feat: a precompile to get ciphertext bytes

Given a contract address and an ebool/euint value stored in it, return
the underlying ciphertext.

Returns an empty response if no such ciphertext exist. Only works via
`eth_call`.

The function selector for it, as of this commit, is `e4b808cb`.

Move protected storage code into its own file.

Add a `Precompiles` section in the getting started doc section.

Nit: rename `arg_types` to `argTypes` in instructions.go for naming
consistency.

docs/SUMMARY.md

@@ -6,6 +6,7 @@
 ## Getting Started
 - [Initial Setup](getting_started/README.md)
 - [Integration](getting_started/Integration.md)
+- [Precompiles](getting_started/Precompiles.md)
 
 ## 🔗 Support
 

docs/getting_started/Precompiles.md

@@ -0,0 +1,61 @@
+# Precompiles
+
+fhevm-go supports a number of functionalities that can be accessed via EVM function selectors. We call these functionalities `precompiles`, even though, technically, there is only one main fhevm-go precompile.
+
+This page describes the required inputs, behaviours and outputs of some of these precompiles.
+
+## GetCiphertext
+
+The `GetCiphertext` precompile returns a serialized TFHE ciphertext from protected storage given:
+ * contract address where the ciphertext is stored at
+ * the ebool/e(u)int value (also called a handle) for which the ciphertext is requested
+
+GetCiphertext only works via `eth_call`.
+
+To call GetCiphertext via `eth_call`, the following Python can serve as an example:
+
+```python
+import http.client
+import json
+
+# This is the address of the main fhevm-go precompile. This value is hardcoded per blockchain.
+fhe_precompile_address = "0x000000000000000000000000000000000000005d"
+
+# The contract address where the ciphertext is stored at.
+contract_address = "ACD7Be4EBF68Bf2A5b6eB0CaFb15460C169BC459"
+# 12 bytes of 0s for padding the contract address.
+address_zero_padding = "000000000000000000000000"
+
+# The ebool/e(u)int value for which the ciphertext is requested.
+handle = "f038cdc8bf630e239f143abeb039b91ec82ec17a8460582e7a409fa551030c06"
+
+# The function selector of GetCiphertext.
+get_ciphertext_selector = "e4b808cb"
+
+# Call the FHE precompile with `data` being the concatenation of:
+#  - getCiphertext function selector;
+#  - 12 bytes of 0s to padd the contract address;
+#  - contract address;
+#  - the handle to the ciphertext.
+payload = {
+    "jsonrpc": "2.0",
+    "method": "eth_call",
+    "params": [
+        {
+            "to": fhe_precompile_address,
+            "data": "0x" + get_ciphertext_selector + address_zero_padding +
+                    contract_address + handle
+        },
+        "latest"
+    ],
+    "id": 1,
+}
+
+con = http.client.HTTPConnection("localhost", 8545)
+con.request("POST", "/", body=json.dumps(payload),
+            headers={"Content-Type": "application/json"})
+resp = json.loads(con.getresponse().read())
+
+# Remove leading "0x" and decode hex to get a byte buffer with the ciphertext.
+ciphertext = bytes.fromhex(resp["result"][2:])
+```

