1 files changed, 156 insertions, 0 deletions
diff --git a/src/lossless/lsb.rs b/src/lossless/lsb.rs
new file mode 100644
index 0000000..59dcc0b
--- /dev/null
+++ b/src/lossless/lsb.rs
@@ -0,0 +1,156 @@
+use std::cmp::Ordering;
+
+use image::{ColorType, DynamicImage, GenericImageView, Pixel};
+use thiserror::Error;
+
+use crate::codec::Codec;
+
+/// Least-significant bit (LSB) steganography encodes data in the least-significant bits of colors
+/// in an image. This implementation reduces the colors in the carrier (irreversibly) in order to
+/// allow a byte of data to fit in each pixel of the image. 3 bits of data are encoded per pixel,
+/// and the 9th bit is used to signal the end of data.
+#[derive(Debug)]
+pub struct LsbCodec;
+
+impl Codec for LsbCodec {
+    type Carrier = DynamicImage;
+    type Payload = Vec<u8>;
+    type Output = Self::Carrier;
+    type Error = LsbError;
+
+    fn encode<C, P>(&self, carrier: C, payload: P) -> Result<Self::Output, Self::Error>
+    where
+        C: Into<Self::Carrier>,
+        P: Into<Self::Payload>,
+    {
+        let mut image: DynamicImage = carrier.into();
+        let payload: Vec<u8> = payload.into();
+
+        if image.pixels().count() < payload.len() {
+            return Err(LsbError::PayloadTooBig);
+        }
+
+        let mut payload_iter = payload.iter();
+
+        match image {
+            DynamicImage::ImageRgba8(ref mut image) => {
+                for pixel in image.pixels_mut() {
+                    if let Some(payload_byte) = payload_iter.next() {
+                        encode_pixel(pixel, *payload_byte, false);
+                    } else {
+                        encode_pixel(pixel, 0, true);
+                    }
+                }
+            },
+            DynamicImage::ImageRgb8(ref mut image) => {
+                for pixel in image.pixels_mut() {
+                    if let Some(payload_byte) = payload_iter.next() {
+                        encode_pixel(pixel, *payload_byte, false);
+                    } else {
+                        encode_pixel(pixel, 0, true);
+                    }
+                }
+            },
+            _ => return Err(LsbError::UnsupportedFormat { format: image.color() })
+        }
+
+        Ok(image)
+    }
+
+    fn decode<E>(&self, carrier: E) -> Result<(Self::Carrier, Self::Payload), LsbError>
+    where
+        E: Into<Self::Output>,
+    {
+        let mut image: DynamicImage = carrier.into();
+        let mut payload: Vec<u8> = Vec::new();
+
+        match image {
+            DynamicImage::ImageRgba8(ref mut image) => {
+                for pixel in image.pixels_mut() {
+                    if let Some(payload_byte) = decode_pixel(pixel) {
+                        payload.push(payload_byte);
+                    } else {
+                        break;
+                    }
+                }
+            },
+            DynamicImage::ImageRgb8(ref mut image) => {
+                for pixel in image.pixels_mut() {
+                    if let Some(payload_byte) = decode_pixel(pixel) {
+                        payload.push(payload_byte);
+                    } else {
+                        break;
+                    }
+                }
+            },
+            _ => return Err(LsbError::UnsupportedFormat { format: image.color() })
+        }
+        
+        Ok((image, payload))
+    }
+}
+
+fn encode_pixel<P: Pixel<Subpixel = u8>>(pixel: &mut P, payload_byte: u8, end_of_data: bool) {
+    let mut bits_remaining: i32 = 8;
+    for channel in pixel.channels_mut() {
+        *channel &= 0b11111000;
+        bits_remaining -= 3;
+        if bits_remaining <= -3 {
+            break;
+        }
+
+        let mask = match bits_remaining.cmp(&0) {
+            Ordering::Less => payload_byte << -bits_remaining,
+            _ => payload_byte >> bits_remaining,
+        } & 0b00000111;
+
+        *channel |= mask;
+    }
+
+    // Add end-of-data marker to final bit if necessary
+    if end_of_data {
+        *pixel.channels_mut().last_mut().unwrap() |= 1;
+    }
+}
+
+fn decode_pixel<P: Pixel<Subpixel = u8>>(pixel: &mut P) -> Option<u8> {
+    
+    // Final bit as end-of-data marker
+    if pixel.channels().last().unwrap() & 1 == 1 {
+        return None;
+    }
+
+    let mut bits_remaining: i32 = 8;
+    let mut payload_byte: u8 = 0;
+    for channel in pixel.channels_mut() {
+        bits_remaining -= 3;
+        if bits_remaining <= -3 {
+            break;
+        }
+
+        let channel_bits = *channel & 0b00000111;
+        *channel &= 0b11111000;
+        let mask = match bits_remaining.cmp(&0) {
+            Ordering::Less => channel_bits >> -bits_remaining,
+            _ => channel_bits << bits_remaining,
+        };
+        payload_byte |= mask;
+    }
+    Some(payload_byte)
+}
+
+/// Errors thrown by the LSB Codec.
+#[derive(Error, Debug)]
+pub enum LsbError {
+
+    /// Error thrown when payload is too big for the carrier.
+    #[error("Payload is too big for the carrier. Choose a smaller payload or an image with greater pixel dimensions.")]
+    PayloadTooBig,
+
+    /// Error thrown when pixel format is unsupported.
+    #[error("Specified image format ({format:?}) is unsupported.")]
+    UnsupportedFormat {
+        /// Provided (invalid) format.
+        format: ColorType
+    },
+}