"""DKIM (DomainKeys Identified Mail) implementation for email authentication."""
import dns.resolver
import base64
import re
from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.primitives.asymmetric import rsa, padding
from cryptography.hazmat.primitives import hashes


class DKIMManager:
    def __init__(self, domain, selector="default"):
        """
        Initialize DKIM manager.
        
        Args:
            domain (str): Email domain
            selector (str): DKIM selector
        """
        self.domain = domain
        self.selector = selector
        self._dkim_record = None
    
    @property
    def dkim_record(self):
        """Get the DKIM record for the domain and selector."""
        if self._dkim_record is None:
            try:
                lookup_domain = f"{self.selector}._domainkey.{self.domain}"
                answers = dns.resolver.resolve(lookup_domain, 'TXT')
                for rdata in answers:
                    self._dkim_record = rdata.to_text().strip('"')
                    break
            except (dns.resolver.NoAnswer, dns.resolver.NXDOMAIN):
                self._dkim_record = ""
        return self._dkim_record
    
    def verify_record_exists(self):
        """Check if a DKIM record exists for the domain and selector."""
        return bool(self.dkim_record)
    
    def analyze_record(self):
        """
        Analyze the DKIM record for common issues.
        
        Returns:
            dict: Analysis results
        """
        if not self.dkim_record:
            return {"exists": False, "error": "No DKIM record found"}
        
        results = {
            "exists": True,
            "record": self.dkim_record,
            "issues": [],
            "parsed": {}
        }
        
        # Parse key-value pairs
        pairs = re.findall(r'([a-z]+)=([^;]+)', self.dkim_record)
        for key, value in pairs:
            results["parsed"][key] = value
        
        # Check for required tags
        required_tags = ["v", "p"]
        for tag in required_tags:
            if tag not in results["parsed"]:
                results["issues"].append(f"Missing required tag: {tag}")
        
        # Check if the record has a public key
        if results["parsed"].get("p", "") == "":
            results["issues"].append("Empty public key (p=)")
            
        return results
    
    def generate_keypair(self, key_size=2048):
        """
        Generate a new RSA key pair for DKIM signing.
        
        Args:
            key_size (int): Size of the key in bits
            
        Returns:
            tuple: (private_key_pem, public_key_txt_record)
        """
        # Generate private key
        private_key = rsa.generate_private_key(
            public_exponent=65537,
            key_size=key_size
        )
        
        # Convert to PEM format
        private_pem = private_key.private_bytes(
            encoding=serialization.Encoding.PEM,
            format=serialization.PrivateFormat.TraditionalOpenSSL,
            encryption_algorithm=serialization.NoEncryption()
        ).decode('utf-8')
        
        # Get public key
        public_key = private_key.public_key()
        public_der = public_key.public_bytes(
            encoding=serialization.Encoding.DER,
            format=serialization.PublicFormat.PKCS1
        )
        
        # Base64 encode the public key
        public_b64 = base64.b64encode(public_der).decode('ascii')
        
        # Generate a TXT record value
        txt_record = f"v=DKIM1; k=rsa; p={public_b64}"
        
        return private_pem, txt_record
    
    def sign_email(self, email_content, private_key_pem):
        """
        Sign an email with DKIM.
        
        Args:
            email_content (str): Email content to sign
            private_key_pem (str): Private key in PEM format
            
        Returns:
            str: DKIM signature header
        """
        # This is a simplified implementation
        # A full implementation would need to extract headers,
        # canonicalize them, and create a proper signature
        
        # Load the private key
        private_key = serialization.load_pem_private_key(
            private_key_pem.encode(),
            password=None
        )
        
        # Create a signature (simplified)
        signature = private_key.sign(
            email_content.encode(),
            padding.PKCS1v15(),
            hashes.SHA256()
        )
        
        # Return a simplified DKIM-Signature header
        b64_sig = base64.b64encode(signature).decode('ascii')
        
        header = (
            f"DKIM-Signature: v=1; a=rsa-sha256; d={self.domain}; "
            f"s={self.selector}; c=relaxed/simple; q=dns/txt; "
            f"h=from:to:subject:date; bh=<body-hash>; b={b64_sig}"
        )
        
        return header