Module crypt 

The core of this program. Encrypt/decrypt, compress/decompress files.

Module Structure

Module	Contents
[header]	Constants (MAGIC, VERSION, SALT_LEN, …) and FileHeader
[key]	Key derivation (Argon2, key splitting, nonce derivation) + key cache
[stream]	Streaming Read → Write encrypt/decrypt primitives
file	File-to-file encrypt/decrypt with atomic writes & metadata preservation
[batch]	Parallel batch operations with shared key cache
[repo]	Repository-level encrypt/decrypt with salt cache integration

See the module-level docs of each submodule for details.

Nonce Derivation (Content-Based with File ID)

Per-chunk nonces are derived from the file’s random File_ID and the chunk’s own plaintext content using keyed Blake3:

1. A random 16-byte File_ID is generated once per file and stored in the header. This ensures that even if two different files have identical plaintext at chunk 0, they produce different nonces and ciphertexts.
2. The Argon2-derived master key is split via blake3::derive_key into Key_ENC (for XChaCha20-Poly1305 encryption) and Key_MAC (for nonce generation).
3. For each chunk i: Nonce_i = Blake3_keyed(Key_MAC, File_ID || M_i || chunk_idx_le)[0..24]
4. The 24-byte nonce is stored in plaintext at the head of each encrypted chunk.

Different plaintext always produces a different nonce (within the same file). The File_ID ensures cross-file uniqueness. The chunk index prevents reordering attacks on identical 64 KB blocks.

Authenticated Additional Data (AAD)

Each chunk’s AAD binds the ciphertext to the full file header so that any tampering with header fields (version, compression flag, salt, file_id, reserved) is detected via Poly1305 authentication failure:

AAD = HEADER (64B) || chunk_idx (8B LE) || is_last_chunk (1B)   // 73 bytes

Each encrypted chunk layout: [NONCE (24B)] [CIPHERTEXT] [TAG (16B)]

Structs

BatchSummary

Summary of a batch encrypt/decrypt run.

FileHeader

Constants

FILE_ID_LEN

HEADER_LEN

MAGIC

NONCE_LEN

SALT_LEN

VERSION

Functions

cache_key

Compute a repo-relative cache key from a file path.

decrypt_file

Decrypt a single file in place.

decrypt_file_to

Decrypt src into dst.

decrypt_file_with_cache

Decrypt a single file with a thread-safe Argon2 key cache and optional salt/file_id cache.

decrypt_into

Decrypt data from reader into writer.

decrypt_repo

Decrypt given files in the repo.

derive_key

encrypt_file

Encrypt a single file in place.

encrypt_file_to

Encrypt src into dst.

encrypt_into

Encrypt data from reader into writer using streaming chunked encryption.

encrypt_repo

Encrypt given files in the repo.

is_encrypted_version