📜 man page   📑 Specification   📄 Input Options   📖 Tutorial   ❓ FAQ

 Table of Contents

• About
• Goals

• Usage
• Input Options
• Debug Mode

• Payload
• Input Keying Material

• Cryptographic Primitives
• Encrypted Data Format

• Low Observability and Minimizing Metadata

• Hidden File System and Container Format
• Storing and Carrying Concealed Encrypted Data

• Time-Lock Encryption

• Tradeoffs and Limitations
• Warnings

• Requirements
• Installation

• TODO
• Feedback

About

tird (an acronym for "this is random data") is a file encryption tool focused on

• minimizing metadata and
• hiding encrypted data.

With tird, you can:

1. Create files filled with random data to use as containers or keyfiles.
2. Overwrite the contents of block devices and regular files with random data. This can be used to prepare containers and to destroy residual data.
3. Encrypt file contents and comments with modern cryptographic primitives. The encrypted data format (called cryptoblob) is a padded uniform random blob (PURB): it looks like random data and has a randomized size. This reduces metadata leakage from file format and length, and also allows cryptoblobs to be hidden among random data. You can use keyfiles and passphrases at your choice to enhance security.
4. Create steganographic (hidden, undetectable) user-driven file systems inside container files and block devices. Unlike VeraCrypt and Shufflecake containers, tird containers do not contain headers at all; the user specifies the location of the data in the container and is responsible for ensuring that this location is separated from the container.
5. Prevent or resist coercive attacks (keywords: key disclosure law, rubber-hose cryptanalysis, xkcd 538). tird provides some forms of plausible deniability out of the box, even if you encrypt files without hiding them in containers.

The author pronounces tird as /tɪrd/ (where /tɪrd/ sounds like тирд in Russian).

Goals

• File Protection: Ensuring protection for individual files, including:
  • Symmetric encryption and authentication.
  • Minimizing metadata leakage.
  • Preventing access to data in cases of user coercion.
  • Plausible deniability of payload existence.
  • Hiding encrypted data.
• Stable Format: Ensuring a stable encryption format with no cryptographic agility for long-term data storage.
• Simplicity: Ensuring simplicity and avoiding feature creep: refusal to implement features that are not directly related to primary security goals.

Usage

You don't need to memorize command-line options to use tird. This tool features a prompt-based CLI: simply start it, select a menu option, and answer the questions that will follow.

$ tird

                       MENU
    ———————————————————————————————————————————
    0. Exit              1. Info & Warnings
    2. Encrypt           3. Decrypt
    4. Embed             5. Extract
    6. Encrypt & Embed   7. Extract & Decrypt
    8. Create w/ Random  9. Overwrite w/ Random
    ———————————————————————————————————————————
A0. Select an option [0-9]:

Input Options

There are 5 groups of input options: A (Action), C (Custom), D (Data), K (Keys), P (Proceed). They are numbered for ease of description.

+——————————————————————————+————————————————————————+
| A0. Select an option     | A. Select an action    |
+——————————————————————————+————————————————————————+
| C0. Use custom settings? |                        |
| C1. Time cost            | C. Set custom settings |
| C2. Max padding size     |                        |
| C3. Set fake MAC tag?    |                        |
+——————————————————————————+————————————————————————+
| D1. Input file path      |                        |
| D2. Comments             | D. Enter data,         |
| D3. Output file path     |    data location,      |
| D4. Output file size     |    data size           |
| D5. Start position       |                        |
| D6. End position         |                        |
+——————————————————————————+————————————————————————+
| K1. Keyfile path         | K. Specify input       |
| K2. Passphrase           |    keying material     |
+——————————————————————————+————————————————————————+
| P0. Proceed?             | P. Confirm to continue |
+——————————————————————————+————————————————————————+

A detailed description of these options with examples can be found here.

Debug Mode

Start tird with the --debug option to look under the hood while the program is running:

$ tird --debug

Enabling debug messages additionally shows:

• File operations:
  • Opening and closing of file descriptors.
  • Real paths to opened files.
  • Movement of file pointers.
• Byte strings related to cryptographic operations: salts, passphrases, digests, keys, nonces, and tags.
• Some other information, including various sizes.

