fix(core): cap imgsecret MAX_DIMENSION at 10000px (audit M3)

crates/relicario-core/src/imgsecret.rs

@@ -65,6 +65,11 @@ const QUANT_STEP: f64 = 50.0;
 /// this cannot hold enough 8x8 blocks for reliable embedding.
 const MIN_DIMENSION: u32 = 100;
 
+/// Maximum image dimension (width or height) in pixels. Images larger than
+/// this are rejected before full decode to prevent DoS via attacker-supplied
+/// oversized JPEGs (audit M3).
+pub const MAX_DIMENSION: u32 = 10_000;
+
 /// Number of secret bits to embed: 256 bits = 32 bytes.
 const SECRET_BITS: usize = 256;
 
@@ -112,6 +117,64 @@ const EMBED_POSITIONS: [(usize, usize); 12] = [
     (2, 3), // zig-zag 14-17
 ];
 
+// ─── Dimension guard ─────────────────────────────────────────────────────────
+
+/// Walk JPEG markers until we hit an SOF (start-of-frame) marker, which
+/// carries the image dimensions in bytes 5..=8 of its segment.
+///
+/// This peek does NOT decode any pixel data, so an oversized JPEG header is
+/// rejected in O(marker-count) time without allocating a frame buffer.
+fn peek_jpeg_dimensions(jpeg: &[u8]) -> Result<(u32, u32)> {
+    let mut i = 0;
+    while i + 1 < jpeg.len() {
+        if jpeg[i] != 0xFF {
+            i += 1;
+            continue;
+        }
+        let marker = jpeg[i + 1];
+        match marker {
+            0xD8 | 0xD9 => {
+                i += 2;
+                continue;
+            } // SOI / EOI
+            0xC0..=0xC3 | 0xC5..=0xC7 | 0xC9..=0xCB | 0xCD..=0xCF => {
+                // SOFn — height in [i+5..i+7], width in [i+7..i+9]
+                if i + 9 >= jpeg.len() {
+                    return Err(RelicarioError::ImgSecret("truncated SOF marker".into()));
+                }
+                let height = u16::from_be_bytes([jpeg[i + 5], jpeg[i + 6]]) as u32;
+                let width = u16::from_be_bytes([jpeg[i + 7], jpeg[i + 8]]) as u32;
+                return Ok((width, height));
+            }
+            _ => {
+                if i + 3 >= jpeg.len() {
+                    return Err(RelicarioError::ImgSecret("truncated marker segment".into()));
+                }
+                let seg_len = u16::from_be_bytes([jpeg[i + 2], jpeg[i + 3]]) as usize;
+                i += 2 + seg_len;
+            }
+        }
+    }
+    Err(RelicarioError::ImgSecret(
+        "no SOF marker found in JPEG".into(),
+    ))
+}
+
+/// Reject JPEGs that claim dimensions exceeding [`MAX_DIMENSION`].
+///
+/// Called at the entry point of both `embed` and `extract` to prevent
+/// attacker-supplied 32000×32000 images from wedging the WASM service worker
+/// during the expensive DCT extraction pass (audit M3).
+fn enforce_dimension_cap(jpeg: &[u8]) -> Result<()> {
+    let (w, h) = peek_jpeg_dimensions(jpeg)?;
+    if w > MAX_DIMENSION || h > MAX_DIMENSION {
+        return Err(RelicarioError::ImgSecret(format!(
+            "image dimensions {w}x{h} exceed {MAX_DIMENSION}x{MAX_DIMENSION} cap"
+        )));
+    }
+    Ok(())
+}
+
 // ─── YChannel ────────────────────────────────────────────────────────────────
 
 /// The luminance (Y) channel of an image, stored as a flat array of f64 values.
@@ -601,6 +664,7 @@ fn reconstruct_jpeg(original_jpeg: &[u8], y_modified: &YChannel) -> Result<Vec<u
 ///   or does not have enough blocks for reliable embedding.
 /// - [`RelicarioError::ImgSecret`] if the image cannot be decoded or re-encoded.
 pub fn embed(carrier_jpeg: &[u8], secret: &[u8; 32]) -> Result<Vec<u8>> {
+    enforce_dimension_cap(carrier_jpeg)?;
     let mut y = extract_y_channel(carrier_jpeg)?;
 
     if (y.width as u32) < MIN_DIMENSION || (y.height as u32) < MIN_DIMENSION {
@@ -672,6 +736,7 @@ pub fn embed(carrier_jpeg: &[u8], secret: &[u8; 32]) -> Result<Vec<u8>> {
 /// - [`RelicarioError::ExtractionFailed`] if no valid secret could be recovered
 ///   (image was never watermarked, or was too heavily recompressed/cropped).
 pub fn extract(jpeg_bytes: &[u8]) -> Result<[u8; 32]> {
+    enforce_dimension_cap(jpeg_bytes)?;
     extract_with_crop_recovery(jpeg_bytes)
 }
 
@@ -1015,6 +1080,30 @@ mod tests {
         assert_eq!(extracted, secret);
     }
 
+    #[test]
+    fn rejects_oversized_image_without_full_decode() {
+        // Synthesize a JPEG header claiming 20000x20000 dimensions.
+        // The actual pixel data is irrelevant — the dimension peek should bail out
+        // before decoding any pixels.
+        let jpeg = build_oversized_jpeg_header(20_000, 20_000);
+        let result = extract(&jpeg);
+        assert!(matches!(result, Err(RelicarioError::ImgSecret(ref msg)) if msg.contains("dimension")));
+    }
+
+    fn build_oversized_jpeg_header(width: u16, height: u16) -> Vec<u8> {
+        // SOI + APP0 JFIF + SOF0 declaring width/height + SOS with minimal data + EOI
+        let mut v = vec![0xFF, 0xD8];                                   // SOI
+        v.extend_from_slice(&[0xFF, 0xE0, 0x00, 0x10]);                 // APP0
+        v.extend_from_slice(b"JFIF\0");
+        v.extend_from_slice(&[0x01, 0x01, 0x00, 0x00, 0x01, 0x00, 0x01, 0x00, 0x00]);
+        v.extend_from_slice(&[0xFF, 0xC0, 0x00, 0x11, 0x08]);           // SOF0
+        v.extend_from_slice(&height.to_be_bytes());
+        v.extend_from_slice(&width.to_be_bytes());
+        v.extend_from_slice(&[0x03, 0x01, 0x22, 0x00, 0x02, 0x11, 0x01, 0x03, 0x11, 0x01]);
+        v.extend_from_slice(&[0xFF, 0xD9]);                              // EOI
+        v
+    }
+
     #[test]
     fn embed_extract_survives_10pct_crop() {
         let jpeg = make_test_jpeg(400, 300);