Add sig strip command

sig/Cargo.toml

@@ -4,3 +4,6 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
+aes = "0.8.4"
+cbc = "0.1.2"
+clap = { version = "4.5.37", features = ["derive"] }

sig/src/cli.rs

@@ -0,0 +1,57 @@
+//! Clap command and argument definitions.
+//!
+//! This module is deliberately limited to command-line shape: subcommand names,
+//! flags, positional arguments, and user-facing help text. It does not know how
+//! to parse or decrypt `.sig` files. Keeping those responsibilities out of the
+//! CLI layer makes the format code easier to test and reuse from future commands.
+
+use std::path::PathBuf;
+
+use clap::{Args, Parser, Subcommand};
+
+/// Top-level command-line parser for the `sig` binary.
+///
+/// `clap` derives `--help`, `--version`, shell-friendly error messages, and the
+/// dispatch table for all subcommands from this struct. New subcommands should
+/// be added to [`Command`] rather than handled manually in `main`.
+#[derive(Parser)]
+#[command(version, about = "Interact with Centauri Carbon 2 .sig packages")]
+pub struct Cli {
+    /// The action to run.
+    #[command(subcommand)]
+    pub command: Command,
+}
+
+/// Supported subcommands.
+///
+/// The tool currently has one operation: `strip`. The enum layout leaves room
+/// for future package inspection, signing, or repacking commands without
+/// changing the top-level parser API.
+#[derive(Subcommand)]
+pub enum Command {
+    /// Strip the .sig wrapper and write the unpacked payload.
+    Strip(StripArgs),
+}
+
+/// Arguments accepted by the `strip` subcommand.
+///
+/// The command mirrors the browser-based unpacker at
+/// <https://docs.opencentauri.cc/extras/cc2_update_decrypt.html>: read a `.sig`
+/// file, remove the header, decrypt the payload when needed, trim it to the
+/// payload size declared in the header, then write the resulting package bytes.
+#[derive(Args)]
+pub struct StripArgs {
+    /// Input `.sig` file.
+    ///
+    /// The file must begin with the Centauri/Elegoo `ELEG` magic value and must
+    /// contain the 512-byte header used by Carbon 2 update packages.
+    pub input: PathBuf,
+
+    /// Output path.
+    ///
+    /// When omitted, the default output is the input filename with a trailing
+    /// `.sig` extension removed. For example, `update.swu.sig` writes
+    /// `update.swu`.
+    #[arg(short, long)]
+    pub output: Option<PathBuf>,
+}

