(t, err, "load state into unicorn")
+
+	oracle, expectedStdOut, expectedStdErr := claimTestOracle(t)
+
 	var stdOutBuf, stdErrBuf bytes.Buffer
 	us, err := NewUnicornState(mu, state, oracle, io.MultiWriter(&stdOutBuf, os.Stdout), io.MultiWriter(&stdErrBuf, os.Stderr))
 	require.NoError(t, err, "hook unicorn to state")
@@ -212,6 +218,6 @@ func TestClaim(t *testing.T) {
 	require.True(t, state.Exited, "must complete program")
 	require.Equal(t, uint8(0), state.ExitCode, "exit with 0")
 
-	require.Equal(t, fmt.Sprintf("computing %d * %d + %d\nclaim %d is good!\n", s, a, b, s*a+b), stdOutBuf.String(), "stdout says hello")
-	require.Equal(t, "started!", stdErrBuf.String(), "stderr silent")
+	require.Equal(t, expectedStdOut, stdOutBuf.String(), "stdout")
+	require.Equal(t, expectedStdErr, stdErrBuf.String(), "stderr")
 }

mipsevm/unicorn.go

@@ -83,6 +83,16 @@ func NewUnicornState(mu uc.Unicorn, state *State, po PreimageOracle, stdOut, std
 		return
 	}
 
+	trackMemAccess := func(effAddr uint32) {
+		if m.memProofEnabled && m.lastMemAccess != effAddr {
+			if m.lastMemAccess != ^uint32(0) {
+				panic(fmt.Errorf("unexpected different mem access at %08x, already have access at %08x buffered", effAddr, m.lastMemAccess))
+			}
+			m.lastMemAccess = effAddr
+			m.memProof = m.state.Memory.MerkleProof(effAddr)
+		}
+	}
+
 	var err error
 	_, err = mu.HookAdd(uc.HOOK_INTR, func(mu uc.Unicorn, intno uint32) {
 		if intno != 17 {
@@ -137,6 +147,7 @@ func NewUnicornState(mu uc.Unicorn, state *State, po PreimageOracle, stdOut, std
 				// leave v0 and v1 zero: read nothing, no error
 			case fdPreimageRead: // pre-image oracle
 				effAddr := a1 & 0xFFffFFfc
+				trackMemAccess(effAddr)
 				mem := st.Memory.GetMemory(effAddr)
 				dat, datLen := readPreimage(st.PreimageKey, st.PreimageOffset)
 				fmt.Printf("reading pre-image data: addr: %08x, offset: %d, datLen: %d, data: %x, key: %s  count: %d\n", a1, st.PreimageOffset, datLen, dat[:datLen], st.PreimageKey, a2)
@@ -190,7 +201,9 @@ func NewUnicornState(mu uc.Unicorn, state *State, po PreimageOracle, stdOut, std
 				}
 				v0 = a2
 			case fdPreimageWrite:
-				mem := st.Memory.GetMemory(a1 & 0xFFffFFfc)
+				effAddr := a1 & 0xFFffFFfc
+				trackMemAccess(effAddr)
+				mem := st.Memory.GetMemory(effAddr)
 				key := st.PreimageKey
 				alignment := a1 & 3
 				space := 4 - alignment
@@ -244,13 +257,7 @@ func NewUnicornState(mu uc.Unicorn, state *State, po PreimageOracle, stdOut, std
 
