ja4sentinel/internal/fingerprint/engine.go

// Package fingerprint provides JA4/JA3 fingerprint generation for TLS ClientHello
package fingerprint

import (
	"encoding/binary"
	"fmt"

	"ja4sentinel/api"

	tlsfingerprint "github.com/psanford/tlsfingerprint"
)

// EngineImpl implements the api.Engine interface for fingerprint generation
type EngineImpl struct{}

// NewEngine creates a new fingerprint engine
func NewEngine() *EngineImpl {
	return &EngineImpl{}
}

// FromClientHello generates JA4 (and optionally JA3) fingerprints from a TLS ClientHello
// Note: JA4 hash portion is extracted for internal use but NOT serialized to LogRecord
// as the JA4 format already includes its own hash portions (per architecture.yml)
func (e *EngineImpl) FromClientHello(ch api.TLSClientHello) (*api.Fingerprints, error) {
	if len(ch.Payload) == 0 {
		return nil, fmt.Errorf("empty ClientHello payload from %s:%d -> %s:%d",
			ch.SrcIP, ch.SrcPort, ch.DstIP, ch.DstPort)
	}

	// Parse the ClientHello using tlsfingerprint
	fp, err := tlsfingerprint.ParseClientHello(ch.Payload)
	if err != nil {
		// Try to sanitize truncated extensions and retry
		if sanitized := sanitizeClientHelloExtensions(ch.Payload); sanitized != nil {
			fp, err = tlsfingerprint.ParseClientHello(sanitized)
		}
		if err != nil {
			return nil, fmt.Errorf("failed to parse ClientHello from %s:%d -> %s:%d (conn_id=%s, payload_len=%d): %w",
				ch.SrcIP, ch.SrcPort, ch.DstIP, ch.DstPort, ch.ConnID, len(ch.Payload), err)
		}
	}

	// Generate JA4 fingerprint
	// Note: JA4 string format already includes the hash portion
	// e.g., "t13d1516h2_8daaf6152771_02cb136f2775" where the last part is the SHA256 hash
	ja4 := fp.JA4String()

	// Generate JA3 fingerprint and its MD5 hash
	ja3 := fp.JA3String()
	ja3Hash := fp.JA3Hash()

	// Extract JA4 hash portion (last segment after underscore)
	// JA4 format: <tls_ver><ciphers><extensions>_<sni_hash>_<cipher_extension_hash>
	// This is kept for internal use but NOT serialized to LogRecord
	ja4Hash := extractJA4Hash(ja4)

	return &api.Fingerprints{
		JA4:     ja4,
		JA4Hash: ja4Hash, // Internal use only - not serialized to LogRecord
		JA3:     ja3,
		JA3Hash: ja3Hash,
	}, nil
}

// extractJA4Hash extracts the hash portion from a JA4 string
// JA4 format: <base>_<sni_hash>_<cipher_hash> -> returns "<sni_hash>_<cipher_hash>"
func extractJA4Hash(ja4 string) string {
	// JA4 string format: t13d1516h2_8daaf6152771_02cb136f2775
	// We extract everything after the first underscore as the "hash" portion
	for i, c := range ja4 {
		if c == '_' {
			return ja4[i+1:]
		}
	}
	return ""
}

// sanitizeClientHelloExtensions fixes ClientHellos with truncated extension data
// by adjusting the extensions length to include only complete extensions.
// Returns a corrected copy, or nil if the payload cannot be fixed.
func sanitizeClientHelloExtensions(data []byte) []byte {
	if len(data) < 5 || data[0] != 0x16 {
		return nil
	}
	recordLen := int(data[3])<<8 | int(data[4])
	if len(data) < 5+recordLen {
		return nil
	}
	payload := data[5 : 5+recordLen]
	if len(payload) < 4 || payload[0] != 0x01 {
		return nil
	}
	helloLen := int(payload[1])<<16 | int(payload[2])<<8 | int(payload[3])
	if len(payload) < 4+helloLen {
		return nil
	}
	hello := payload[4 : 4+helloLen]

	// Skip through ClientHello fields to reach extensions
	offset := 2 + 32 // version + random
	if len(hello) < offset+1 {
		return nil
	}
	offset += 1 + int(hello[offset]) // session ID
	if len(hello) < offset+2 {
		return nil
	}
	csLen := int(hello[offset])<<8 | int(hello[offset+1])
	offset += 2 + csLen // cipher suites
	if len(hello) < offset+1 {
		return nil
	}
	offset += 1 + int(hello[offset]) // compression methods
	if len(hello) < offset+2 {
		return nil
	}
	extLenOffset := offset // position of extensions length field
	declaredExtLen := int(hello[offset])<<8 | int(hello[offset+1])
	offset += 2
	extStart := offset

	if len(hello) < extStart+declaredExtLen {
		return nil
	}
	extData := hello[extStart : extStart+declaredExtLen]

	// Walk extensions, find how many complete ones exist
	validLen := 0
	pos := 0
	for pos < len(extData) {
		if pos+4 > len(extData) {
			break
		}
		extBodyLen := int(extData[pos+2])<<8 | int(extData[pos+3])
		if pos+4+extBodyLen > len(extData) {
			break // this extension is truncated
		}
		pos += 4 + extBodyLen
		validLen = pos
	}

	if validLen == declaredExtLen {
		return nil // no truncation found, nothing to fix
	}

	// Build a corrected copy with adjusted extensions length
	fixed := make([]byte, len(data))
	copy(fixed, data)

	// Absolute offset of extensions length field within data
	extLenAbs := 5 + 4 + extLenOffset
	diff := declaredExtLen - validLen

	// Update extensions length
	binary.BigEndian.PutUint16(fixed[extLenAbs:], uint16(validLen))
	// Update ClientHello handshake length
	newHelloLen := helloLen - diff
	fixed[5+1] = byte(newHelloLen >> 16)
	fixed[5+2] = byte(newHelloLen >> 8)
	fixed[5+3] = byte(newHelloLen)
	// Update TLS record length
	newRecordLen := recordLen - diff
	binary.BigEndian.PutUint16(fixed[3:5], uint16(newRecordLen))

	return fixed[:5+newRecordLen]
}