sig/src/crypto.rs

@@ -0,0 +1,157 @@
+//! Centauri Carbon 2 `.sig` package parsing and AES-CBC decryption.
+//!
+//! The Carbon 2 update `.sig` container starts with a fixed 512-byte header.
+//! The header begins with the big-endian magic value `ELEG`, carries a package
+//! type byte whose high bit marks encrypted payloads, stores the unwrapped
+//! payload size as a little-endian `u64`, and stores the AES-CBC IV at offset
+//! `0xA0`. The wrapped payload starts immediately after the header.
+//!
+//! This module implements the same unpacking behavior as the public web tool at
+//! <https://docs.opencentauri.cc/extras/cc2_update_decrypt.html>. It does not
+//! attempt to validate signatures or hashes yet; it only removes the wrapper and
+//! decrypts encrypted payloads.
+
+use std::error::Error;
+
+use aes::Aes256;
+use cbc::cipher::{BlockDecryptMut, KeyIvInit, block_padding::NoPadding};
+
+/// Length of the fixed `.sig` header in bytes.
+pub const HEADER_LEN: usize = 512;
+
+/// Big-endian `ELEG` magic value found at the beginning of supported packages.
+const MAGIC: u32 = 0x454C4547;
+
+/// AES-256-CBC key used by Carbon 2 update packages.
+///
+/// This key is taken from the referenced browser implementation. The IV is not
+/// fixed; each package supplies its IV in the `.sig` header at bytes
+/// `0xA0..0xB0`.
+const AES_KEY: [u8; 32] = [
+    0xD1, 0x4E, 0x15, 0x08, 0x43, 0xE9, 0xD1, 0x68, 0x93, 0x89, 0x07, 0x56, 0xD8, 0xF7, 0x7F, 0x67,
+    0x4E, 0x16, 0x1A, 0x8B, 0xEB, 0xB8, 0xF7, 0x20, 0x73, 0x7E, 0xE6, 0x0E, 0x7F, 0x8C, 0x7E, 0x68,
+];
+
+type Aes256CbcDec = cbc::Decryptor<Aes256>;
+
+/// Parsed subset of a Carbon 2 `.sig` header.
+///
+/// The header contains additional metadata such as package type, version,
+/// encrypted byte ranges, and SHA-256 material. `strip` only needs the fields
+/// represented here: whether the payload is encrypted, how many bytes should be
+/// emitted after unwrap, and which IV to use for AES-CBC.
+#[derive(Debug)]
+struct Header {
+    /// Whether the high bit of the package type byte is set.
+    is_encrypted: bool,
+
+    /// Number of plaintext payload bytes to write after removing encryption and
+    /// any trailing block padding.
+    filesize: usize,
+
+    /// Per-package AES-CBC initialization vector.
+    iv: [u8; 16],
+}
+
+/// Unpack a complete `.sig` file held in memory.
+///
+/// The input must include both the 512-byte header and the wrapped payload. The
+/// returned bytes are suitable to write directly as the stripped output package,
+/// such as a `.swu`, `.zip`, or `.json` file.
+///
+/// For encrypted packages, this function decrypts the bytes after the header
+/// with AES-256-CBC and then truncates the plaintext to the `filesize` recorded
+/// in the header. For unencrypted packages, it simply copies exactly `filesize`
+/// bytes from the post-header payload.
+pub fn unpack_sig(raw: &[u8]) -> Result<Vec<u8>, Box<dyn Error>> {
+    if raw.len() < HEADER_LEN {
+        return Err("input is too small to contain a .sig header".into());
+    }
+
+    let header = parse_header(&raw[..HEADER_LEN])?;
+    let payload = &raw[HEADER_LEN..];
+
+    if header.is_encrypted {
+        decrypt_payload(payload, &header)
+    } else {
+        strip_plain_payload(payload, header.filesize)
+    }
+}
+
+/// Parse the fixed-width `.sig` header.
+///
+/// Offsets are based on the browser implementation:
+///
+/// - `0x00..0x04`: big-endian `ELEG` magic.
+/// - `0x04`: package type; bit `0x80` indicates encryption.
+/// - `0x08..0x10`: little-endian plaintext payload size.
+/// - `0xA0..0xB0`: AES-CBC IV.
+fn parse_header(header: &[u8]) -> Result<Header, Box<dyn Error>> {
+    if header.len() < HEADER_LEN {
+        return Err("header must be at least 512 bytes".into());
+    }
+
+    let magic = u32::from_be_bytes(header[0..4].try_into()?);
+    if magic != MAGIC {
+        return Err(format!("invalid .sig magic: 0x{magic:08x}").into());
+    }
+
+    let package_type = header[0x04];
+    let is_encrypted = package_type & 0x80 != 0;
+    let filesize = u64::from_le_bytes(header[0x08..0x10].try_into()?)
+        .try_into()
+        .map_err(|_| "payload filesize does not fit on this platform")?;
+    let iv = header[0xA0..0xB0].try_into()?;
+
+    Ok(Header {
+        is_encrypted,
+        filesize,
+        iv,
+    })
+}
+
+/// Return the plaintext payload for packages that are marked unencrypted.
+///
+/// Even unencrypted packages still include the `.sig` header. The header's
+/// `filesize` field defines how many bytes after the header are part of the
+/// actual package payload.
+fn strip_plain_payload(payload: &[u8], filesize: usize) -> Result<Vec<u8>, Box<dyn Error>> {
+    if filesize > payload.len() {
+        return Err(format!(
+            "header payload size {} exceeds payload size {}",
+            filesize,
+            payload.len()
+        )
+        .into());
+    }
+
+    Ok(payload[..filesize].to_vec())
+}
+
+/// Decrypt and trim an encrypted `.sig` payload.
+///
+/// The browser version appends an extra encrypted PKCS#7 padding block to work
+/// around Web Crypto's required padding behavior. Rust's `cbc` crate can decrypt
+/// without padding, so this implementation decrypts the payload as-is and then
+/// trims to the plaintext length declared in the header.
+fn decrypt_payload(payload: &[u8], header: &Header) -> Result<Vec<u8>, Box<dyn Error>> {
+    if payload.len() % 16 != 0 {
+        return Err("encrypted payload length is not a multiple of the AES block size".into());
+    }
+
+    let mut buffer = payload.to_vec();
+    let decrypted = Aes256CbcDec::new(&AES_KEY.into(), &header.iv.into())
+        .decrypt_padded_mut::<NoPadding>(&mut buffer)
+        .map_err(|_| "AES-CBC decryption failed")?;
+
+    if header.filesize > decrypted.len() {
+        return Err(format!(
+            "header payload size {} exceeds decrypted payload size {}",
+            header.filesize,
+            decrypted.len()
+        )
+        .into());
+    }
+
+    Ok(decrypted[..header.filesize].to_vec())
+}

