Added embed entropy

Cargo.lock

@@ -128,6 +128,12 @@ dependencies = [
  "subtle",
 ]
 
+[[package]]
+name = "entropy"
+version = "0.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d68716e45ef572f351be6fad93a7bbf35242b4289a2ff75434032e5d73d74cc2"
+
 [[package]]
 name = "fdeflate"
 version = "0.3.4"
@@ -230,6 +236,7 @@ dependencies = [
  "argon2",
  "bitvec",
  "crc",
+ "entropy",
  "getopts",
  "png",
  "rand",

Cargo.toml

@@ -15,6 +15,7 @@ path = "src/png_data/main.rs"
 argon2 = "0.5.3"
 bitvec = "1.0.1"
 crc = "3.2.1"
+entropy = "0.4.2"
 getopts = "0.2.21"
 png = "0.17.13"
 rand = "0.8.5"

README.md

@@ -4,7 +4,7 @@
 
 ![TeX Live english documentation](doc/texlive_en.png)
 
-`png_data` transforms a file as a png image.
+`png_data` encodes a file into a png image.
 
 ### Encoding
 `png_data -l rgb8 -e file.pdf -o output.png -c "(.pdf) documentation"`
@@ -52,6 +52,10 @@ Where:
  * `output.png` the resulting PNG file
  * `"(.tar) archive"` an optional comment
 
+**Additional Options**
+ * `-s|--seed TXT` Sets the random seed for determining the payload blocks. By default the random seed is "WIDTHxHEIGHT" where WIDTH and HEIGHT are the original image's dimensions.
+ * `-n|--entropy` Fills unused payload blocks with random data that tries to match the payload's entropy. This feature is experimental and may not fully protect against entropy based steganography-detection. We highlihy recommend that the payload has maximal entropy, which can be achieved by compressing it.
+
 ### Decoding an image
 `png_embed -l lo2 -d image.png -o embed.tar`
 Where:
@@ -59,12 +63,18 @@ Where:
  * `image.png` the PNG containing an embed
  * `embed.tar` the extracted embedded file
 
+**Additional Options**
+ * `-s|--seed TXT` Sets the random seed for determining the payload blocks. By default the random seed is "WIDTHxHEIGHT" where WIDTH and HEIGHT are the original image's dimensions.
+
 ### Getting header information
 `png_embed -l lo2 -z output.png`
  * `lo2` is the `Lo` algorithm using the 2 lowest bits
  * `output.png` a `png_embed` encoded image
 This will display the header of the encoded file, as well as the comment.
 
+**Additional Options**
+ * `-s|--seed TXT` Sets the random seed for determining the payload blocks. By default the random seed is "WIDTHxHEIGHT" where WIDTH and HEIGHT are the original image's dimensions.
+
 # License
 
 png_data is licensed under the GNU AGPL version 3 or later. See [LICENSE.md](LICENSE.md) for more information.

examples/test.sh

@@ -7,7 +7,7 @@ PNG_EMBED=../target/debug/png_embed
 echo "Encoding..."
 for i in {1..7}; do
 	echo "Writing dec-lo${i}.."
-	$PNG_EMBED -l lo${i} -e embed.png input.png -o out-lo${i}.png
+	$PNG_EMBED -l lo${i} -n -e embed.png input.png -o out-lo${i}.png
 done
 
 echo "Decoding..."

src/png_data/header.rs

@@ -10,7 +10,8 @@ pub trait Decode {
 
 	/// Decode the data from an iterator
 	fn decode<I>(it: &mut I) -> Result<Self::Type, String>
-		where I: Iterator<Item = (usize, u8)>;
+	where
+		I: Iterator<Item = (usize, u8)>;
 }
 
 /// The program's version.
@@ -69,7 +70,7 @@ impl Header {
 }
 
 impl Encode for Header {
-    fn encode(&self, vec: &mut Vec<u8>) {
+	fn encode(&self, vec: &mut Vec<u8>) {
 		// Version
 		vec.extend_from_slice((self.version as u16).to_le_bytes().as_slice());
 
@@ -87,14 +88,16 @@ impl Encode for Header {
 		if let Some(comment) = &self.comment {
 			vec.extend_from_slice(comment.as_bytes());
 		}
-    }
+	}
 }
 
 impl Decode for Header {
-    type Type = Header;
+	type Type = Header;
 
-    fn decode<I>(it: &mut I) -> Result<Self::Type, String>
-		    where I: Iterator<Item = (usize, u8)> {
+	fn decode<I>(it: &mut I) -> Result<Self::Type, String>
+	where
+		I: Iterator<Item = (usize, u8)>,
+	{
 		let mut count = 0;
 		let mut next = || -> Result<u8, String> {
 			let result = it
@@ -117,7 +120,7 @@ impl Decode for Header {
 
 			Some(
 				String::from_utf8(comment_data)
-				.map_err(|e| format!("Failed to retrieve comment: {e}"))?,
+					.map_err(|e| format!("Failed to retrieve comment: {e}"))?,
 			)
 		} else {
 			None
@@ -129,7 +132,7 @@ impl Decode for Header {
 			data_crc,
 			comment,
 		})
-    }
+	}
 }
 