 	_, err = mu.HookAdd(uc.HOOK_MEM_READ, func(mu uc.Unicorn, access int, addr64 uint64, size int, value int64) {
 		effAddr := uint32(addr64 & 0xFFFFFFFC) // pass effective addr to tracer
-		if m.memProofEnabled && m.lastMemAccess != effAddr {
-			if m.lastMemAccess != ^uint32(0) {
-				panic(fmt.Errorf("unexpected different mem access at %08x, already have access at %08x buffered", effAddr, m.lastMemAccess))
-			}
-			m.lastMemAccess = effAddr
-			m.memProof = m.state.Memory.MerkleProof(effAddr)
-		}
+		trackMemAccess(effAddr)
 	}, 0, ^uint64(0))
 	if err != nil {
 		return nil, fmt.Errorf("failed to set up mem-write hook: %w", err)
@@ -283,13 +290,7 @@ func NewUnicornState(mu uc.Unicorn, state *State, po PreimageOracle, stdOut, std
 		} else {
 			log.Fatal("bad size write to ram")
 		}
-		if m.memProofEnabled && m.lastMemAccess != effAddr {
-			if m.lastMemAccess != ^uint32(0) {
-				panic(fmt.Errorf("unexpected different mem access at %08x, already have access at %08x buffered", effAddr, m.lastMemAccess))
-			}
-			m.lastMemAccess = effAddr
-			m.memProof = m.state.Memory.MerkleProof(effAddr)
-		}
+		trackMemAccess(effAddr)
 		// only set memory after making the proof: we need the pre-state
 		st.Memory.SetMemory(effAddr, post)
 	}, 0, ^uint64(0))

mipsevm/witness.go

@@ -2,8 +2,12 @@ package mipsevm
 
 import (
 	"encoding/binary"
+	"errors"
+	"fmt"
 
 	"github.com/ethereum/go-ethereum/crypto"
+
+	"github.com/ethereum-optimism/cannon/preimage"
 )
 
 type StepWitness struct {
@@ -12,7 +16,7 @@ type StepWitness struct {
 	memProof []byte
 
 	preimageKey    [32]byte // zeroed when no pre-image is accessed
-	preimageValue  []byte
+	preimageValue  []byte   // including the 8-byte length prefix
 	preimageOffset uint32
 }
 
@@ -37,10 +41,42 @@ func (wit *StepWitness) EncodeStepInput() []byte {
 	return input
 }
 
-func (wit *StepWitness) EncodePreimageOracleInput() []byte {
+func (wit *StepWitness) HasPreimage() bool {
+	return wit.preimageKey != ([32]byte{})
+}
+
+func (wit *StepWitness) EncodePreimageOracleInput() ([]byte, error) {
 	if wit.preimageKey == ([32]byte{}) {
-		return nil
+		return nil, errors.New("cannot encode pre-image oracle input, witness has no pre-image to proof")
+	}
+
+	switch preimage.KeyType(wit.preimageKey[0]) {
+	case preimage.LocalKeyType:
+		// We have no on-chain form of preparing the bootstrap pre-images onchain yet.
+		// So instead we cheat them in.
+		// In production usage there should be an on-chain contract that exposes this,
+		// rather than going through the global keccak256 oracle.
+		var input []byte
+		input = append(input, CheatBytes4...)
+		input = append(input, uint32ToBytes32(wit.preimageOffset)...)
+		input = append(input, wit.preimageKey[:]...)
+		var tmp [32]byte
+		copy(tmp[:], wit.preimageValue[wit.preimageOffset:])
+		input = append(input, tmp[:]...)
+		input = append(input, uint32ToBytes32(uint32(len(wit.preimageValue))-8)...)
+		// TODO: do we want to pad the end to a multiple of 32 bytes?
+		return input, nil
+	case preimage.Keccak256KeyType:
+		var input []byte
+		input = append(input, LoadKeccak256PreimagePartBytes4...)
+		input = append(input, uint32ToBytes32(wit.preimageOffset)...)
+		input = append(input, uint32ToBytes32(32+32)...) // partOffset, calldata offset
+		input = append(input, uint32ToBytes32(uint32(len(wit.preimageValue))-8)...)
+		input = append(input, wit.preimageValue[8:]...)
+		// TODO: do we want to pad the end to a multiple of 32 bytes?
+		return input, nil
+	default:
+		return nil, fmt.Errorf("unsupported pre-image type %d, cannot prepare preimage with key %x offset %d for oracle",
+			wit.preimageKey[0], wit.preimageKey, wit.preimageOffset)
 	}
-	// TODO
-	return nil
 }