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#!/usr/bin/env python
r'''
Extract SSL/DTLS keys from programs that use OpenSSL.
Example usage, attach to an existing process, put keys in premaster.txt:
PYTHONPATH=. \
gdb -q -ex 'py import sslkeylog as skl; skl.start("premaster.txt")' \
-p `pidof curl`
Run a new program while outputting keys to the default stderr
(you can set envvar SSLKEYLOGFILE to override this):
PYTHONPATH=. gdb -q -ex 'py import sslkeylog as skl; skl.start()' \
-ex r --args curl https://example.com
Recommended configuration: copy this file to ~/.gdb/sslkeylog.py and put
the following in your ~/.gdbinit:
python
import sys, os.path
#sys.dont_write_bytecode = True # avoid *.pyc clutter
sys.path.insert(0, os.path.expanduser('~/.gdb'))
import sslkeylog as skl
# Override default keylog (SSLKEYLOGFILE env or stderr)
skl.keylog_filename = '/tmp/premaster.txt'
end
Then you can simply execute:
gdb -q -ex 'py skl.start()' -p `pidof curl`
To stop capturing keys, detach GDB or invoke 'skl.stop()'
'''
import gdb
from os import getenv
# Default filename for new Keylog instances.
keylog_filename = getenv('SSLKEYLOGFILE', '/dev/stderr')
_SSL_KEYLOG_HEADER = b'# Automatically generated by sslkeylog.py\n'
def _read_as_hex(value, size):
addr = value.address
data = gdb.selected_inferior().read_memory(addr, size)
return b''.join(b'%02X' % ord(x) for x in data)
def _ssl_get_master_key(ssl_ptr):
session = ssl_ptr['session']
if session != 0 and session['master_key_length'] > 0:
return _read_as_hex(session['master_key'], 48)
return None
def get_keylog_line(ssl_ptr):
'''
Returns (client_random, master_key) for the current SSL session.
'''
mk = _ssl_get_master_key(ssl_ptr)
s3 = ssl_ptr['s3']
if s3 == 0 or mk is None:
return
cr = _read_as_hex(s3['client_random'], 32)
# Maybe optimize storage by using Session ID if available?
#sid = _read_as_hex(self.ssl_ptr['session']['session_id'], 32)
return (cr, mk)
class SKLFinishBreakpoint(gdb.FinishBreakpoint):
'''Breaks on points where new key material is possibly available.'''
def __init__(self, ssl_ptr, key_listener):
# Mark as internal, it is expected to be gone as soon as this quits.
gdb.FinishBreakpoint.__init__(self, internal=True)
self.ssl_ptr = ssl_ptr
self.key_listener = key_listener
def stop(self):
# Attempt to recover key material.
info = get_keylog_line(self.ssl_ptr)
if info:
# Line consists of a cache key and actual key log line
self.key_listener.notify(*info)
return False # Continue execution
class SKLBreakpoint(gdb.Breakpoint):
'''Breaks at function entrance and registers a finish breakpoint.'''
def __init__(self, spec, key_listener):
gdb.Breakpoint.__init__(self, spec)
self.key_listener = key_listener
def stop(self):
# Retrieve SSL* parameter.
ssl_ptr = gdb.selected_frame().read_var('s')
# Proceed with handshakes (finish function) before checking for keys.
SKLFinishBreakpoint(ssl_ptr, self.key_listener)
# Increase hit count for debugging (info breakpoints)
# This number will be decremented when execution continues.
self.ignore_count += 1
return False # Continue execution
class Keylog(object):
'''Listens for new key material and writes them to a file.'''
def __init__(self, keylog_file):
self.keylog_file = keylog_file
# Remember written lines to avoid printing duplicates.
self.written_items = set()
def notify(self, client_random, master_key):
'''Puts a new entry in the key log if not already known.'''
if client_random not in self.written_items:
line = b'CLIENT_RANDOM %s %s\n' % (client_random, master_key)
self.keylog_file.write(line)
# Assume client random is random enough as cache key.
cache_key = client_random
self.written_items.add(cache_key)
def close():
self.keylog_file.close()
@classmethod
def create(cls, filename):
def needs_header(f):
try:
# Might fail for pipes (such as stdin).
return f.tell() == 0
except:
return False
# Byte output is needed for unbuffered I/O
f = open(filename, 'ab', 0)
if needs_header(f):
f.write(_SSL_KEYLOG_HEADER)
return cls(f)
# A shared Keylog instance.
_keylog_file = None
def start(sslkeylogfile=None, cont=True):
'''
:param sslkeylogfile: optional SSL keylog file name (overrides
SSLKEYLOGFILE environment variable and its fallback value).
:param cont: True to continue this process when paused.
'''
global keylog_filename
if sslkeylogfile:
keylog_filename = sslkeylogfile
enable()
# Continue the process when it was already started before.
if cont and gdb.selected_thread():
gdb.execute('continue')
def stop():
'''Remove all breakpoints and close the key logfile.'''
global _keylog_file
if not _keylog_file:
print('No active keylog session')
return
disable()
_keylog_file.close()
print('Logged %d entries in total' % _keylog_file.written_items)
_keylog_file = None
# Remember enabled breakpoints
_locations = { name: None for name in (
'SSL_connect',
'SSL_do_handshake',
'SSL_accept',
'SSL_read',
'SSL_write',
)}
def enable():
'''Enable all SSL-related breakpoints.'''
global _keylog_file
if not _keylog_file:
_keylog_file = Keylog.create(keylog_filename)
print('Started logging SSL keys to %s' % keylog_filename)
for name, breakpoint in _locations.items():
if breakpoint:
print('Breakpoint for %s is already active, ignoring' % name)
continue
_locations[name] = SKLBreakpoint(name, _keylog_file)
def disable():
'''Disable all SSL-related breakpoints.'''
for name, breakpoint in _locations.items():
if breakpoint:
msg = 'Deleting breakpoint %d' % breakpoint.number
msg += ' (%s)' % breakpoint.location
if breakpoint.hit_count > 0:
msg += ' (called %d times)' % breakpoint.hit_count
print(msg)
breakpoint.delete()
_locations[name] = None
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