Fixes #2337: External encryption method (#2367)

Signed-off-by: AbstractionFactory <179820029+abstractionfactory@users.noreply.github.com>

internal/encryption/method/external/testmethod/data/testmethod.go

@@ -0,0 +1,71 @@
+// Copyright (c) The OpenTofu Authors
+// SPDX-License-Identifier: MPL-2.0
+// Copyright (c) 2023 HashiCorp, Inc.
+// SPDX-License-Identifier: MPL-2.0
+
+package main
+
+import (
+	"encoding/json"
+	"io"
+	"log"
+	"os"
+)
+
+type Header struct {
+	Magic   string `json:"magic"`
+	Version int    `json:"version"`
+}
+
+type Input struct {
+	Key     []byte `json:"key,omitempty"`
+	Payload []byte `json:"payload"`
+}
+
+type Output struct {
+	// Payload is the payload that has been encrypted/decrypted by the external method.
+	Payload []byte `json:"payload"`
+}
+
+// main implements a simple XOR-encryption. This is meant as an example and not suitable for any production use.
+func main() {
+	// Write logs to stderr
+	log.Default().SetOutput(os.Stderr)
+
+	// Write header
+	header := Header{
+		"OpenTofu-External-Encryption-Method",
+		1,
+	}
+	marshalledHeader, err := json.Marshal(header)
+	if err != nil {
+		log.Fatalf("%v", err)
+	}
+	_, _ = os.Stdout.Write(append(marshalledHeader, []byte("\n")...))
+
+	// Read input
+	input, err := io.ReadAll(os.Stdin)
+	if err != nil {
+		log.Fatalf("Failed to read stdin: %v", err)
+	}
+	var inputData Input
+	if err = json.Unmarshal(input, &inputData); err != nil {
+		log.Fatalf("Failed to parse stdin: %v", err)
+	}
+
+	// Create output as an XOR of the key and input
+	outputPayload := make([]byte, len(inputData.Payload))
+	for i, b := range inputData.Payload {
+		outputPayload[i] = inputData.Key[i%len(inputData.Key)] ^ b
+	}
+
+	// Write output
+	output := Output{
+		Payload: outputPayload,
+	}
+	outputData, err := json.Marshal(output)
+	if err != nil {
+		log.Fatalf("Failed to stringify output: %v", err)
+	}
+	_, _ = os.Stdout.Write(outputData)
+}