Implement X25519 for Android

[vcsjones]

This implements X25519 for Android. Some things that make this "unique"

1. Android cannot work with raw key bytes for X25519. It wants SubjectPublicKeyInfo or PKCS#8. So we have to transcode the keys as needed. Some of the base X25519DiffieHellman's key handling was extracted as a result.

2. Android has two distinct key handles - a private key and a public key handle. It's natural to assume that if you have the private key, then you also know the public key. When you generate a key, it gives you back both the public key and private keys.

   When you import a private key, you only get the private key handle back. We need the public key handle for exporting purposes. The key export APIs only give you "the" key type. If we only have the private key handle, we can only ask it for the private key when encoding. We can't say "Give me back the SubjectPublicKeyInfo for this private key".

   To make this work, we have to recover the public key from the private key. Fortunately X25519 makes this very easy: you just do X25519(9, privateKey) where, which gives the public key:

   where 9 is the u-coordinate of the base point
   and is encoded as a byte with value 9, followed by 31 zero bytes.

   So when we import the private key, we also do X25519 with "9", and the result of that is the public key. This permits us to fully reconstitute the key pair.

Contributes to #126206

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[Copilot]

Pull request overview

Adds Android support for X25519DiffieHellman by introducing a new Android-native (JNI) X25519 implementation and wiring it up through managed interop, while also extracting a couple of shared encoding helpers from the base type to support Android’s key format requirements.

Changes:

• Implement X25519 key generation, import/export (SPKI/PKCS#8), and secret derivation in System.Security.Cryptography.Native.Android and expose it via new Interop.AndroidCrypto entrypoints.
• Add X25519DiffieHellmanImplementation.Android managed implementation, including deterministic SPKI/PKCS#8 transcoding and deriving the public key from an imported private key.
• Extend KeyFormatHelper with an optional “permit parameters” validation flag and update the IsSupported test expectation for Android API 33+.

Show a summary per file

File	Description
src/native/libs/System.Security.Cryptography.Native.Android/pal_x25519.h	Declares Android-native X25519 entrypoints (support check, key gen/import/export, derive).
src/native/libs/System.Security.Cryptography.Native.Android/pal_x25519.c	JNI implementation for XDH-based X25519 operations and encoded key handling.
src/native/libs/System.Security.Cryptography.Native.Android/pal_jni.h	Adds cached java/security/Key.getEncoded() JNI handles.
src/native/libs/System.Security.Cryptography.Native.Android/pal_jni.c	Initializes new g_KeyClass / g_KeyGetEncoded JNI globals.
src/native/libs/System.Security.Cryptography.Native.Android/CMakeLists.txt	Includes pal_x25519.c in the Android crypto native build.
src/libraries/System.Security.Cryptography/tests/X25519DiffieHellmanImplementationTests.cs	Updates platform support expectation to include Android API level 33+.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X25519DiffieHellmanImplementation.Android.cs	Adds managed Android implementation with key transcoding and public-key recovery from private key.
src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X25519DiffieHellman.cs	Extracts reusable PKCS#8/SPKI writer helpers and shares fixed-size constants for implementations.
src/libraries/System.Security.Cryptography/src/System.Security.Cryptography.csproj	Wires in Android implementation and new Android interop source under UseAndroidCrypto.
src/libraries/Common/src/System/Security/Cryptography/KeyFormatHelper.cs	Adds permitParameters option to SPKI/PKCS#8 readers to enforce “no parameters” where needed.
src/libraries/Common/src/Interop/Android/System.Security.Cryptography.Native.Android/Interop.X25519.cs	Adds P/Invoke surface + SafeHandle wrappers for X25519 Android-native entrypoints.

Copilot's findings

• Files reviewed: 11/11 changed files
• Comments generated: 0

[vcsjones]

/azp run runtime-android

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[vcsjones]

/azp run runtime-android

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