secure_watermark

A Flutter package for visible & invisible watermarking. Part of the FlutterPlaza Security Suite.

• Visible watermarks deter screenshots by tiling user-identifying text over content
• Forensic (invisible) watermarks embed traceable data directly into image pixels using spread-spectrum LSB modulation — enabling post-leak identification even when visible watermarks are cropped out

Visible Watermark

Tile user-identifying text (email, user ID, timestamp) across any widget with configurable opacity, rotation, font size, and staggered brick pattern.

Forensic (Invisible) Watermark

Embed traceable data into image pixels. The watermarked image looks identical to the original — but the hidden payload can be extracted with the correct secret key.

Features

Visible watermark (Watermark widget):

• Diagonal tiled text overlay with configurable opacity, rotation, font size, and spacing
• Staggered brick pattern (alternate rows offset) for crop resistance
• IgnorePointer — watermark does not intercept touch events
• RepaintBoundary — watermark repaints are isolated from child content
• enabled flag to toggle visibility without removing from widget tree

Forensic (invisible) watermark (ForensicWatermark + ForensicWatermarkImage):

• Spread-spectrum LSB embedding in blue channel at PRNG-selected pixel positions
• Majority voting with configurable redundancy for error correction
• Magic number validation for wrong-key detection
• ForensicWatermarkImage widget with background isolate processing
• CLI tools for embedding and extraction (dart run secure_watermark:embed / extract)

Pure Dart/Flutter — works on all platforms (no plugin, no method channels).

Installation

dependencies:
  secure_watermark: ^0.3.1

Usage

Visible watermark overlay

import 'package:secure_watermark/secure_watermark.dart';

Watermark(
  text: 'user@example.com  2026-02-15',
  style: const WatermarkStyle(opacity: 0.2, rotate: -45),
  enabled: true,
  child: MyProtectedContent(),
)

Toggle watermark at runtime

Watermark(
  text: 'user@example.com',
  enabled: isWatermarkVisible, // controlled by setState, provider, etc.
  child: MyProtectedContent(),
)

Customize style

const style = WatermarkStyle(
  opacity: 0.2,        // 0.0–1.0 (default: 0.15)
  rotate: -45,         // degrees (default: -30)
  fontSize: 14,        // logical pixels (default: 16)
  rowSpacing: 60,      // vertical gap between rows (default: 80)
  columnSpacing: 100,  // horizontal gap between columns (default: 120)
  fontWeight: FontWeight.bold,
  staggered: true,     // brick pattern offset (default: true)
);

Forensic watermark (embed invisible data)

import 'package:secure_watermark/secure_watermark.dart';

// Embed a payload into an image
final watermarked = ForensicWatermark.embed(
  imageBytes: pngBytes,         // PNG-encoded Uint8List
  payload: 'user@example.com',  // data to hide
  key: 'secret-key-123',        // secret key for PRNG seeding
);

// Extract the payload later
final payload = ForensicWatermark.extract(
  imageBytes: watermarked,
  key: 'secret-key-123',
);
print(payload); // user@example.com

Forensic watermark widget

ForensicWatermarkImage(
  imageBytes: pngBytes,
  payload: 'user@example.com',
  secretKey: 'key-123',
  config: const ForensicConfig(redundancy: 5),
  placeholder: const CircularProgressIndicator(),
  errorBuilder: (ctx, error) => Text('Failed: $error'),
  fit: BoxFit.contain,
)

CLI tools

# Embed
dart run secure_watermark:embed -i photo.png -o watermarked.png -p "user@example.com" -k "secret"

# Extract
dart run secure_watermark:extract -i watermarked.png -k "secret"

API Reference

Watermark

Property	Type	Default	Description
text	String	required	Text to tile as the watermark
style	WatermarkStyle	WatermarkStyle()	Visual configuration
enabled	bool	true	Whether the overlay is visible
child	Widget	required	Content to display beneath the watermark

WatermarkStyle

Property	Type	Default	Description
opacity	double	0.15	Text opacity (0.0–1.0)
rotate	double	-30	Rotation angle in degrees
textColor	Color	Color(0xFF9E9E9E)	Text color (opacity applied separately)
fontSize	double	16	Font size in logical pixels
rowSpacing	double	80	Vertical spacing between rows
columnSpacing	double	120	Horizontal spacing between columns
fontWeight	FontWeight	FontWeight.normal	Font weight
staggered	bool	true	Offset alternate rows for brick pattern

ForensicWatermark

Method	Returns	Description
embed(imageBytes, payload, key, [config])	Uint8List	Embeds payload into PNG, returns watermarked PNG bytes
extract(imageBytes, key, [config])	String?	Extracts payload, returns null on wrong key or no watermark

ForensicConfig

Property	Type	Default	Description
redundancy	int	5	Odd positive int — copies for majority voting
bitsPerChannel	int	1	LSBs per channel (only 1 supported)

ForensicWatermarkImage

Property	Type	Default	Description
imageBytes	Uint8List	required	Source PNG image
payload	String	required	Data to embed invisibly
secretKey	String	required	Secret key for PRNG seeding
config	ForensicConfig	ForensicConfig()	Algorithm configuration
placeholder	Widget?	null	Shown during processing
errorBuilder	Widget Function(BuildContext, Object)?	null	Called on failure
fit	BoxFit	BoxFit.contain	Image fit mode
alignment	Alignment	Alignment.center	Image alignment

How it works

The Watermark widget uses a Stack to layer a CustomPaint overlay on top of the child content. The WatermarkPainter:

1. Builds a ui.Paragraph with the watermark text
2. Translates and rotates the canvas around its center
3. Tiles the paragraph in a grid covering the full diagonal extent (ensuring no gaps after rotation)
4. In staggered mode, offsets alternate rows by half the column spacing — creating a brick pattern that is harder to crop out

The overlay is wrapped in IgnorePointer (touches pass through) and RepaintBoundary (repaints are isolated from the child).

Forensic watermark algorithm

The ForensicWatermark uses spread-spectrum LSB (Least Significant Bit) modulation:

Embedding:

1. Decode PNG to RGBA pixels
2. Convert payload to UTF-8 bytes, prepend 16-bit magic number (0x574D) + 32-bit length header
3. Seed a PRNG with a DJB2 hash of the secret key
4. Generate unique random pixel positions (Set-based collision avoidance)
5. Modify the LSB of the blue channel at each position to encode bits
6. Repeat the payload redundancy times for error correction
7. Re-encode to PNG

Extraction:

1. Decode PNG, seed PRNG with same key
2. Read header bits across all redundancy copies, majority-vote each bit
3. Verify magic number — return null if wrong (wrong key)
4. Read payload bits, majority-vote, decode UTF-8

Example app

The example/ directory contains a full demo app with two tabs:

• Visible — interactive controls for the Watermark widget (opacity, angle, font size, staggered pattern)
• Forensic — embed/extract invisible watermarks with configurable payload, key, and redundancy

cd example
flutter run

Related packages

• no_screenshot — Screenshot & recording prevention
• no_tapjack — Tapjacking & overlay attack detection

License

BSD 3-Clause License. See LICENSE for details.