Add WebAssembly (Blazor WASM) support — System.Security.Cryptography.ChaCha20Poly1305 is [UnsupportedOSPlatform("browser")]

[petre-c]

Summary

AgeSharp 0.2.0-preview.2 does not work in Blazor WebAssembly (or any other browser-WASM .NET host). Every age operation – encrypt and decrypt, passphrase mode and X25519 – throws:

PlatformNotSupportedException: Cryptography_AlgorithmNotSupported, ChaCha20Poly1305

Root cause: Age/Crypto/CryptoHelper.cs uses System.Security.Cryptography.ChaCha20Poly1305 directly. That BCL class is annotated [UnsupportedOSPlatform("browser")] (API ref), so on mono-wasm the constructor raises PlatformNotSupportedException. .IsSupported returns false and there is no native browser substitute (WebCrypto doesn't expose ChaCha20-Poly1305).

Affected files:

• Age/Crypto/CryptoHelper.cs
• Age/Crypto/EncryptStream.cs
• Age/Crypto/DecryptStream.cs
• Age/Crypto/StreamEncryption.cs

Why this matters

age is an obvious fit for client-side encryption in Blazor WASM apps (offline wallet tools, password-manager-style apps, Tails-friendly tooling). We hit this in PeteSparrowBTC/Seed-Phrase-Storage-SLIP39 – a SLIP-39 + age wallet-backup tool that needs to run airgapped in a browser – and had to fork to ship.

Suggested fix

Swap System.Security.Cryptography.ChaCha20Poly1305 for Org.BouncyCastle.Crypto.Modes.ChaCha20Poly1305 (pure-managed C#, works in WASM, already a transitive dep via the scrypt / X25519 paths).

The mechanical, minimum-diff approach is a small adapter class (BcChaCha20Poly1305) that mirrors the BCL surface (Encrypt(nonce, plaintext, ciphertext, tag, aad) / Decrypt(...) + IDisposable) and delegates to BC internally; then in each consuming file, a per-file type alias:

using ChaCha20Poly1305 = Age.Crypto.BcChaCha20Poly1305;

This keeps EncryptStream / DecryptStream / StreamEncryption / CryptoHelper source code completely unchanged downstream of the type alias.

Ready-made patch (PR-ready)

A working patch lives on the PeteSparrowBTC:wasm-bouncy-chacha branch (one commit). It:

• Adds Age/Crypto/BcChaCha20Poly1305.cs – the adapter.
• Adds the type alias to the 4 affected files.
• Translates BC's InvalidCipherTextException to BCL's AuthenticationTagMismatchException so existing catch blocks in CryptoHelper continue to work unchanged.

Test results on that branch:

• Age.Tests (323 tests): all pass on the patched branch.
• Age.TestKit (61 failures): pre-existing on upstream main (verified via git stash round-trip), unrelated to this patch. Worth a separate triage.
• Round-trip verified end-to-end in Blazor WASM by the consuming project: an age v1 ciphertext produced under WASM decrypts cleanly back to the original payload, and the derived BIP-32 master fingerprint matches the known-answer vector (73c5da0a for the abandon-seed). The previous PlatformNotSupportedException: ChaCha20Poly1305 is gone.

Happy to open a PR.

Environment

• AgeSharp 0.2.0-preview.2 (NuGet)
• BouncyCastle.Cryptography 2.6.2 (already a transitive dep)
• .NET SDK 10.0.203 on Windows 11
• Blazor WASM (Microsoft.AspNetCore.Components.WebAssembly 10.0.0)

[petre-c]

Follow-up — patch now validated on CI with full CCTV cross-impl coverage

After landing the patch in the consuming project, we ran AgeSharp's own Age.TestKit (143 wire-format reference vectors from filippo.io/age) on a Linux CI runner through the patched BcChaCha20Poly1305 adapter. Result:

Passed!  - Failed: 0, Passed: 143, Skipped: 0, Total: 143, Duration: 998 ms - Age.TestKit.dll (net10.0)

143 / 143 pass on Linux — every age v1 reference ciphertext generated by the Go reference implementation decrypts correctly through the BouncyCastle-backed adapter. The cipher swap is wire-format equivalent.

CI run: https://github.com/PeteSparrowBTC/Seed-Phrase-Storage-SLIP39/actions/runs/27400022461 (job → AgeSharp CCTV vectors (cross-impl interop baseline check)).

Correction to the original issue body

I originally reported "Age.TestKit: 61 failures (pre-existing on upstream main)". That was correct on Windows but misleading — those 61 failures happen identically on unpatched upstream main (Windows) and disappear on Linux. So the failures are environmental, not a baseline of upstream bugs. Root cause is Git's autocrlf normalization on Windows checkout corrupting the byte-exact testdata reference vectors — filed separately as #3.

TL;DR

• Patched fork passes all 143 wire-format reference vectors on Linux.
• Patched fork passes Age.Tests (323 / 323) on Linux as before.
• Original WASM crash (PlatformNotSupportedException: ChaCha20Poly1305) is resolved end-to-end (verified via a Blazor WASM Owner→Recoverer round-trip in a real browser).

PR-ready branch is still https://github.com/PeteSparrowBTC/AgeSharp/tree/wasm-bouncy-chacha. Happy to open a PR.