fhevm/contracts_test.go

@@ -4101,3 +4101,213 @@ func TestDecryptInTransactionDisabled(t *testing.T) {
 		t.Fatalf("unexpected error for disabling decryption transactions, got %s", err.Error())
 	}
 }
+
+func TestFheLibGetCiphertextInvalidInputSize(t *testing.T) {
+	environment := newTestEVMEnvironment()
+	addr := common.Address{}
+	environment.ethCall = true
+	readOnly := true
+	input := make([]byte, 0)
+	zeroPadding := make([]byte, 12)
+	signature := crypto.Keccak256([]byte("getCiphertext(address,uint256)"))[0:4]
+	input = append(input, signature...)
+	input = append(input, zeroPadding...)
+	// missing input data...
+	_, err := FheLibRun(environment, addr, addr, input, readOnly)
+	if err == nil {
+		t.Fatalf("getCiphertext expected failure on bad input size")
+	}
+}
+
+func TestFheLibGetCiphertextNonEthCall(t *testing.T) {
+	environment := newTestEVMEnvironment()
+	pc := uint64(0)
+	depth := 1
+	environment.depth = depth
+	plaintext := uint64(2)
+	ct := verifyCiphertextInTestMemory(environment, plaintext, depth, FheUint32)
+	ctHash := ct.GetHash()
+	scope := newTestScopeConext()
+	loc := uint256.NewInt(10)
+	value := uint256FromBig(ctHash.Big())
+
+	// Setup and call SSTORE - it requires a location and a value to set there.
+	scope.pushToStack(value)
+	scope.pushToStack(loc)
+	_, err := OpSstore(&pc, environment, scope)
+	if err != nil {
+		t.Fatalf(err.Error())
+	}
+
+	// Call getCiphertext.
+	addr := common.Address{}
+	environment.ethCall = false
+	readOnly := true
+	input := make([]byte, 0)
+	zeroPadding := make([]byte, 12)
+	signature := crypto.Keccak256([]byte("getCiphertext(address,uint256)"))[0:4]
+	input = append(input, signature...)
+	input = append(input, zeroPadding...)
+	input = append(input, testContractAddress{}.Address().Bytes()...)
+	input = append(input, ctHash.Bytes()...)
+	_, err = FheLibRun(environment, addr, addr, input, readOnly)
+	if err == nil {
+		t.Fatalf("getCiphertext expected failure non-EthCall")
+	}
+}
+
+func TestFheLibGetCiphertextNonExistentHandle(t *testing.T) {
+	environment := newTestEVMEnvironment()
+	pc := uint64(0)
+	depth := 1
+	environment.depth = depth
+	plaintext := uint64(2)
+	ct := verifyCiphertextInTestMemory(environment, plaintext, depth, FheUint32)
+	ctHash := ct.GetHash()
+	scope := newTestScopeConext()
+	loc := uint256.NewInt(10)
+	value := uint256FromBig(ctHash.Big())
+
+	// Setup and call SSTORE - it requires a location and a value to set there.
+	scope.pushToStack(value)
+	scope.pushToStack(loc)
+	_, err := OpSstore(&pc, environment, scope)
+	if err != nil {
+		t.Fatalf(err.Error())
+	}
+
+	// Change ctHash to something that doesn't exist
+	ctHash[0]++
+
+	// Call getCiphertext.
+	addr := common.Address{}
+	environment.ethCall = true
+	readOnly := true
+	input := make([]byte, 0)
+	zeroPadding := make([]byte, 12)
+	signature := crypto.Keccak256([]byte("getCiphertext(address,uint256)"))[0:4]
+	input = append(input, signature...)
+	input = append(input, zeroPadding...)
+	input = append(input, testContractAddress{}.Address().Bytes()...)
+	input = append(input, ctHash.Bytes()...)
+	out, err := FheLibRun(environment, addr, addr, input, readOnly)
+	if err != nil {
+		t.Fatalf(err.Error())
+	}
+	if len(out) != 0 {
+		t.Fatalf("getCiphertext expected empty output on non-existent handle")
+	}
+}
+
+func TestFheLibGetCiphertextWrongContractAddress(t *testing.T) {
