path: root/sigsum-witness.py

#! /usr/bin/env python3

# Sign the most recently published tree head from a given ST log,
# after verifying a consistency proof from an already verified tree
# head to this new tree head.

# A verified tree head is expected to be found in the file
# ~/.config/sigsum-witness/signed-tree-head . It's updated once a
# newer tree head has been verified successfully.

# If the config file ~/.config/sigsum-witness/sigsum-witness.conf
# exists and is readable, options are read from it. Options read from
# the config file can be overridden on the command line.

# Pubkey from secret key:
# sigkey = nacl.signing.SigningKey('badc0ffee123456...', nacl.encoding.HexEncoder)
# sigkey.verify_key.encode(nacl.encoding.HexEncoder)

import sys
import os
from stat import *
import argparse
import requests
import struct
from binascii import hexlify, unhexlify
import nacl.encoding
import nacl.signing
from hashlib import sha256
import time
from math import floor
from pathlib import PurePath
from tools.libsigntools import ssh_to_sign

BASE_URL_DEFAULT = 'http://poc.sigsum.org:4780/'
CONFIG_DIR_DEFAULT = os.path.expanduser('~/.config/sigsum-witness/')

ERR_OK                         = 0
ERR_USAGE                      = os.EX_USAGE
ERR_TREEHEAD_READ              = 2
ERR_TREEHEAD_FETCH             = 3
ERR_TREEHEAD_SIGNATURE_INVALID = 4
ERR_TREEHEAD_INVALID           = 5
ERR_CONSISTENCYPROOF_FETCH     = 6
ERR_CONSISTENCYPROOF_INVALID   = 7
ERR_LOGKEY                     = 8
ERR_LOGKEY_FORMAT              = 9
ERR_SIGKEYFILE                 = 10
ERR_SIGKEYFILE_MISSING         = 11
ERR_SIGKEY_FORMAT              = 12
ERR_NYI                        = 13
ERR_COSIG_POST                 = 14

class Parser:
    def __init__(self):
        p = argparse.ArgumentParser(
            description='Sign the most recently published tree head from a given sigsum log, after verifying it against an older tree.')

        p.add_argument('--bootstrap-log',
                       action='store_true',
                       help="Sign and save fetched tree head without verifying a consistency proof against a previous tree head. "
                       "NOTE: Requires user intervention.")

        p.add_argument('-d', '--base-dir',
                       default=CONFIG_DIR_DEFAULT,
                       help="Configuration directory ({})".format(CONFIG_DIR_DEFAULT))

        p.add_argument('-g', '--generate-signing-key',
                       action='store_true',
                       help="Generate signing key if missing. NOTE: Requires user intervention.")

        p.add_argument('-l', '--log-verification-key',
                       help="Log verification key")

        p.add_argument('--save-config',
                       action='store_true',
                       help="Save command line options to the configuration file")

        p.add_argument('-s', '--sigkey-file',
                       default='signing-key',
                       help="Signing key file, relative to $base_dir if not an absolute path (signing-key)")

        p.add_argument('-u', '--base-url',
                       default=BASE_URL_DEFAULT,
                       help="Log base URL ({})".format(BASE_URL_DEFAULT))

        self.parser = p

def parse_config(filename):
    try:
        lines = []
        with open(filename, 'r') as f:
            line = f.readline()
            while line:
                lines.append(line.strip())
                line = f.readline()
            g_args.parser.parse_args(lines, namespace=g_args)
    except FileNotFoundError:
        pass

def parse_args(argv):
    g_args.parser.parse_args(namespace=g_args)

def parse_keyval(text):
    dictx = {}
    for line in text.split():
        (key, val) = line.split('=')
        if not key in dictx:
            dictx[key] = val
        else:
            if type(dictx[key]) is list:
                dictx[key] += [val]
            else:
                dictx[key] = [dictx[key], val]
    return dictx

class TreeHead:
    def __init__(self, sth_data):
        self._text = parse_keyval(sth_data)
        assert(len(self._text) == 4)
        assert('timestamp' in self._text)
        assert('tree_size' in self._text)
        assert('root_hash' in self._text)
        assert('signature' in self._text)

    @property
    def timestamp(self):
        return int(self._text['timestamp'])

    @property
    def tree_size(self):
        return int(self._text['tree_size'])

