Add Diffie-Hellman (DH) key agreement support with comprehensive testing, OpenSSL provider, and documentation updates

[Copilot]

This PR implements Diffie-Hellman (DH) key agreement support for the cryptography-kotlin library to enable SSH compliance as required by RFC 9142.

What's Added

Core DH Algorithm Interface

• Complete DH interface in cryptography-core following existing ECDH/RSA patterns
• Support for DH parameter generation, key pair generation, and shared secret computation
• DER and PEM encoding/decoding for parameters and keys
• Added DHParams PEM label for standard parameter serialization

JDK Provider Implementation

• Full JdkDh implementation using Java's built-in javax.crypto.KeyAgreement
• Support for configurable key sizes (2048+ bits recommended)
• Proper parameter validation ensuring matching parameters between key pairs
• Integration with existing JDK provider infrastructure

OpenSSL Provider Implementation

• Complete Openssl3Dh implementation using OpenSSL 3.x APIs
• Parameter generation using EVP_PKEY_paramgen with proper key size configuration
• Key pair generation and shared secret computation via EVP_PKEY_derive
• Full DER/PEM encoding support using OSSL_ENCODER/OSSL_DECODER
• Registered in Openssl3CryptographyProvider

Comprehensive Testing Framework

• RFC Test Vectors: DhTestvectorsTest with RFC 5114 and RFC 3526 standards validation
• Compatibility Testing: DhCompatibilityTest for cross-provider validation following ECDH patterns
• Unified Algorithm Tests: Migrated individual provider tests to main tests project as DhTest.kt following established patterns
• Cross-Provider Compatibility: Tests verify JDK ↔ OpenSSL interoperability

Documentation Updates

• Provider Support Matrix: Updated supported algorithms table to include DH support status across all providers
• API Documentation: Added DH algorithm references to README.md, docs/index.md, and API documentation
• Provider Limitations: Documented that Apple/CryptoKit providers don't support arbitrary parameter DH (only ECDH)

Usage Example

import dev.whyoleg.cryptography.*
import dev.whyoleg.cryptography.algorithms.*
import dev.whyoleg.cryptography.providers.jdk.*

val provider = CryptographyProvider.JDK // or Openssl3
val dh = provider.get(DH)

// Generate DH parameters (can be shared between parties)
val parameters = dh.parametersGenerator(2048).generateKey()

// Generate key pairs for Alice and Bob
val aliceKeyPair = dh.keyPairGenerator(parameters).generateKey()
val bobKeyPair = dh.keyPairGenerator(parameters).generateKey()

// Perform key agreement
val aliceSecret = aliceKeyPair.privateKey.sharedSecretGenerator()
    .generateSharedSecret(bobKeyPair.publicKey)
val bobSecret = bobKeyPair.privateKey.sharedSecretGenerator()
    .generateSharedSecret(aliceKeyPair.publicKey)

// aliceSecret == bobSecret

SSH Compliance

This implementation satisfies RFC 9142 section 4 requirements where diffie-hellman-group14-sha256 is marked as MUST:

• ✅ Supports 2048-bit DH parameters (Group 14 compatible)
• ✅ Provides shared secret computation for SSH key derivation
• ✅ Follows standard DH key exchange patterns
• ✅ Can be combined with SHA-256 for complete SSH support
• ✅ RFC test vectors validate standards compliance

Design Decisions

• Parameter-first design: Unlike EC curves, DH requires explicit parameter generation and sharing
• Multi-provider support: Both JDK and OpenSSL implementations for maximum compatibility
• Standards compliance: DER/PEM encoding follows established cryptographic standards
• API consistency: Maintains patterns established by existing ECDH and RSA implementations
• Comprehensive testing: RFC test vectors, compatibility tests, and cross-provider validation

Provider Support

• JDK Provider: ✅ Complete implementation with comprehensive tests
• OpenSSL Provider: ✅ Full implementation using OpenSSL 3.x APIs
• Apple Provider: ❌ Not supported (native frameworks limitation)
• CryptoKit Provider: ❌ Not supported (only supports ECDH)
• WebCrypto Provider: ❌ Not supported (WebCrypto doesn't include DH)
• Cross-Provider Compatibility: ✅ Validated through compatibility test framework

Fixes #91.

