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package server
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import (
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"bytes"
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"crypto/rand"
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"encoding/binary"
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"encoding/hex"
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"errors"
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"fmt"
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)
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// ClientHello contains every field in a ClientHello message
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type ClientHello struct {
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handshakeType byte
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length int
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clientVersion []byte
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random []byte
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sessionIdLen int
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sessionId []byte
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cipherSuitesLen int
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cipherSuites []byte
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compressionMethodsLen int
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compressionMethods []byte
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extensionsLen int
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extensions map[[2]byte][]byte
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}
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var u16 = binary.BigEndian.Uint16
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var u32 = binary.BigEndian.Uint32
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func parseExtensions(input []byte) (ret map[[2]byte][]byte, err error) {
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defer func() {
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if r := recover(); r != nil {
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err = errors.New("Malformed Extensions")
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}
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}()
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pointer := 0
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totalLen := len(input)
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ret = make(map[[2]byte][]byte)
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for pointer < totalLen {
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var typ [2]byte
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copy(typ[:], input[pointer:pointer+2])
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pointer += 2
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length := int(u16(input[pointer : pointer+2]))
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pointer += 2
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data := input[pointer : pointer+length]
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pointer += length
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ret[typ] = data
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}
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return ret, err
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}
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func parseKeyShare(input []byte) (ret []byte, err error) {
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defer func() {
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if r := recover(); r != nil {
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err = errors.New("malformed key_share")
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}
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}()
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totalLen := int(u16(input[0:2]))
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// 2 bytes "client key share length"
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pointer := 2
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for pointer < totalLen {
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if bytes.Equal([]byte{0x00, 0x1d}, input[pointer:pointer+2]) {
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// skip "key exchange length"
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pointer += 2
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length := int(u16(input[pointer : pointer+2]))
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pointer += 2
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if length != 32 {
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return nil, fmt.Errorf("key share length should be 32, instead of %v", length)
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}
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return input[pointer : pointer+length], nil
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}
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pointer += 2
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length := int(u16(input[pointer : pointer+2]))
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pointer += 2
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_ = input[pointer : pointer+length]
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pointer += length
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}
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return nil, errors.New("x25519 does not exist")
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}
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// AddRecordLayer adds record layer to data
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func AddRecordLayer(input []byte, typ []byte, ver []byte) []byte {
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length := make([]byte, 2)
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binary.BigEndian.PutUint16(length, uint16(len(input)))
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ret := make([]byte, 5+len(input))
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copy(ret[0:1], typ)
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copy(ret[1:3], ver)
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copy(ret[3:5], length)
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copy(ret[5:], input)
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return ret
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}
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// PeelRecordLayer peels off the record layer
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func PeelRecordLayer(data []byte) []byte {
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ret := data[5:]
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return ret
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}
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// ParseClientHello parses everything on top of the TLS layer
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// (including the record layer) into ClientHello type
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func ParseClientHello(data []byte) (ret *ClientHello, err error) {
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defer func() {
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if r := recover(); r != nil {
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err = errors.New("Malformed ClientHello")
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}
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}()
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data = PeelRecordLayer(data)
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pointer := 0
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// Handshake Type
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handshakeType := data[pointer]
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if handshakeType != 0x01 {
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return ret, errors.New("Not a ClientHello")
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}
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pointer += 1
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// Length
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length := int(u32(append([]byte{0x00}, data[pointer:pointer+3]...)))
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pointer += 3
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if length != len(data[pointer:]) {
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return ret, errors.New("Hello length doesn't match")
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}
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// Client Version
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clientVersion := data[pointer : pointer+2]
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pointer += 2
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// Random
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random := data[pointer : pointer+32]
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pointer += 32
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// Session ID
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sessionIdLen := int(data[pointer])
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pointer += 1
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sessionId := data[pointer : pointer+sessionIdLen]
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pointer += sessionIdLen
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// Cipher Suites
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cipherSuitesLen := int(u16(data[pointer : pointer+2]))
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pointer += 2
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cipherSuites := data[pointer : pointer+cipherSuitesLen]
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pointer += cipherSuitesLen
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// Compression Methods
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compressionMethodsLen := int(data[pointer])
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pointer += 1
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compressionMethods := data[pointer : pointer+compressionMethodsLen]
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pointer += compressionMethodsLen
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// Extensions
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extensionsLen := int(u16(data[pointer : pointer+2]))
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pointer += 2
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extensions, err := parseExtensions(data[pointer:])
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ret = &ClientHello{
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handshakeType,
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length,
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clientVersion,
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random,
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sessionIdLen,
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sessionId,
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cipherSuitesLen,
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cipherSuites,
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compressionMethodsLen,
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compressionMethods,
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extensionsLen,
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extensions,
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}
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return
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}
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func xor(a []byte, b []byte) {
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for i := range a {
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a[i] ^= b[i]
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}
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}
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func composeServerHello(ch *ClientHello, sharedSecret []byte, sessionKey []byte) []byte {
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var serverHello [11][]byte
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serverHello[0] = []byte{0x02} // handshake type
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serverHello[1] = []byte{0x00, 0x00, 0x76} // length 77
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serverHello[2] = []byte{0x03, 0x03} // server version
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xor(sharedSecret, sessionKey)
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serverHello[3] = sharedSecret // random
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serverHello[4] = []byte{0x20} // session id length 32
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serverHello[5] = ch.sessionId // session id
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serverHello[6] = []byte{0xc0, 0x30} // cipher suite TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384
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serverHello[7] = []byte{0x00} // compression method null
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serverHello[8] = []byte{0x00, 0x2e} // extensions length 46
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keyShare, _ := hex.DecodeString("00330024001d0020")
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keyExchange := make([]byte, 32)
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rand.Read(keyExchange)
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serverHello[9] = append(keyShare, keyExchange...)
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serverHello[10], _ = hex.DecodeString("002b00020304")
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var ret []byte
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for _, s := range serverHello {
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ret = append(ret, s...)
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}
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return ret
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}
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// ComposeReply composes the ServerHello, ChangeCipherSpec and Finished messages
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// together with their respective record layers into one byte slice. The content
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// of these messages are random and useless for this plugin
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func ComposeReply(ch *ClientHello, sharedSecret []byte, sessionKey []byte) []byte {
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TLS12 := []byte{0x03, 0x03}
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shBytes := AddRecordLayer(composeServerHello(ch, sharedSecret, sessionKey), []byte{0x16}, TLS12)
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ccsBytes := AddRecordLayer([]byte{0x01}, []byte{0x14}, TLS12)
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ret := append(shBytes, ccsBytes...)
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return ret
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}
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