"""
SSH Key Exchange Implementation

Implements SSH key exchange algorithms including Curve25519, ECDH,
and Diffie-Hellman for secure session key establishment.
"""

from typing import Optional, Tuple, Any
from ..exceptions import CryptoException


class KeyExchange:
    """
    SSH key exchange implementation.
    
    Handles key exchange algorithms and session key derivation
    according to SSH protocol specifications.
    """
    
    def __init__(self, transport: Any) -> None:
        """
        Initialize key exchange with transport.
        
        Args:
            transport: SSH transport instance
        """
        self._transport = transport
        self._algorithm: Optional[str] = None
    
    def start_kex(self) -> None:
        """
        Start key exchange process.
        
        Raises:
            CryptoException: If key exchange fails
        """
        # Implementation will be added in later tasks
        raise NotImplementedError("KeyExchange.start_kex will be implemented in task 3.2")
    
    def generate_keys(self) -> Tuple[bytes, bytes, bytes, bytes]:
        """
        Generate session keys from shared secret.
        
        Returns:
            Tuple of (encryption_key, mac_key, iv, mac_iv)
            
        Raises:
            CryptoException: If key generation fails
        """
        # Implementation will be added in later tasks
        raise NotImplementedError("KeyExchange.generate_keys will be implemented in task 3.2")