Underlay

Underlay is a local .NET desktop app for hiding encrypted text messages inside ordinary PNG images.

The goal is not just to encrypt message content, but to avoid obvious technical markers that reveal a message exists. Underlay encrypts text with AES-256-GCM using an Argon2id-derived key, then embeds the encrypted payload into adaptive PNG pixel data. There are no accounts, servers, telemetry, analytics, or network workflows.

Status

Current release: 0.1.0-beta

Supported v1 beta workflow:

• Select a PNG cover image.
• Enter a text message and password.
• Save an encoded PNG.
• Reveal the message later from the encoded PNG with the same password.

Important v1 limitations:

• PNG only for the user-facing workflow.
• JPEG support is experimental research only.
• Resizing, cropping, screenshots, format conversion, and social-platform processing are expected to destroy hidden content.
• Large 12MP-class PNGs can take tens of seconds to encode or decode.
• Advanced targeted steganalysis is out of scope.
• Password exchange is out of scope.
• Do not share the original cover image together with the encoded output image.

Build And Run

Requirements:

• .NET 8 SDK for building and testing.
• .NET 8 Desktop Runtime for running the framework-dependent Windows beta package.

From the repository root:

dotnet restore Underlay.sln
dotnet build Underlay.sln --configuration Release --no-restore
dotnet run --project src\Underlay.App\Underlay.App.csproj --configuration Release

Run the test suite:

dotnet test Underlay.sln --configuration Release --no-build

Create the Windows x64 beta publish folder:

dotnet publish src\Underlay.App\Underlay.App.csproj --configuration Release --runtime win-x64 --self-contained false --output artifacts\publish\underlay-v1-beta-win-x64

Run the packaged app:

.\artifacts\publish\underlay-v1-beta-win-x64\Underlay.exe

Documentation

• User guide
• Threat model
• Technical whitepaper
• Privacy statement
• Release notes
• Changelog
• Security policy
• Contributing guide

Security Model Summary

Underlay's encrypted payload format is exactly:

[salt:16][nonce:12][ciphertext:N][authTag:16]

There are no magic bytes, version fields, fixed app identifiers, or Underlay signatures in the encrypted payload. Decode failures are intentionally uniform:

No message found

The same message is shown for a wrong password, a clean image, corrupted payload, unsupported data, or extraction failure. This avoids leaking whether hidden data exists.

License

Underlay is released under the MIT License.