    @property
    def root_hash(self):
        return unhexlify(self._text['root_hash'])

    def text(self):
        text = 'timestamp={}\n'.format(self._text['timestamp'])
        text += 'tree_size={}\n'.format(self._text['tree_size'])
        text += 'root_hash={}\n'.format(self._text['root_hash'])
        text += 'signature={}\n'.format(self._text['signature'])
        return text.encode('ascii')

    def to_signed_data(self, pubkey):
        namespace = 'tree_head:v0:{}@sigsum.org'.format(hexlify(sha256(pubkey.encode()).digest()).decode())
        msg = struct.pack('!QQ', self.timestamp, self.tree_size)
        msg += unhexlify(self._text['root_hash'])
        assert(len(msg) == 8 + 8 + 32)
        return ssh_to_sign(namespace, 'sha256', sha256(msg).digest())

    def signature_valid(self, pubkey):
        # Guard against tree head with >1 signature -- don't try to
        # validate a cosigned tree head.
        assert(type(self._text['signature']) is str)
        sig = unhexlify(self._text['signature'])
        assert(len(sig) == 64)
        data = self.to_signed_data(pubkey)
        try:
            verified_data = pubkey.verify(sig + data)
        except nacl.exceptions.BadSignatureError:
            return False
        assert(verified_data == data)
        return True

    def timestamp_valid(self, now):
        ts_sec = self.timestamp
        ts_asc = time.ctime(ts_sec)
        acceptable_drift = 10
        if ts_sec < now - 5 * 60 - acceptable_drift:
            return (ERR_OK,
                    "WARNING: Tree head timestamp older than five minutes: {} ({})".format(ts_sec, ts_asc))
        if ts_sec > now + acceptable_drift:
            return (ERR_OK,
                    "WARNING: Tree head timestamp from the future: {} ({})".format(ts_sec, ts_asc))

    def history_valid(self, prev):
        if self.tree_size < prev.tree_size:
            return (ERR_TREEHEAD_INVALID,
                    "ERROR: Log is shrinking: {} < {} ".format(self.tree_size,
                                                               prev.tree_size))

        if self.timestamp < prev.timestamp:
            return (ERR_TREEHEAD_INVALID,
                    "ERROR: Log is time traveling: {} < {} ".format(time.ctime(self.timestamp),
                                                                    time.ctime(prev.timestamp)))

        if self.timestamp == prev.timestamp and \
           self.root_hash == prev.root_hash and \
           self.tree_size == prev.tree_size:
            return (ERR_OK,
                    "INFO: Fetched head of tree of size {} already seen".format(prev.tree_size))

        if self.root_hash == prev.root_hash and \
           self.tree_size != prev.tree_size:
            return (ERR_TREEHEAD_INVALID,
                    "ERROR: Tree size has changed but hash has not: "
                    "{}: {} != {}".format(self.root_hash,
                                          self.tree_size,
                                          prev.tree_size))

        if self.root_hash != prev.root_hash and \
           self.tree_size == prev.tree_size:
            return (ERR_TREEHEAD_INVALID,
                    "ERROR: Hash has changed but tree size has not: "
                    "{}: {} != {}".format(self.tree_size,
                                          self.root_hash,
                                          prev.root_hash))

        # New timestamp but same hash and size is ok but there's no
        # consistency to prove.
        if self.root_hash == prev.root_hash:
            assert(self.tree_size == prev.tree_size)
            assert(self.timestamp != prev.timestamp)
            print("INFO: Signing re-published head of tree of size {}".format(self.tree_size))
            return None         # Success

        proof, err = fetch_consistency_proof(prev.tree_size, self.tree_size)
        if err: return err
        if not consistency_proof_valid(prev, self, proof):
            errmsg = "ERROR: failing consistency proof check for {}->{}\n".format(prev.tree_size,
                                                                                  self.tree_size)
            errmsg += "DEBUG: {}:{}->{}:{}\n  {}".format(prev.tree_size,
                                                         prev.root_hash,
                                                         self.tree_size,
                                                         self.root_hash,
                                                         proof.path())
            return ERR_CONSISTENCYPROOF_INVALID, errmsg

        return None             # Success

class ConsistencyProof():
    def __init__(self, old_size, new_size, consistency_proof_data):
        self._old_size = old_size
        self._new_size = new_size
        self._text = parse_keyval(consistency_proof_data)
        assert(len(self._text) == 1)
        assert('consistency_path' in self._text)

