muenc

A command-line tool for streaming authenticated file encryption with versioned, self-describing algorithm metadata.

Highlights

• Streams large files with bounded memory and 1 MiB buffers.
• Authenticates ciphertext before publishing any plaintext output.
• Writes through temporary files, so failures do not destroy an existing output and input may safely equal output.
• Detects whether a private key is encrypted and only then asks for its passphrase.
• Never accepts passphrases as command-line arguments, where they can leak through shell history or process listings.
• Supports RSA-OAEP-SHA256 + AES-256-GCM and X25519-HKDF-SHA256 + ChaCha20-Poly1305.
• Automatically selects encryption from the public-key type and decryption from the file header.
• Reads the original unversioned RSA/AES format produced by earlier releases.
• Keeps algorithms isolated under src/crypto/algorithms for future PQC support.

Build

cargo build --release

This produces the muenc binary (target\release\muenc.exe on Windows).

The project currently uses vendored OpenSSL. On Windows, ensure a native Windows Perl (for example Strawberry Perl) appears before MSYS Perl in PATH when compiling OpenSSL.

Usage

Generate a key pair:

muenc generate --output keys

If --output is omitted, keys are stored in ~/.muenc. On Unix, the private key is created with mode 0600. Existing keys are preserved unless replacement is explicitly authorized with --force; replacing a private key can make data encrypted with the old key unrecoverable.

RSA/AES remains the generation default for CLI compatibility. Select the modern X25519/ChaCha20 suite with:

muenc generate --output keys --algorithm x25519-chacha20-poly1305

The program securely prompts for a private-key passphrase. Press Enter at the first prompt to create an unencrypted private key; otherwise, enter it again for confirmation.

Encrypt a file:

muenc encrypt --input plain.txt --key keys\public_key.pem

If --key is omitted, encryption uses ~/.muenc/public_key.pem.

The default output is plain.txt.enc. Use --output - for stdout or --input - for stdin.

Decrypt a file:

muenc decrypt --input plain.txt.enc --key keys\private_key.pem

If --key is omitted, decryption uses ~/.muenc/private_key.pem.

The program inspects the private key. An unencrypted key needs no input; an encrypted key triggers a hidden passphrase prompt. The default output removes .enc, or appends .dec if that suffix is absent.

Run muenc --help or add --help after a subcommand for all options.

The public key determines the encryption suite; no algorithm argument is required for encryption. The versioned encrypted-file header determines the suite during decryption.

Versioned file format

Every preamble is 13 bytes. The legacy format is version zero, so its bytes remain exactly unchanged:

00 00 00 00 "MUENC" 00 00 [version: u16 big-endian]

Version 1 then stores:

[key-establishment: u16]
[data-cipher: u16]
[key-material-length: u32]
[nonce-length: u16]
[tag-length: u16]
[key-establishment material]
[nonce]
[ciphertext]
[authentication tag]

All integers are big-endian. The complete version-1 header—including algorithm identifiers, lengths, key material, and nonce—is authenticated as AEAD associated data.

IDs	Encryption suite
key 1, cipher 1	RSA-OAEP-SHA256 + AES-256-GCM
key 2, cipher 2	X25519-HKDF-SHA256 + ChaCha20-Poly1305

Both version-1 suites currently use a 12-byte nonce and a 16-byte authentication tag.

Version 0 is still parsed as [legacy magic][RSA-OAEP-SHA1 wrapped key][16-byte nonce][ciphertext][16-byte tag], allowing existing files to decrypt without conversion.

Architecture

• cli: command parsing and secure passphrase prompts.
• application: key generation and streaming encrypt/decrypt use cases.
• crypto: suite interfaces, registry, shared OpenSSL streaming, and isolated algorithm implementations.
• file_format: compatibility-critical format parsing and writing.
• io: buffered input, progress, and transactional output.

Additional algorithms implement EncryptionSuite and register an ID pair. The versioned header already provides the dispatch point needed for later PQC key-establishment algorithms.

Security behavior

• A wrong key, damaged header or ciphertext, or modified authentication tag returns a normal error and leaves the destination untouched.
• Decryption to stdout is also staged until authentication succeeds.
• Private keys receive mode 0600 on Unix.
• Losing a private key or its passphrase makes its encrypted files unrecoverable.

Development

Unit tests live beside each module and cover version parsing, algorithm dispatch, CLI safety, key detection, path rules, and transactional I/O. End-to-end tests cover both suites, authenticated headers, passphrases, and legacy compatibility.

cargo fmt --check
cargo clippy --all-targets -- -D warnings
cargo test

License

See LICENSE.