StegoBox

An Android application that embeds arbitrary data — files, archives, or plain text — inside an image, protected with authenticated encryption.

The project is deliberately stated without exaggeration. It does not claim to be undetectable, and it does not claim that a hidden payload survives image re-processing. The limitations section below describes exactly what the tool does and does not provide.

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English

Overview

StegoBox packs a payload into a carrier image and restores it later. The payload is compressed, encrypted, and integrity-checked before it is stored, so extracting the container does not reveal its contents without the password.

Two carriers are available, with very different properties:

Carrier	Location of the payload	Capacity	Survives re-encoding
Tail-append	Appended after the image bytes	Unlimited, bounded only by file size	No
Pixel LSB	Least significant bits of the R/G/B channels	width x height x 3 / 8 bytes	No

Neither carrier is designed to resist image processing. Any re-encode, thumbnail generation, or platform transcode removes the payload. Payloads are therefore intended for lossless channels (direct file transfer, storage, local network).

Cryptography

All primitives are provided by Bouncy Castle through its lightweight API. No JCE provider is registered, which avoids conflicts with the provider Android ships.

• Ciphers: AES-256-GCM, ChaCha20-Poly1305, SM4-GCM, or no encryption
• Key derivation: PBKDF2-HMAC-SHA256, 200,000 iterations, random 16-byte salt
• Nonce: random 12 bytes per container
• Integrity: SHA-256 of the plaintext, in addition to the AEAD authentication tag

Key and receive modes

Key mode determines how the password is produced:

Key mode	Behaviour
Manual password	The user supplies the password; the receiver supplies it again
Random password	The application generates a 16-character password, which can be displayed or copied
No password	Encryption is disabled; only the SHA-256 integrity check remains

Receive mode determines how the receiver obtains the password:

Receive mode	Behaviour
Manual entry	The container is encrypted with the password and the receiver must enter it
Automatic entry	The password is stored inside the container. The receiver needs no input, but the container is not confidential: anyone holding the file can decrypt it

Container format (version 3)

magic        5 bytes   "SGBX3"
version      1 byte
carrier      1 byte    0 = tail-append, 1 = pixel LSB
cipher       1 byte    0 = none, 1 = AES-GCM, 2 = ChaCha20-Poly1305, 3 = SM4-GCM
keyMode      1 byte    0 = none, 1 = password, 2 = embedded
iterations   4 bytes   big-endian
salt        16 bytes
nonce       12 bytes
payloadLen   8 bytes   big-endian, length of the stored payload (ciphertext plus tag)
sha256      32 bytes   digest of the plaintext
nameLen      2 bytes   big-endian
name         n bytes   UTF-8; empty when the payload is a multi-file bundle
passwordLen  2 bytes   present only when keyMode = embedded
password     n bytes
payload      payloadLen bytes

Features

• Any payload is accepted and treated as a byte stream: documents, images, archives, applications
• Multiple files are packed into a ZIP archive with automatically de-duplicated entry names
• Restoration preserves the original file name for single files and the directory structure for bundles
• Payloads are processed as streams through a temporary file, so hiding or extracting a file of several hundred megabytes does not exhaust memory
• Pixel LSB output is analysed with a chi-square test and the estimated detectability is reported in the log, so the user is told how visible the embedding is rather than being reassured
• Output can be written to a chosen directory, or directly to Download/StegoBox when the all-files access permission has been granted
• A self-test screen exercises every cipher and key mode, including the tail-append and LSB paths

Building

Requirements: JDK 17, Android SDK with platform 35 and matching build tools, and the Gradle wrapper included in the repository (Gradle 8.9, Android Gradle Plugin 8.7.3).

./gradlew :app:assembleRelease

Release signing must be configured in app/build.gradle with a keystore of your own.

Note for restricted environments: the aapt2 binary bundled with the Android Gradle Plugin may fail to start inside containers without a compatible loader. In that case point the build at a working binary, for example:

android.aapt2FromMavenOverride=/usr/bin/aapt2

Limitations

• The container is not stealthy. The magic string and the payload are visible with a hex editor or strings, and the pixel LSB carrier is detectable with standard steganalysis.
• Payloads are removed by any re-encoding of the image, including thumbnailing and platform transcoding. A lossless channel is required.
• The pixel LSB carrier requires a lossless output format and is bounded by the image dimensions.
• The automatic-entry receive mode stores the password inside the container and therefore provides no confidentiality.
• A temporary file the size of the payload is written to the application cache during hiding and extraction. The cache partition must have sufficient free space.

Dependencies

• Bouncy Castle (org.bouncycastle:bcprov-jdk18on) for AES, ChaCha20, SM4, PBKDF2 and SHA-256
• Material Components for Android and AndroidX for the user interface
• Material Icons (Apache License 2.0) for the interface icons

License

Apache License 2.0 with an additional non-commercial condition — see LICENSE and NOTICE.