sig/src/main.rs

@@ -1,3 +1,61 @@
-fn main() {
+//! Command-line entry point for the `sig` utility.
-    println!("Hello, world!");
+//!
+//! The binary intentionally keeps very little logic here. Clap-owned command
+//! definitions live in [`cli`], while the Centauri Carbon 2 `.sig` format and
+//! cryptographic details live in [`crypto`]. That split keeps argument parsing
+//! separate from file-format handling so future commands can reuse the same
+//! unpacking code without duplicating CLI concerns.
+
+use std::{error::Error, fs, path::PathBuf};
+
+use clap::Parser;
+
+mod cli;
+mod crypto;
+
+#[cfg(test)]
+mod tests;
+
+use cli::{Cli, Command};
+
+fn main() -> Result<(), Box<dyn Error>> {
+    let cli = Cli::parse();
+
+    match cli.command {
+        Command::Strip(args) => strip(args)?,
+    }
+
+    Ok(())
+}
+
+/// Execute the `strip` subcommand.
+///
+/// `strip` removes the 512-byte `.sig` wrapper and writes the contained package
+/// bytes. For encrypted packages, [`crypto::unpack_sig`] performs the AES-CBC
+/// decryption before returning the payload.
+fn strip(args: cli::StripArgs) -> Result<(), Box<dyn Error>> {
+    let raw = fs::read(&args.input)?;
+    let unpacked = crypto::unpack_sig(&raw)?;
+    let output_path = args
+        .output
+        .unwrap_or_else(|| default_output_path(&args.input));
+
+    fs::write(&output_path, unpacked)?;
+    println!("wrote {}", output_path.display());
+
+    Ok(())
+}
+
+/// Derive the default output path used by the web unpacker.
+///
+/// A conventional input like `firmware.zip.sig` becomes `firmware.zip`. If the
+/// input does not use the `.sig` extension, the tool still writes beside the
+/// original file but changes the extension to `.decrypted` to avoid overwriting
+/// the input by accident.
+fn default_output_path(input: &std::path::Path) -> PathBuf {
+    if input.extension().is_some_and(|ext| ext == "sig") {
+        input.with_extension("")
+    } else {
+        input.with_extension("decrypted")
+    }
 }

