Unity 6 + Speech-to-Text V2: RSA Key Import Error

Google Cloud Build with AI AI APIs
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The Problem

Getting this error when trying to authenticate with service account in Unity 6.0.58:

Unable to import PKCS#8 private key with the current runtime

Happens when calling RSA.ImportPkcs8PrivateKey() to sign JWT for OAuth token exchange.

Setup

  • Unity 6.0.58, Meta Quest (Android)

  • Trying to use service account JSON to generate OAuth tokens client-side

  • Need access tokens for Speech-to-Text V2 API calls

Questions

  1. Does Unity’s Mono/.NET support PKCS#8 key import? Or do I need BouncyCastle?

  2. Should I even do this client-side? Is server-side token generation the recommended pattern for Unity apps?

  3. Can I just use API keys instead? Documentation shows API key option but I’m getting auth errors with that approach.

  4. Any Unity samples/SDKs for Google Cloud authentication?

Code Context

Loading service account JSON → building JWT → signing with private key (fails here) → exchange for access token → use for API calls.

The RSA import methods available in .NET Standard 2.1+ don’t seem to exist in Unity 6’s runtime.

Platform: Android (Quest), .NET Standard 2.1, IL2CPP

Any guidance appreciated!