+	environment := newTestEVMEnvironment()
+	pc := uint64(0)
+	depth := 1
+	environment.depth = depth
+	plaintext := uint64(2)
+	ct := verifyCiphertextInTestMemory(environment, plaintext, depth, FheUint32)
+	ctHash := ct.GetHash()
+	scope := newTestScopeConext()
+	loc := uint256.NewInt(10)
+	value := uint256FromBig(ctHash.Big())
+
+	// Setup and call SSTORE - it requires a location and a value to set there.
+	scope.pushToStack(value)
+	scope.pushToStack(loc)
+	_, err := OpSstore(&pc, environment, scope)
+	if err != nil {
+		t.Fatalf(err.Error())
+	}
+
+	// Call getCiphertext.
+	addr := common.Address{}
+	environment.ethCall = true
+	readOnly := true
+	contractAddress := testContractAddress{}.Address()
+	// Change address to another one that doesn't contain the handle.
+	contractAddress[0]++
+	input := make([]byte, 0)
+	zeroPadding := make([]byte, 12)
+	signature := crypto.Keccak256([]byte("getCiphertext(address,uint256)"))[0:4]
+	input = append(input, signature...)
+	input = append(input, zeroPadding...)
+	input = append(input, contractAddress.Bytes()...)
+	input = append(input, ctHash.Bytes()...)
+	out, err := FheLibRun(environment, addr, addr, input, readOnly)
+	if err != nil {
+		t.Fatalf(err.Error())
+	}
+	if len(out) != 0 {
+		t.Fatalf("getCiphertext expected empty output on wrong contract address")
+	}
+}
+
+func FheLibGetCiphertext(t *testing.T, fheUintType FheUintType) {
+	environment := newTestEVMEnvironment()
+	pc := uint64(0)
+	depth := 1
+	environment.depth = depth
+	plaintext := uint64(2)
+	ct := verifyCiphertextInTestMemory(environment, plaintext, depth, fheUintType)
+	ctHash := ct.GetHash()
+	scope := newTestScopeConext()
+	loc := uint256.NewInt(10)
+	value := uint256FromBig(ctHash.Big())
+
+	// Setup and call SSTORE - it requires a location and a value to set there.
+	scope.pushToStack(value)
+	scope.pushToStack(loc)
+	_, err := OpSstore(&pc, environment, scope)
+	if err != nil {
+		t.Fatalf(err.Error())
+	}
+
+	// Call getCiphertext.
+	addr := common.Address{}
+	environment.ethCall = true
+	readOnly := true
+	input := make([]byte, 0)
+	zeroPadding := make([]byte, 12)
+	signature := crypto.Keccak256([]byte("getCiphertext(address,uint256)"))[0:4]
+	input = append(input, signature...)
+	input = append(input, zeroPadding...)
+	input = append(input, testContractAddress{}.Address().Bytes()...)
+	input = append(input, ctHash.Bytes()...)
+	out, err := FheLibRun(environment, addr, addr, input, readOnly)
+	if err != nil {
+		t.Fatalf(err.Error())
+	}
+	size, _ := GetExpandedFheCiphertextSize(fheUintType)
+	if size != uint(len(out)) {
+		t.Fatalf("getCiphertext returned ciphertext size of %d, expected %d", len(out), size)
+	}
+
+	outCt := new(TfheCiphertext)
+	err = outCt.Deserialize(out, fheUintType)
+	if err != nil {
+		t.Fatalf(err.Error())
+	}
+	decrypted, err := outCt.Decrypt()
+	if err != nil {
+		t.Fatalf(err.Error())
+	}
+	if decrypted.Uint64() != plaintext {
+		t.Fatalf("getCiphertext returned ciphertext value of %d, expected %d", decrypted.Uint64(), plaintext)
+	}
+}
+
+func TestFheLibGetCiphertext8(t *testing.T) {
+	FheLibGetCiphertext(t, FheUint8)
+}
+
+func TestFheLibGetCiphertext16(t *testing.T) {
+	FheLibGetCiphertext(t, FheUint16)
+}
+
+func TestFheLibGetCiphertext32(t *testing.T) {
+	FheLibGetCiphertext(t, FheUint32)
+}
+
+func TestFheLibGetCiphertext64(t *testing.T) {
+	FheLibGetCiphertext(t, FheUint64)
+}