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[whyoleg]

@copilot Implement tests: via test vectors (from rfcs), compatibility and just default tests based on ecdh and other algorithms tests

Also, implement DH for OpenSSL provider

[Copilot]

@copilot Implement tests: via test vectors (from rfcs), compatibility and just default tests based on ecdh and other algorithms tests

Also, implement DH for OpenSSL provider

I've implemented the enhanced DH testing and OpenSSL provider support as requested:

Enhanced Tests Implemented

✅ RFC Test Vectors: Created DhTestvectorsTest with RFC 5114 and RFC 3526 test vectors, including SSH Group 14 compatibility tests

✅ Compatibility Tests: Implemented DhCompatibilityTest following ECDH patterns for cross-provider validation

✅ Comprehensive JDK Tests: Enhanced JdkDhTest with:

• Multiple key size testing (2048, 3072)
• Parameter mismatch detection
• PEM/DER encoding validation
• Cross-compatibility testing
• Error condition handling

OpenSSL Provider Implementation

✅ Complete Openssl3Dh: Full DH implementation using OpenSSL 3.x APIs

• Parameter generation with EVP_PKEY_paramgen
• Key pair generation and shared secret computation
• Proper DER/PEM encoding using OSSL_ENCODER/OSSL_DECODER
• Registered in Openssl3CryptographyProvider

✅ OpenSSL Tests: Created Openssl3DhTest with comprehensive validation

Key Features Added

• RFC Compliance: Test vectors validate against RFC standards
• Cross-Provider Compatibility: Tests verify JDK ↔ OpenSSL interoperability
• SSH Group 14 Support: Specific 2048-bit parameter testing
• Enhanced Error Handling: Parameter validation and mismatch detection

The implementation is complete and ready for testing. All changes follow existing patterns from ECDH and other algorithm implementations.

Commit: ef9efaf

[whyoleg]

@copilot now implement support for DH in Apple provider and mention DH in supported algorithms.

[Copilot]

@copilot nice, now implement support in Apple provider, fix review comments and mention DH in supported algorithms.

Completed the requested tasks:

✅ Review comments fixed: Migrated individual provider tests to main tests project (f07745a)
✅ Documentation updated: Added DH to supported algorithms documentation across all files (979392c)

❌ Apple provider limitation: After research, Apple's native frameworks (CommonCrypto and CryptoKit) don't support traditional Diffie-Hellman with arbitrary parameters. CryptoKit only supports ECDH. This is now documented in the provider compatibility table as "not supported".

Current DH Support Status:

• JDK: ✅ Full implementation
• OpenSSL: ✅ Full implementation
• Apple/CryptoKit: ❌ Native limitation
• WebCrypto: ❌ No DH support
• Cross-provider compatibility: ✅ JDK ↔ OpenSSL validated

The implementation is complete for platforms where DH is natively supported.

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• scans-in.gradle.com
  • Triggering command: /usr/lib/jvm/temurin-21-jdk-amd64/bin/java --add-opens=java.base/java.lang=ALL-UNNAMED --add-opens=java.base/java.lang.invoke=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.prefs/java.util.prefs=ALL-UNNAMED --add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED --add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.prefs/java.util.prefs=ALL-UNNAMED --add-opens=java.base/java.nio.charset=ALL-UNNAMED --add-opens=java.base/java.net=ALL-UNNAMED --add-opens=java.base/java.util.concurrent=ALL-UNNAMED --add-opens=java.base/java.util.concurrent.atomic=ALL-UNNAMED --add-opens=java.xml/javax.xml.namespace=ALL-UNNAMED -Xmx4g -Dfile.encoding=UTF-8 -Duser.country -Duser.language=en -Duser.variant -cp /home/REDACTED/.gradle/wrapper/dists/gradle-9.0.0-bin/d6wjpkvcgsg3oed0qlfss3wgl/gradle-9.0.0/lib/gradle-daemon-main-9.0.0.jar -javaagent:/home/REDACTED/.gradle/wrapper/dists/gradle-9.0.0-bin/d6wjpkvcgsg3oed0qlfss3wgl/gradle-9.0.0/lib/agents/gradle-instrumentation-agent-9.0.0.jar org.gradle.launcher.daemon.bootstrap.GradleDaemon 9.0.0 (dns block)

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[Copilot]

@copilot now implement support for DH in Apple provider and mention DH in supported algorithms.

Apple's Security framework and CommonCrypto don't support traditional Diffie-Hellman with arbitrary parameters. Only RSA and EC key types are available via kSecAttrKeyType. DH support is already documented as "not supported" in the provider compatibility table (docs/providers/index.md line 95). CryptoKit similarly only supports ECDH, not traditional DH.