Feat: Added Core ZkEmail Functionality

[shreyas-londhe]

Add DKIM Email Verification and Pattern Matching

Implements core functionality for verifying DKIM-signed emails and extracting patterns using zkVM-compatible regex matching.

Features

• DKIM signature verification using RSA-SHA256
• Email body hash validation
• Pattern matching using pre-compiled DFAs for zkVM compatibility
• Support for searching in both headers and body
• Optional pattern match revelation

Implementation Details

• Uses cfdkim for email canonicalization
• Implements zkVM-friendly pattern matching using regex-automata
• Supports both forward and reverse DFA matching
• Handles UTF-8 encoded content

Testing

• Verifies DKIM signatures on real email samples
• Tests pattern matching for reset codes and usernames
• Includes test email fixtures

Usage Example

let eml_content =
    fs::read("./test_emails/test.eml").context("Failed to read test email")?;
let verifier = EmailVerifier::from_eml(&eml_content).await?;

// Test for reset code
let pattern = r"t7bezzrn"; // The actual reset code from the email
let dfa = DfaRegex::new(pattern)?;
let fwd_bytes = dfa.forward().to_bytes_native_endian().0;
let rev_bytes = dfa.reverse().to_bytes_native_endian().0;

let result = verifier.verify_pattern(
    &fwd_bytes, &rev_bytes, false, // search in body
    true,  // reveal match
)?;

assert!(result.is_some(), "Reset code should be found");
assert_eq!(result.unwrap(), "t7bezzrn", "Should match exact reset code");

// Test for username
let pattern = r"@0xDMello"; // The username from the email
let dfa = DfaRegex::new(pattern)?;
let fwd_bytes = dfa.forward().to_bytes_native_endian().0;
let rev_bytes = dfa.reverse().to_bytes_native_endian().0;

let result = verifier.verify_pattern(
    &fwd_bytes, &rev_bytes, false, // search in body
    true,  // reveal match
)?;

assert!(result.is_some(), "Username should be found");
assert_eq!(result.unwrap(), "@0xDMello", "Should match exact username");