feat: ECDSA key handling and ByteBuffer conversions for OpenSSH format

[nedithgar]

This pull request introduces support for handling ECDSA keys (P-256, P-384, and P-521) in OpenSSH format by adding parsing, serialization, and validation functionalities. It also includes test coverage for these features to ensure correctness. Below is a summary of the most important changes:

ECDSA Key Support

• Added ByteBufferConvertible conformances for P256, P384, and P521 private and public keys to enable reading and writing keys in OpenSSH format. This includes handling curve-specific constraints and key serialization requirements. (Sources/Citadel/Algorithms/ECDSA.swift, Sources/Citadel/Algorithms/ECDSA.swiftR1-R265)
• Implemented OpenSSHPrivateKey conformances for P256, P384, and P521 private keys, defining key prefixes and types for OpenSSH integration. (Sources/Citadel/Algorithms/ECDSA.swift, Sources/Citadel/Algorithms/ECDSA.swiftR1-R265)

ByteBuffer Enhancements

• Added readSSHBignum and writeSSHBignum methods to ByteBuffer for reading and writing SSH bignum values, which are used in ECDSA key serialization. (Sources/Citadel/ByteBufferHelpers.swift, Sources/Citadel/ByteBufferHelpers.swiftR152-R171)
• Imported BigInt to support SSH bignum operations. (Sources/Citadel/ByteBufferHelpers.swift, Sources/Citadel/ByteBufferHelpers.swiftR3)

OpenSSH Key Type Updates

• Made OpenSSH.KeyType and OpenSSH enums public and added new key types for ECDSA (ecdsaP256, ecdsaP384, ecdsaP521). (Sources/Citadel/OpenSSHKey.swift, [1] [2]

ECDSA Key Parsing APIs

• Added initializers to P256, P384, and P521 private keys for creating keys from OpenSSH private key strings or data, with optional decryption support. (Sources/Citadel/SSHCert.swift, Sources/Citadel/SSHCert.swiftR124-R192)

Unit Tests

• Introduced ECDSAKeyTests to validate parsing, serialization, and signing functionality for P-256, P-384, and P-521 private keys. Tests include scenarios for valid keys, encrypted keys, invalid formats, and curve mismatches. (Tests/CitadelTests/ECDSAKeyTests.swift, Tests/CitadelTests/ECDSAKeyTests.swiftR1-R122)

[Copilot]

Pull Request Overview

This pull request adds comprehensive ECDSA key support for OpenSSH format, enabling parsing, serialization, and validation of P-256, P-384, and P-521 elliptic curve keys. The implementation provides a complete ECDSA key handling solution with proper SSH format compliance.

Key changes include:

• ECDSA key type support with ByteBuffer serialization/deserialization for all three curve types
• SSH bignum handling methods for proper numeric data encoding/decoding
• Public API extensions for creating ECDSA keys from OpenSSH formatted strings

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
Sources/Citadel/Algorithms/ECDSA.swift	Implements ByteBufferConvertible and OpenSSHPrivateKey conformances for P256/P384/P521 keys with SSH format support
Sources/Citadel/ByteBufferHelpers.swift	Adds SSH bignum read/write methods and string helper functions for key serialization
Sources/Citadel/OpenSSHKey.swift	Makes OpenSSH types public and adds ECDSA key type enumerations
Sources/Citadel/SSHCert.swift	Provides convenient initializers for creating ECDSA keys from OpenSSH strings
Tests/CitadelTests/ECDSAKeyTests.swift	Comprehensive test suite covering key parsing, validation, and error handling scenarios

Comments suppressed due to low confidence (1)

Tests/CitadelTests/ECDSAKeyTests.swift:90

• This test uses an empty string which doesn't properly test encrypted key parsing. Consider using a real encrypted key or testing the specific error type that should be thrown for invalid formats.

        XCTAssertThrowsError(try P256.Signing.PrivateKey(sshECDSA: "", decryptionKey: passphrase.data(using: .utf8)))