Sends a TLS 1.2 Client Hello and extracts the resulting random bytes from the Server Hello

tlsrand.py

#!/usr/bin/env python3
# Usage: ./tlsrand server port

import socket
import sys

hello = bytearray([3, 3])               # Version: TLS 1.2

hello += bytearray([0]*32)              # Random

hello += bytearray([
    0,                                  # Session ID Length

    0, 6,                               # Cipher Suites Length
    0xc0, 0x2f,                         # TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256
    0, 0x2f,                            # TLS_RSA_WITH_AES_128_CBC_SHA
    0, 0x39,                            # TLS_DHE_RSA_WITH_AES_256_CBC_SHA

    1,                                  # Compression Methods Length: 1
    0,                                  # Compression Method: null

    0, 0                                # Extensions Length
])

msg = bytearray([1])                    # Handshake Type: Client Hello
msg += bytearray([0, 0, len(hello)]) + hello

record = bytearray([
    0x16,                               # Content Type: Handshake
    3, 3                                # Version TLS 1.2
])
record += bytearray([0, len(msg)]) + msg

s = socket.socket()
s.connect((sys.argv[1], int(sys.argv[2])))
s.sendall(record)
buf = s.recv(4096)
s.close()

if buf[0] == 0x16 and buf[5] == 2:      # Response is: Handshake + Server Hello
    sys.stdout.buffer.write(buf[15:15+28])