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中文

项目简介

StegoBox 是一个 Android 应用，可以把任意数据（文件、压缩包或纯文本）嵌入图片中，并使用可认证 的加密保护载荷。

本项目不做夸大表述：它不宣称"无法被检测"，也不宣称载荷能在图片被重新处理后存活。下文的 "局限性"一节准确说明了它能做什么、不能做什么。

载体

StegoBox 提供两种载体，特性差异很大：

载体	载荷位置	容量	抗重编码
尾部追加	追加在图片字节之后	无上限，仅受文件大小限制	否
像素 LSB	R/G/B 三个通道的最低有效位	宽 × 高 × 3 / 8 字节	否

两种载体都不具备抗图片处理能力：任何重新编码、缩略图生成或平台转码都会清除载荷。因此载荷 只适用于无损通道（直接文件传输、存储、局域网）。

密码学

所有密码学原语由 Bouncy Castle 的轻量级 API 提供，不注册 JCE provider，从而避免与 Android 内置的实现冲突。

• 算法：AES-256-GCM、ChaCha20-Poly1305、SM4-GCM，或不加密
• 密钥派生：PBKDF2-HMAC-SHA256，20 万次迭代，随机 16 字节盐
• 随机数：每个容器随机 12 字节 nonce
• 完整性：除 AEAD 认证标签外，另对明文计算 SHA-256

密钥模式与接收模式

密钥模式决定口令的来源：

密钥模式	行为
手写口令	由用户输入口令，接收方需要再次输入
随机口令	应用生成 16 位口令，可显示或复制
无口令	不加密，仅保留 SHA-256 完整性校验

接收模式决定接收方如何获得口令：

接收模式	行为
手动输入	容器使用口令加密，接收方必须输入口令
自动输入	口令保存在容器内，接收方无需输入；但容器不具备机密性，任何持有文件的人都能解密

容器格式（版本 3）

magic        5 字节   "SGBX3"
version      1 字节
carrier      1 字节   0 = 尾部追加，1 = 像素 LSB
cipher       1 字节   0 = 不加密，1 = AES-GCM，2 = ChaCha20-Poly1305，3 = SM4-GCM
keyMode      1 字节   0 = 无，1 = 口令，2 = 随文件保存
iterations   4 字节   大端
salt        16 字节
nonce       12 字节
payloadLen   8 字节   大端，存储载荷长度（密文 + 认证标签）
sha256      32 字节   明文的摘要
nameLen      2 字节   大端
name         n 字节   UTF-8；载荷为多文件包时为空
passwordLen  2 字节   仅当 keyMode 为"随文件保存"时存在
password     n 字节
payload      payloadLen 字节

功能

• 载荷一律按字节流处理：文档、图片、压缩包、应用均可
• 多文件自动打包为 ZIP，并自动处理重名条目
• 还原时，单文件保留原文件名，多文件保留原目录结构
• 载荷通过临时文件以流式方式处理，隐藏或提取数百 MB 的文件不会耗尽内存
• 像素 LSB 输出会经过卡方检验，并在日志中给出可检测性评估，如实告知嵌入的可见程度
• 输出可写入指定目录；授予"所有文件"权限后可直接写入 Download/StegoBox
• 内置自检界面，覆盖全部算法与密钥模式，以及两种载体

编译

需要 JDK 17、Android SDK（platform 35 及对应的 build tools），以及仓库内置的 Gradle Wrapper （Gradle 8.9、Android Gradle Plugin 8.7.3）。

./gradlew :app:assembleRelease

发布签名请在 app/build.gradle 中配置为你自己的 keystore。

受限环境提示：在缺少兼容加载器的容器中，Android Gradle Plugin 自带的 aapt2 可能无法启动。 此时可将构建指向可用的二进制文件，例如：

android.aapt2FromMavenOverride=/usr/bin/aapt2

局限性

• 容器不具备隐蔽性。魔数与载荷用十六进制编辑器或 strings 即可看到，像素 LSB 载体可被常规 隐写分析检出。
• 图片一旦被重新编码（包括生成缩略图、平台转码），载荷即被清除；必须使用无损通道。
• 像素 LSB 载体要求无损输出格式，且容量受图片尺寸限制。
• "自动输入"接收模式把口令保存在容器内，不具备机密性。
• 隐藏与提取过程中会在应用缓存目录写入与载荷等大的临时文件，请确保缓存分区有足够空间。

依赖

• Bouncy Castle（org.bouncycastle:bcprov-jdk18on）：AES、ChaCha20、SM4、PBKDF2、SHA-256
• Material Components for Android 与 AndroidX：界面
• Material Icons（Apache License 2.0）：界面图标

许可证

Apache License 2.0，附加非商业使用条件 —— 见 LICENSE 与 NOTICE。