 /*

src/png_data/main.rs

@@ -45,7 +45,7 @@ fn str_to_layout(layout: &str) -> Result<(ColorType, BitDepth), String> {
 	let split = layout
 		.char_indices()
 		.find(|(_, c)| c.is_ascii_digit())
-		.ok_or(format!("Unable to find number for layout's bit depth"))?
+		.ok_or("Unable to find number for layout's bit depth".to_string())?
 		.0;
 	match layout.split_at(split) {
 		("rgb", bits) => match bits {
@@ -99,8 +99,7 @@ fn best_layout(size: u64, bits_per_pixel: u8) -> (u32, u32) {
 }
 
 /// Gets the minimum image buffer size in bytes
-fn minimum_size(color: ColorType, depth: BitDepth, width: u32, height: u32) -> usize
-{
+fn minimum_size(color: ColorType, depth: BitDepth, width: u32, height: u32) -> usize {
 	let samples = width as usize * color.samples();
 	(match depth {
 		BitDepth::Sixteen => samples * 2,
@@ -137,15 +136,12 @@ fn encode(input: String, output: String, layout: String, matches: Matches) -> Re
 	eprintln!("==============");
 
 	let bits_per_pixel = bits_per_pixel(layout.0, layout.1);
-	let (width, height) = best_layout(
-		(data.len() + input_data.len()) as u64,
-		bits_per_pixel
-	);
+	let (width, height) = best_layout((data.len() + input_data.len()) as u64, bits_per_pixel);
 
 	// Encode
 	let output_file = File::create(&output)
 		.map_err(|err| format!("Failed to open output file `{output}`: {err}"))?;
-	let ref mut w = BufWriter::new(output_file);
+	let w = &mut BufWriter::new(output_file);
 	let mut encoder = png::Encoder::new(w, width, height);
 	encoder.set_color(layout.0);
 	encoder.set_depth(layout.1);
@@ -189,9 +185,9 @@ fn decode_header(input: String, _matches: Matches) -> Result<(), String> {
 
 	data.resize(info.buffer_size(), 0);
 
-
 	let mut it = data.iter().enumerate().map(|(idx, byte)| (idx, *byte));
-	let header = Header::decode(&mut it).map_err(|err| format!("Failed to decode header: {err}"))?;
+	let header =
+		Header::decode(&mut it).map_err(|err| format!("Failed to decode header: {err}"))?;
 	eprintln!("=== HEADER ===");
 	eprintln!("Version: {:#?}", header.version);
 	eprintln!(
@@ -219,10 +215,8 @@ fn decode(input: String, output: String, _matches: Matches) -> Result<(), String
 
 	data.resize(info.buffer_size(), 0);
 
-
 	let mut it = data.iter().enumerate().map(|(idx, byte)| (idx, *byte));
-	let header = 
-	{
+	let header = {
 		//let mut temp_it = std::mem::take(&mut it);
 		Header::decode(&mut it).map_err(|err| format!("Failed to decode header: {err}"))?
 	};
@@ -236,14 +230,18 @@ fn decode(input: String, output: String, _matches: Matches) -> Result<(), String
 	eprintln!("==============");
 
 	// Check crc
-	let data_start = it.next().ok_or(format!("Failed to get data start"))?.0;
-	let file_data = &data[data_start..data_start+header.data_len as usize];
+	let data_start = it.next().ok_or("Failed to get data start".to_string())?.0;
+	let file_data = &data[data_start..data_start + header.data_len as usize];
 	let crc = Crc::<u32>::new(&crc::CRC_32_CKSUM).checksum(file_data);
 	if crc != header.data_crc {
-		Err(format!("Data CRC[{crc:X}] does not match header CRC[{:X}]", header.data_crc))?;
+		Err(format!(
+			"Data CRC[{crc:X}] does not match header CRC[{:X}]",
+			header.data_crc
+		))?;
 	}
 
-	std::fs::write(&output, file_data).map_err(|err| format!("Failed to write to output file `{output}`: {err}"))?;
+	std::fs::write(&output, file_data)
+		.map_err(|err| format!("Failed to write to output file `{output}`: {err}"))?;
 	println!("File written to `{output}`");
 
 	Ok(())

src/png_embed/block.rs

@@ -4,6 +4,7 @@ use rand::prelude::SliceRandom;
 use rand::Rng;
 
 use crate::embed::EmbedAlgorithm;
+use crate::ent::EntropyGenerator;
 
 /// Gets the best blocksize (i.e. that minimize remaining space) for a certain data length.
 /// The blocksize is a number in range [16, 65536]
@@ -27,6 +28,7 @@ pub struct BlockPlacement<'a> {
 	algorithm: &'a EmbedAlgorithm,
 	data: &'a mut [u8],
 	block_size: usize,
+	pub used_blocks: usize,
 	pub blocks: Vec<usize>,
 }
 
@@ -68,14 +70,16 @@ impl<'a> BlockPlacement<'a> {
 		blocks.shuffle(rng);
 