Payload

The payload that will be encrypted during cryptoblob creation consists of:

• Contents of one file: This may be a regular file or a block device (an entire disk or partition). Maximum size: 16 exbibytes minus 864 bytes.
• Comments (optional): An arbitrary string of up to 512 bytes. Decrypted comments will be displayed during decryption.

Specifying the payload in the UI looks as follows:

D1. File to encrypt: foo
    I: path: 'foo'; size: 1 B
D2. Comments (optional, up to 512 B): foo file, secret data
    I: comments will be shown as ['foo file, secret data']

Input Keying Material

tird provides the option to use passphrases and the contents of keyfiles to derive one-time keys.

• Keyfiles: Specify none, one, or multiple keyfile paths. A keyfile path may be:
  • A regular file. The contents of the keyfile will be hashed, and its digest will be used for further key stretching and key derivation.
  • A block device. Handled the same as a regular keyfile: contents will be hashed.
  • A directory. All files within the directory will be hashed and used as keyfiles.
• Passphrases: Specify none, one, or multiple passphrases of up to 2048 bytes.

The order of input does not matter.

Specifying IKM in the UI looks as follows:

K1. Keyfile path (optional): foo
    I: path: 'foo'; size: 1 B
    I: reading and hashing contents of 'foo'
    I: keyfile accepted
K1. Keyfile path (optional):
K2. Passphrase (optional):
K2. Confirm passphrase:
    I: passphrase accepted

Cryptographic Primitives

The following cryptographic primitives are utilized by tird:

• ChaCha20 cipher (RFC 8439) for data encryption.
• BLAKE2 (RFC 7693) for hashing and authentication.
• Argon2 memory-hard function (RFC 9106) for key stretching and key derivation.

For more details, refer to the specification.

Encrypted Data Format

The format of the encrypted data is quite simple and consists of ciphertext with a MAC tag, located somewhere among the surrounding random data:

+—————————————+————————————+—————————+—————————————+
| Random data | Ciphertext | MAC tag | Random data |
+—————————————+————————————+—————————+—————————————+
|               Random-looking data                |
+——————————————————————————————————————————————————+

 Show more detailed scheme

+————————————————————————————————————————————————————+
| CSPRNG output:                                     |
|     Salt for key stretching used with Argon2, 16 B |
+————————————————————————————————————————————————————+
| CSPRNG output:                                     |
|     Randomized padding (header padding): 0-20% of  |
|     the (unpadded size + 255 B) by default         |
+————————————————————————————————————————————————————+
| ChaCha20 output:                                   |
|     Ciphertext, 512+ B, consists of:               |
|     - Encrypted constant-padded comments, 512 B    |
|     - Encrypted payload file contents, 0+ B        |
+————————————————————————————————————————————————————+
| BLAKE2 or CSPRNG output:                           |
|     MAC tag or Fake MAC tag, 64 B                  |
+————————————————————————————————————————————————————+
| CSPRNG output:                                     |
|     Randomized padding (footer padding): 0-20% of  |
|     the (unpadded size + 255 B) by default         |
+————————————————————————————————————————————————————+
| CSPRNG output:                                     |
|     Salt for prehashing IKM used with BLAKE2, 16 B |
+————————————————————————————————————————————————————+

Data encrypted with tird cannot be distinguished from random data without knowledge of the keys. It also does not contain identifiable headers. tird produces cryptoblobs that contain bilateral randomized padding with uniform random data (PURBs). This minimizes metadata leaks from the file format and makes it possible to hide cryptoblobs among other random data. Bilateral padding also conceals the exact location of the ciphertext and MAC tag within the cryptoblob.

Low Observability and Minimizing Metadata

While the content of an encrypted message is protected, its size, its provenance, its destination… are not. Data is hidden, metadata is shown.

— Loup Vaillant

Vs.