sig/src/tests.rs

@@ -0,0 +1,76 @@
+//! Tests for the Carbon 2 `.sig` unpacking path.
+
+/// Encrypted `ota-package-list.json.sig` fixture bytes.
+///
+/// The bytes are embedded directly so the test does not depend on external test
+/// fixture files being present at runtime.
+const ENCRYPTED_OTA_PACKAGE_LIST: &[u8] = &[
+    0x45, 0x4c, 0x45, 0x47, 0x83, 0x01, 0x02, 0x00, 0xaf, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x6f, 0x74, 0x61, 0x2d, 0x70, 0x61, 0x63, 0x6b, 0x61, 0x67, 0x65, 0x2d, 0x6c, 0x69, 0x73, 0x74,
+    0x2e, 0x6a, 0x73, 0x6f, 0x6e, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xb0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x90, 0x8e, 0x89, 0x12, 0x45, 0xea, 0x37, 0x14, 0xad, 0x5b, 0x8e, 0x5f, 0xa9, 0xcf, 0xbb, 0xc9,
+    0xb0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+    0xbf, 0x17, 0x4e, 0xde, 0xae, 0x8d, 0x1e, 0x00, 0xe2, 0xdd, 0x44, 0x99, 0x42, 0x32, 0x8e, 0x1a,
+    0x79, 0xba, 0x2e, 0x50, 0x3d, 0x16, 0x28, 0xaa, 0x50, 0xf8, 0x2d, 0xaf, 0x35, 0x4d, 0x1c, 0xec,
+    0x5d, 0xe0, 0x8d, 0x4c, 0x80, 0x44, 0xda, 0xc7, 0x63, 0x5c, 0xa7, 0x0f, 0x99, 0xe3, 0x81, 0xb8,
+    0x27, 0xef, 0xd2, 0x6f, 0xc0, 0x49, 0x91, 0x52, 0xa1, 0x0c, 0x32, 0xea, 0xc1, 0x5c, 0xb8, 0xd1,
+    0xb8, 0xe4, 0x65, 0x97, 0x4e, 0x4f, 0x2c, 0x7d, 0x8c, 0xb4, 0xf2, 0x65, 0x92, 0xbc, 0x90, 0x97,
+    0x6d, 0xc1, 0xc1, 0x13, 0xa0, 0xf4, 0xd1, 0xe4, 0xfe, 0x28, 0x77, 0xbc, 0xfe, 0x1f, 0x90, 0x52,
+    0x72, 0x03, 0x56, 0xc0, 0xce, 0xbb, 0x63, 0x0d, 0xf2, 0x52, 0x81, 0x52, 0xaa, 0xb9, 0xc3, 0x59,
+    0xf2, 0x58, 0xa8, 0xb6, 0xb1, 0x12, 0xcd, 0x5d, 0x99, 0x11, 0x85, 0x80, 0x2e, 0xa7, 0x1d, 0x84,
+    0xff, 0x94, 0x4f, 0xc7, 0x94, 0x14, 0xe2, 0x48, 0xb8, 0x50, 0xc8, 0xff, 0xec, 0x23, 0xf3, 0xdb,
+    0x11, 0x9c, 0xca, 0xc3, 0x6a, 0xd1, 0x42, 0xbc, 0x5b, 0x4b, 0x8e, 0xcf, 0xb3, 0x74, 0x1f, 0x34,
+    0x80, 0xad, 0xa6, 0x39, 0x98, 0x15, 0x30, 0x31, 0xca, 0x21, 0x03, 0x67, 0x69, 0x50, 0x04, 0xd9,
+    0xc5, 0x4a, 0xea, 0xd7, 0xed, 0xdc, 0x32, 0xa3, 0x33, 0xc8, 0x87, 0x21, 0xf8, 0x21, 0x96, 0xad,
+    0xf8, 0xf5, 0xb4, 0xe1, 0xe9, 0xc3, 0x72, 0x2f, 0x88, 0x03, 0xa8, 0x89, 0x65, 0x39, 0xb6, 0x53,
+    0x98, 0xef, 0xcb, 0x5a, 0x54, 0x13, 0x4c, 0x4a, 0x55, 0xe9, 0x91, 0x90, 0xcb, 0x39, 0xf0, 0x00,
+    0xee, 0x31, 0xed, 0xa4, 0x43, 0x06, 0x93, 0x75, 0x38, 0x6a, 0xe3, 0xe0, 0x54, 0x8f, 0x0a, 0x3a,
+    0xf3, 0x79, 0x21, 0x9f, 0x41, 0x97, 0xac, 0x80, 0x8b, 0x87, 0xd0, 0x44, 0x11, 0x04, 0xc6, 0x76,
+    0x82, 0x5a, 0x35, 0x77, 0x8f, 0x15, 0x05, 0x05, 0x55, 0xfb, 0xeb, 0x46, 0x1a, 0xe8, 0xb8, 0x73,
+    0x0d, 0xa7, 0xd8, 0xd1, 0x18, 0x3e, 0x46, 0x24, 0x86, 0x67, 0xc3, 0x0c, 0xe5, 0x7d, 0x56, 0x77,
