AES-256-GCM暗号化・復号テスト

You are an experienced full-stack web developer. Build a web tool called "AES暗号化テストツール" that meets the requirements below.

## Goal

Build a tester that encrypts and decrypts text with AES-256-GCM using a password.

## Core features

- Encrypt and decrypt modes
- Derive the key from the password with PBKDF2 and a high iteration count
- Random salt and 12-byte IV for every encryption
- Output salt + IV + ciphertext as one Base64 string
- Copy result

## Inputs

- Plain text or Base64 ciphertext
- Password

## Outputs

- Base64 ciphertext or decrypted plain text
- Summary of algorithm and parameters

## Edge cases and handling rules

- Show 'authentication failed' for a wrong password or tampered ciphertext
- Reject empty passwords
- Explain that the same plain text encrypts differently each time
- Use the Web Crypto API so keys and text never leave the browser

## Implementation requirements

- Build it as a single page (HTML, CSS, JavaScript) that runs entirely in the browser, and never send the user's input to a server.
- Make the UI responsive and easy to use on mobile; label every input and keep everything keyboard accessible.
- Add a Copy button for results and show clear, friendly error messages for invalid input.
- Keep the core logic in functions separate from the UI so it can be unit-tested.

## Definition of done (test cases)

- Encrypt then decrypt with the same password returns the original
- Changing one character of the ciphertext makes decryption fail
- Encrypting the same input twice gives different output

First outline your implementation plan in at most five lines, then write the complete code, and finally explain how to check the test cases above.

---
name: build-encryption-tester
description: Use when building or changing a AES暗号化テストツール tool. Build a tester that encrypts and decrypts text with AES-256-GCM using a password.
---

# Build: AES暗号化テストツール

## Goal

Build a tester that encrypts and decrypts text with AES-256-GCM using a password.

## Workflow

1. Review the required features, inputs and outputs below and ask about anything missing first.
2. Implement the core logic as pure functions separate from the UI.
3. Check the logic against the test cases in the verification list.
4. Build a responsive UI with an input form, a result area and a Copy button.
5. Reproduce each edge case listed under Rules and refine the error messages.

## Core features

- Encrypt and decrypt modes
- Derive the key from the password with PBKDF2 and a high iteration count
- Random salt and 12-byte IV for every encryption
- Output salt + IV + ciphertext as one Base64 string
- Copy result

## Inputs

- Plain text or Base64 ciphertext
- Password

## Outputs

- Base64 ciphertext or decrypted plain text
- Summary of algorithm and parameters

## Rules

- Show 'authentication failed' for a wrong password or tampered ciphertext
- Reject empty passwords
- Explain that the same plain text encrypts differently each time
- Use the Web Crypto API so keys and text never leave the browser
- Build it as a single page (HTML, CSS, JavaScript) that runs entirely in the browser, and never send the user's input to a server.

## Verification

- [ ] Encrypt then decrypt with the same password returns the original
- [ ] Changing one character of the ciphertext makes decryption fail
- [ ] Encrypting the same input twice gives different output