 		// Only keep the first blocks_num blocks
-		blocks.resize(blocks_num, 0);
+		//blocks.resize(blocks_num, 0);
 
-		Ok(Self {
+		let s = Self {
 			algorithm,
 			data,
 			block_size,
+			used_blocks: blocks_num,
 			blocks,
-		})
+		};
+		Ok(s)
 	}
 
 	// Embeds the data into the original image
@@ -93,7 +97,7 @@ impl<'a> BlockPlacement<'a> {
 		let mut index = 0;
 		match self.algorithm {
 			EmbedAlgorithm::Lo(bits) => {
-				for block in &self.blocks {
+				for block in &self.blocks[0..self.used_blocks] {
 					for i in 0..self.block_size {
 						let pos = block * self.block_size + i;
 						let hi = std::cmp::min(*bits as usize, embed.len() - index);
@@ -107,6 +111,26 @@ impl<'a> BlockPlacement<'a> {
 			}
 		}
 	}
+
+	/// Fills unused blocks with randomly generated data
+	pub fn fill_unused<R>(&mut self, mut gen: EntropyGenerator<R>)
+	where
+		R: Rng,
+	{
+		match self.algorithm {
+			EmbedAlgorithm::Lo(bits) => {
+				let mask: u8 = (1 << bits) - 1;
+				for block in &self.blocks[self.used_blocks..] {
+					for i in 0..self.block_size {
+						let pos = block * self.block_size + i;
+
+						self.data[pos] &= !mask;
+						self.data[pos] |= gen.next() & mask;
+					}
+				}
+			}
+		}
+	}
 }
 
 // Iterator over blocks in the resulting image

src/png_embed/ent.rs

@@ -0,0 +1,53 @@
+use rand::distributions::WeightedIndex;
+use rand::prelude::Distribution;
+use rand::Rng;
+
+pub struct EntropyGenerator<R>
+where
+	R: Rng,
+{
+	rng: R,
+	dist: WeightedIndex<f64>,
+}
+
+/// Genrates random bytes with a set entropy
+impl<R: Rng> EntropyGenerator<R> {
+	// FIXME: Bad entropy
+	pub fn new(entropy: f64, rng: R) -> Self {
+		// FIXME: Does not work for entropy below 1.0
+		let n = (2.0f64.powf(entropy)).round() as usize;
+
+		let mut probabilities = std::iter::repeat(1.0f64).take(n).collect::<Vec<_>>();
+		let sum = probabilities.iter().sum::<f64>();
+		probabilities.iter_mut().for_each(|p| *p /= sum);
+
+		let dist = WeightedIndex::new(&probabilities).unwrap();
+
+		Self { rng, dist }
+	}
+
+	pub fn next(&mut self) -> u8 { self.dist.sample(&mut self.rng) as u8 }
+}
+
+#[cfg(test)]
+mod tests {
+	use entropy::shannon_entropy;
+	use rand::SeedableRng;
+	use rand_chacha::ChaCha8Rng;
+
+	use super::*;
+
+	#[test]
+	fn test_entropy() {
+		for i in 1..8 {
+			let mut gen = EntropyGenerator::new(i as f64, ChaCha8Rng::from_entropy());
+
+			let mut data = Vec::with_capacity(1024);
+			for _ in 0..1024 {
+				data.push(gen.next());
+			}
+
+			assert!((shannon_entropy(data) - i as f32).abs() < 0.2);
+		}
+	}
+}

src/png_embed/main.rs

@@ -1,7 +1,8 @@
-pub mod block;
-pub mod embed;
-pub mod header;
-pub mod image;
+mod block;
+mod embed;
+mod ent;
+mod header;
+mod image;
 
 use std::env;
 use std::fs::File;
@@ -16,6 +17,7 @@ use block::BlockPlacement;
 use block::BlockPlacementIterator;
 use crc::Crc;
 use embed::EmbedAlgorithm;
+use ent::EntropyGenerator;
 use getopts::Matches;
 use getopts::Options;
 use header::Decode;
@@ -152,6 +154,14 @@ fn encode(
 	eprintln!("==============");
 
 	placement.write_embed(embed_data.as_slice().view_bits::<Lsb0>());
+	if matches.opt_present("n") {
+		let ent = entropy::shannon_entropy(&embed_data);
+		println!("Payload entropy: {ent}\nFilling image remainder with random data...");
+		placement.fill_unused(EntropyGenerator::new(
+			ent as f64,
+			ChaCha8Rng::from_entropy(),
+		))
+	}
 
 	let outfile = File::create(&output).unwrap();
 	let w = &mut BufWriter::new(Box::new(outfile) as Box<dyn Write>);
@@ -256,6 +266,11 @@ fn main() -> ExitCode {
 	);
 	opts.optflag("z", "info", "Read header");
 	opts.optopt("l", "algorithm", "Embed algorithm", "lo3");
+	opts.optflag(
+		"n",
+		"entropy",
+		"Attempts to hide payload by modifying the file's entropy",
+	);
 	opts.optflag("h", "help", "Print this help menu");
 	opts.optflag("v", "version", "Print program version and licenses");