• PURB format:
  • Encrypted files look like random data.
  • Encrypted files have a randomized size: do not reveal the payload size.
• Do not prove that the entered keys are incorrect.
• Prompt-based CLI: no leakage of used options through shell history.
• The output file path is user-defined and is not related to the input file path by default.
• Optional: hiding encrypted data in containers.

Hidden File System and Container Format

tird employs a technique that is described as follows:

Concealing data within encrypted data or within random data. The message to conceal is encrypted, then used to overwrite part of a much larger block of encrypted data or a block of random data (an unbreakable cipher like the one-time pad generates ciphertexts that look perfectly random without the private key).

You can encrypt files and embed cryptoblobs into containers starting at arbitrary positions. After writing the cryptoblob, you will need to remember its location in the container (the starting and ending positions), which will be used later to extract the cryptoblobs. In this way, you can create a hidden, headerless, user-driven file system inside a container:

• It is hidden because it is impossible to distinguish between random container data and cryptoblob data, as well as to determine the location of written cryptoblobs without knowing the positions and keys.
• It is headerless because containers do not contain any headers; all data about cryptoblob locations must be stored separately by the user.
• The starting position of the cryptoblob in the container is user-defined, and the user must store both the starting and ending positions separately from the container. This is why it is called a user-driven file system.

Any file, disk, or partition larger than the minimum cryptonlob size (863 B) can be a valid container. Cryptoblobs can be embedded into any area.

Examples of Valid Containers Include:

1. Specially generated files with random data.
2. tird cryptoblobs, as they contain unauthenticated padding of random data by default, which can be used to embed smaller cryptoblobs.
3. Disk areas containing random data. For example, you can overwrite a disk with random data, format it in FAT32 or exFAT, and use a large portion of the disk, leaving a few dozen MB from the beginning. The disk will appear empty unless you add some files to it.
4. LUKS encrypted volumes.
5. VeraCrypt containers, even those that already contain hidden volumes.

Example of Container Structure:

+—————————+—————————————+ <— Position 0 of the container
|         |             |
|         | Random data |
|         |             |
|         +—————————————+ <— Cryptoblob1 start position
| Header- |             |
| less    | Cryptoblob1 |
|         |             |
| Layer   +—————————————+ <— Cryptoblob1 end position
|         | Random data |
| Cake    +—————————————+ <— Cryptoblob2 start position
|         |             |
|         | Cryptoblob2 |
|         |             |
|         +—————————————+ <— Cryptoblob2 end position
|         | Random data |
+—————————+—————————————+

Visualization of Embedding

The next image visualizes how hard it is to distinguish one random data entry from another and the process of embedding cryptoblobs in a container.

 Show Images

Empty container with random data:

One cryptoblob embedded in the container:

Two cryptoblobs embedded in the container:

Three cryptoblobs embedded in the container:

Animation: visualization of embedding:

Storing and Carrying Concealed Encrypted Data

Please look at the following screenshot.

It looks like this 16 GB volume contains only one 8.7 MiB file. Is it really true? Maybe yes, maybe no.

The file system tells us that there is only one file here. But is there really only one file on the volume? We cannot determine this using the file system. In fact, data may be located outside the file system and be undetectable by file system tools. The 15.2 GiB of space marked as free may be occupied by a hidden file system. This "free" space may be taken up by hidden encrypted data.

Can we disprove the existence of this data? Yes, for example, by examining the entropy level of this free space using binwalk. Low entropy indicates a likely absence of hidden data. High entropy does not, by itself, prove the presence of encrypted hidden data. Areas with high entropy can be either just residual data or hidden encrypted data.

If you are interested in hiding data outside the visible file system, then tird is at your service to provide an Invisibility Cloak for your files.

Time-Lock Encryption

Time-lock encryption (TLE) can be used to prevent an adversary from quickly accessing plaintexts in the event of an IKM compromise (in case of user coercion, for example). In our implementation, it is actually a PoW-based time-lock key derivation. The "Time cost" input option specifies the number of Argon2 passes. If you specify a sufficiently high number of passes, it will take a significant amount of time to perform them. However, an attacker will require the same amount of time when using similar hardware. The execution of Argon2 cannot be accelerated through parallelization, so it is expected that the time spent by an attacker will be approximately the same as that spent by the defender.

