The Chitu CTB format (.ctb) is an encrypted binary format used by printers with Chitu controller boards. This format is commonly found in Anycubic, Creality, Phrozen, and many other MSLA resin printers.
Overview
CTB stands for “ChiTu Box” format, version 5. It uses AES-256 encryption to protect the format specification and includes comprehensive metadata for print settings.
Key Features
- Format Version: V5 (latest)
- Encryption: AES-256-CBC with hardcoded key and IV
- Compression: Custom run-length encoding (RLE)
- Layer Data: XOR-encrypted RLE compressed bitmaps
- Preview Images: Two sizes (large and small thumbnails)
- Checksum: SHA-256 verification for data integrity
File Structure
The file begins with a magic number and version information:
Magic: 0x12FD0107
Version: 5 (FORMAT_VERSION)
Main Sections
-
Settings Section (encrypted)
- Print parameters
- Layer configuration
- Resin settings
- Machine information
-
Signature Section (encrypted)
- SHA-256 checksum verification
- Encrypted signature data
-
Preview Images
- Large preview (variable size)
- Small preview (variable size)
- RGB565 format
-
Layer Data
- XOR-encrypted RLE compressed pixels
- Per-layer positioning information
- Page-based storage (4GB pages)
Encryption Details
AES-256-CBC
The CTB format uses AES-256 in CBC mode with a fixed key and initialization vector:
// From format/ctb_format/src/crypto.rs
const ENCRYPT_KEY: &[u8; 32] = [
0xD0, 0x5B, 0x8E, 0x33, 0x71, 0xDE, 0x3D, 0x1A,
0xE5, 0x4F, 0x22, 0xDD, 0xDF, 0x5B, 0xFD, 0x94,
0xAB, 0x5D, 0x64, 0x3A, 0x9D, 0x7E, 0xBF, 0xAF,
0x42, 0x03, 0xF3, 0x10, 0xD8, 0x52, 0x2A, 0xEA
];
const ENCRYPT_IV: &[u8; 16] = [
0x0F, 0x01, 0x0A, 0x05, 0x05, 0x0B, 0x06, 0x07,
0x08, 0x06, 0x0A, 0x0C, 0x0C, 0x0D, 0x09, 0x0F
];
XOR Layer Encryption
Layer data is additionally XOR-encrypted with a key stored in the file (default: 0x67):
const DEFAULT_XOR_KEY: u32 = 0x67;
Run-Length Encoding
CTB uses a sophisticated RLE scheme optimized for horizontal pixel runs:
[value_byte] [length_bytes...]
- Bit 7 of value: Set if run length > 1
- Bits 0-6 of value: Grayscale value (right-shifted)
- Length encoding: Variable-length (1-4 bytes)
Length Encoding
| Length Range | Bytes | Format |
|---|
| 1 | 0 | (implicit from bit 7 = 0) |
| 2-127 | 1 | 0xxxxxxx |
| 128-16,383 | 2 | 10xxxxxx xxxxxxxx |
| 16,384-2,097,151 | 3 | 110xxxxx xxxxxxxx xxxxxxxx |
| 2,097,152+ | 4 | 1110xxxx xxxxxxxx xxxxxxxx xxxxxxxx |
Example
From format/ctb_format/src/layer_coding.rs:76-100:
pub fn add_run(&mut self, length: u64, value: u8) {
if length == 0 { return; }
self.data.push((value >> 1) | (0x80 * (length > 1) as u8));
if length <= 1 {
// pass
} else if length <= 0x7F {
self.data.push(length as u8);
} else if length <= 0x3FFF {
self.data.push((length >> 8) as u8 | 0x80);
self.data.push(length as u8);
} else if length <= 0x1FFFFF {
self.data.push((length >> 16) as u8 | 0xc0);
self.data.push((length >> 8) as u8);
self.data.push(length as u8);
} else if length <= 0xFFFFFFF {
self.data.push((length >> 24) as u8 | 0xe0);
self.data.push((length >> 16) as u8);
self.data.push((length >> 8) as u8);
self.data.push(length as u8);
}
}
Print Parameters
The CTB format stores extensive print configuration:
- Platform size (X, Y, Z) in millimeters
- Platform resolution (X, Y) in pixels
- Machine name
- Projector type
Layer Settings
- Layer height
- Exposure time (bottom and regular)
- Light-off delay (bottom and regular)
- Bottom layer count
- Transition layer count
Movement Settings
- Lift height (primary and secondary)
- Lift speed (primary and secondary)
- Retract height (primary and secondary)
- Retract speed (primary and secondary)
- Rest times (various stages)
Light Settings
- Light PWM (0-255, bottom and regular)
- Per-layer settings flag
- Anti-aliasing level
Resin Parameters
Stored in a separate section (from format/ctb_format/src/resin.rs):
pub struct ResinParameters {
pub resin_color: Vector4<f32>, // RGBA color
pub machine_name: String,
pub resin_type: String, // e.g., "Normal"
pub resin_name: String, // e.g., "Standard"
pub resin_density: f32, // g/cm³
}
Compatibility
Supported Printers
The CTB format is used by a wide range of printers:
- Anycubic: Photon Mono, Mono X, Mono SE, Photon Ultra
- Creality: HALOT series, LD-002H
- Phrozen: Sonic Mini 4K, Sonic Mighty 4K
- Elegoo: Some models support CTB in addition to GOO
- Voxelab: Proxima
- QIDI: Shadow series
The CTB format requires exact resolution matching. The sliced file will not print if the printer’s resolution does not match the file’s resolution settings.
Disclaimer
The CTB format includes a mandatory disclaimer string:
"Layout and record format for the ctb and cbddlp file types are the
copyrighted programs or codes of CBD Technology (China) Inc.. The
Customer or User shall not in any manner reproduce, distribute, modify,
decompile, disassemble, decrypt, extract, reverse engineer, lease, assign,
or sublicense the said programs or codes."
Implementation in Mslicer
Mslicer’s CTB implementation (format/ctb_format/):
- Full encryption/decryption support
- Optimized RLE encoder/decoder
- Complete metadata serialization
- Preview image generation
- SHA-256 checksum verification
Reading CTB Files
use ctb_format::File;
use common::serde::SliceDeserializer;
let bytes = std::fs::read("model.ctb")?;
let mut des = SliceDeserializer::new(&bytes);
let file = File::deserialize(&mut des)?;
Writing CTB Files
use ctb_format::File;
use common::serde::DynamicSerializer;
let mut ser = DynamicSerializer::new();
file.serialize(&mut ser);
std::fs::write("output.ctb", ser.into_inner())?;
File Size
CTB files are typically very compact due to the efficient RLE compression:
- Small models: 1-5 MB
- Medium models: 5-20 MB
- Large models: 20-100 MB
- Full-plate prints: 100-500 MB
The encryption overhead is minimal (only the settings section is encrypted, not the layer data).
Credits
Mslicer’s CTB implementation is based on documentation from the UVtools project. Special thanks to the UVtools contributors for reverse-engineering this format.
See Also