+    0x3b, 0xd3, 0xd6, 0x95, 0x15, 0x65, 0xf5, 0xb8, 0x68, 0x8d, 0xc9, 0x0b, 0x16, 0x45, 0x91, 0xef,
+    0xcc, 0xe5, 0x54, 0x16, 0x9b, 0x8c, 0x2c, 0x76, 0x4b, 0xa2, 0x44, 0xbc, 0x49, 0xda, 0x33, 0x04,
+    0xc5, 0xb6, 0xd7, 0xc6, 0x3b, 0x49, 0x95, 0x08, 0x8c, 0xd3, 0x7e, 0x2b, 0x65, 0xe6, 0x83, 0x62,
+    0x81, 0xd3, 0x11, 0x24, 0x5a, 0xac, 0x9b, 0xd3, 0xcd, 0x19, 0x06, 0xde, 0xf3, 0x0a, 0xb4, 0x5f,
+    0x18, 0xc6, 0x06, 0x60, 0x67, 0xca, 0x0c, 0x36, 0x34, 0xb8, 0x92, 0x69, 0xde, 0xf6, 0xe7, 0x64,
+    0x58, 0x80, 0x4f, 0x5c, 0xf5, 0x44, 0x62, 0x21, 0x2c, 0x9d, 0xe4, 0x18, 0x71, 0xba, 0x18, 0x43,
+    0x0f, 0x44, 0xa8, 0x1b, 0x35, 0x22, 0x36, 0x77, 0x0c, 0xc6, 0x22, 0x46, 0x4c, 0x11, 0xec, 0xfd,
+    0xb9, 0xef, 0x51, 0xb5, 0xb2, 0x8e, 0x3f, 0x2f, 0xa5, 0x7c, 0x3c, 0x46, 0x57, 0xfd, 0x5c, 0x8c,
+    0x6f, 0xc4, 0x13, 0x99, 0xf0, 0x06, 0x01, 0x5f, 0x92, 0x51, 0xc6, 0x94, 0x29, 0xf5, 0xb4, 0xd0,
+    0x5e, 0x2f, 0x1e, 0x0a, 0x93, 0x8d, 0x08, 0x00, 0xcd, 0x71, 0x4d, 0x6c, 0x11, 0xad, 0xe3, 0x78,
+    0xba, 0xd3, 0x6d, 0xe8, 0xf5, 0x93, 0xd6, 0x83, 0x94, 0x9d, 0xf1, 0xf0, 0xde, 0x04, 0xcc, 0x41,
+];
+
+/// Expected plaintext `ota-package-list.json` fixture bytes.
+const EXPECTED_OTA_PACKAGE_LIST: &[u8] = &[
+    0x7b, 0x22, 0x70, 0x61, 0x63, 0x6b, 0x61, 0x67, 0x65, 0x73, 0x22, 0x3a, 0x20, 0x5b, 0x7b, 0x22,
+    0x66, 0x69, 0x6c, 0x65, 0x22, 0x3a, 0x20, 0x22, 0x63, 0x63, 0x32, 0x5f, 0x65, 0x65, 0x62, 0x30,
+    0x30, 0x31, 0x5f, 0x30, 0x31, 0x2e, 0x30, 0x33, 0x2e, 0x30, 0x32, 0x2e, 0x33, 0x36, 0x5f, 0x32,
+    0x30, 0x32, 0x36, 0x30, 0x33, 0x32, 0x36, 0x31, 0x37, 0x31, 0x37, 0x34, 0x35, 0x2e, 0x73, 0x77,
+    0x75, 0x2e, 0x73, 0x69, 0x67, 0x22, 0x2c, 0x20, 0x22, 0x68, 0x61, 0x73, 0x68, 0x22, 0x3a, 0x20,
+    0x22, 0x34, 0x36, 0x61, 0x61, 0x34, 0x35, 0x39, 0x62, 0x36, 0x31, 0x33, 0x34, 0x30, 0x61, 0x38,
+    0x34, 0x66, 0x66, 0x35, 0x32, 0x65, 0x32, 0x38, 0x36, 0x35, 0x33, 0x34, 0x37, 0x64, 0x34, 0x32,
+    0x61, 0x35, 0x36, 0x38, 0x33, 0x35, 0x31, 0x39, 0x31, 0x39, 0x34, 0x34, 0x31, 0x66, 0x62, 0x39,
+    0x64, 0x62, 0x36, 0x65, 0x31, 0x38, 0x35, 0x34, 0x32, 0x34, 0x32, 0x32, 0x30, 0x62, 0x31, 0x39,
+    0x32, 0x22, 0x7d, 0x5d, 0x2c, 0x20, 0x22, 0x76, 0x65, 0x72, 0x73, 0x69, 0x6f, 0x6e, 0x22, 0x3a,
+    0x20, 0x22, 0x30, 0x31, 0x2e, 0x30, 0x33, 0x2e, 0x30, 0x32, 0x2e, 0x33, 0x36, 0x22, 0x7d,
+];
+
+/// Decrypting the signed OTA package list should reproduce the known plaintext
+/// JSON fixture byte-for-byte.
+#[test]
+fn decrypts_ota_package_list_fixture() {
+    let actual = super::crypto::unpack_sig(ENCRYPTED_OTA_PACKAGE_LIST)
+        .expect("failed to unpack embedded .sig fixture");
+
+    assert_eq!(actual, EXPECTED_OTA_PACKAGE_LIST);
+}