This TLE implementation works offline, unlike tlock.

Use custom options and set the desired "Time cost" value:

C0. Use custom settings? (Y/N, default=N): y
    I: use custom settings: True
    W: decryption will require the same [C1] and [C2] values!
C1. Time cost (default=4): 1000000
    I: time cost: 1,000,000

Plausible TLE: The adversary does not know the actual value of the time cost, so you can plausibly misrepresent the number of passes. The adversary cannot refute your claim until they attempt to decrypt the cryptoblob using the specified time cost value.

Tradeoffs and Limitations

• tird does not support:
  • Public-key cryptography.
  • File compression.
  • ASCII armored output.
  • Reed–Solomon error correction.
  • Splitting the output into chunks.
  • The use of standard streams for processing files.
  • Low-level block device reading and writing on MS Windows. As a result, these devices cannot be used as keyfiles, cannot be overwritten, and cannot be encrypted or embedded.
• tird does not provide:
  • A graphical user interface.
  • A password generator.
• tird cannot handle (encrypt/embed) more than one file in one pass. Encryption of directories and multiple files is not supported.
• tird does not fake file access, modification, and creation timestamps (atime, mtime, ctime).
• tird's encryption speed is not very high (up to 420 MiB/s in my tests).

Warnings

Crypto can help, but it won’t save you from misuse, vulnerabilities, social engineering, or physical threats.

— Loup Vaillant

• ⚠️ The author does not have a background in cryptography.
• ⚠️ The code has 0% test coverage.
• ⚠️ tird has not been independently audited.
• ⚠️ tird is ineffective in a compromised environment; executing it in such cases may cause disastrous data leaks.
• ⚠️ tird is unlikely to be effective when used with short and predictable keys.
• ⚠️ Sensitive data may leak into swap space.
• ⚠️ tird does not erase its sensitive data from memory after use.
• ⚠️ tird always releases unverified plaintext, violating the Cryptographic Doom Principle; decrypted output is untrusted until the MAC tag is verified.
• ⚠️ Padding contents are never authenticated; authentication only applies to the ciphertext, salts, and certain sizes.
• ⚠️ Padding sizes depend on secret values.
• ⚠️ tird does not sort digests of keyfiles and passphrases in constant-time.
• ⚠️ Overwriting file contents does not guarantee secure destruction of data on the media.
• ⚠️ You cannot prove to an adversary that your random data does not contain encrypted information.
• ⚠️ tird protects data, not the user; it cannot prevent torture if you are under suspicion.
• ⚠️ Development is not complete, and there may be backward compatibility issues.

Requirements

• Python >= 3.9.2
• cryptography >= 2.1 (provides ChaCha20)
• PyNaCl >= 1.2.0 (provides Argon2 and BLAKE2)

Installation

Installation from PyPI

Install python3 and python3-pip (or python-pip), then run

$ pip install tird

Building and Installing the Package on Debian-based Linux Distros

It's easy to build a deb package for Debian and Ubuntu-based distros with the latest git snapshot.

1. Install the build dependencies:

$ sudo apt install make fakeroot

2. Clone the repository (if git is already installed) and enter the directory:

$ git clone https://github.com/hakavlad/tird.git && cd tird

3. Build the package:

$ make build-deb

4. Install or reinstall the package:

$ sudo make install-deb

Standalone Executables

Standalone executables (made with PyInstaller) are also available (see Releases) for Windows and Linux (amd64). Please use at your own risk.

 How to Verify Signatures

Use Minisign to verify signatures. You can find my public key here.

For example:

$ minisign -Vm  tird-v0.19.0-linux-amd64.zip -P RWQLYkPbRQ8b56zEe8QdbjLFqC9UrjOaYxW5JxwsWV7v0ct/F/XfJlel

This requires the signature tird-v0.19.0-linux-amd64.zip.minisig to be present in the same directory.

TODO

Write or improve the documentation:

• Features
• User Guide
• Specification
• Design Rationale

Feedback

Please feel free to ask questions, leave feedback, or provide critiques in the Discussions section.