    def old_size(self):
        return self._old_size
    def new_size(self):
        return self._new_size
    def path(self):
        if type(self._text['consistency_path']) is list:
            return [unhexlify(e) for e in self._text['consistency_path']]
        else:
            return [unhexlify(self._text['consistency_path'])]

def make_base_dir_maybe():
    dirname = os.path.expanduser(g_args.base_dir)
    try:
        os.stat(dirname)
    except FileNotFoundError:
        os.makedirs(dirname, mode=0o700)

def read_tree_head(filename):
    try:
        with open(filename, mode='r') as f:
            return TreeHead(f.read())
    except FileNotFoundError:
        return None

def read_tree_head_and_verify(log_verification_key):
    fn = str(PurePath(os.path.expanduser(g_args.base_dir), 'signed-tree-head'))
    tree_head = read_tree_head(fn)
    if not tree_head:
        return None, (ERR_TREEHEAD_READ,
                      "ERROR: unable to read file {}".format(fn))

    if not tree_head.signature_valid(log_verification_key):
        return None, (ERR_TREEHEAD_SIGNATURE_INVALID,
                      "ERROR: signature of stored tree head invalid")

    return tree_head, None

def store_tree_head(tree_head):
    path = str(PurePath(os.path.expanduser(g_args.base_dir), 'signed-tree-head'))
    with open(path, mode='w+b') as f:
        f.write(tree_head.text())

def fetch_tree_head_and_verify(log_verification_key):
    try:
        req = requests.get(g_args.base_url + 'sigsum/v0/get-tree-head-to-cosign')
    except requests.ConnectionError as err:
        return None, (ERR_TREEHEAD_FETCH, f"ERROR: unable to fetch new tree head: {err}")
    if req.status_code != 200:
        return None, (ERR_TREEHEAD_FETCH,
                      "ERROR: unable to fetch new tree head: {}".format(req.status_code))

    tree_head = TreeHead(req.content.decode())
    if not tree_head.signature_valid(log_verification_key):
        return None, (ERR_TREEHEAD_SIGNATURE_INVALID,
                      "ERROR: signature of fetched tree head invalid")

    return tree_head, None

def fetch_consistency_proof(first, second):
    url = g_args.base_url + 'sigsum/v0/get-consistency-proof/{}/{}'.format(first, second)
    try:
        req = requests.get(url)
    except requests.ConnectionError as err:
        return None, (ERR_TREEHEAD_FETCH, f"ERROR: unable to fetch consistency proof: {err}")
    if req.status_code != 200:
        return None, (ERR_CONSISTENCYPROOF_FETCH,
                      "ERROR: unable to fetch consistency proof: {}".format(req.status_code))
    return ConsistencyProof(first, second, req.content.decode()), None

def numbits(n):
    p = 0
    while n > 0:
        if n & 1:
            p += 1
        n >>= 1
    return p

# Implements the algorithm for consistency proof verification outlined
# in RFC6962-BIS, see
# https://datatracker.ietf.org/doc/html/draft-ietf-trans-rfc6962-bis-39#section-2.1.4.2
def consistency_proof_valid(first, second, proof):
    assert(first.tree_size == proof.old_size())
    assert(second.tree_size == proof.new_size())

    path = proof.path()
    if len(path) == 0:
        return False
    if numbits(first.tree_size) == 1:
        path = [first.root_hash] + path

    fn = first.tree_size - 1
    sn = second.tree_size - 1
    while fn & 1:
        fn >>= 1
        sn >>= 1

    fr = path[0]
    sr = path[0]

    for c in path[1:]:
        if sn == 0:
            return False

        if fn & 1 or fn == sn:
            fr = sha256(b'\x01' + c + fr).digest()
            sr = sha256(b'\x01' + c + sr).digest()
            while fn != 0 and fn & 1 == 0:
                fn >>= 1
                sn >>= 1
        else:
            sr = sha256(b'\x01' + sr + c).digest()

        fn >>= 1
        sn >>= 1

    return sn == 0 and fr == first.root_hash and sr == second.root_hash

def sign_send_store_tree_head(signing_key, log_key, tree_head):
    signature = signing_key.sign(tree_head.to_signed_data(log_key)).signature
    hash = sha256(signing_key.verify_key.encode())

    post_data = 'cosignature={}\n'.format(hexlify(signature).decode('ascii'))
    post_data += 'key_hash={}\n'.format(hash.hexdigest())

    try:
        req = requests.post(g_args.base_url + 'sigsum/v0/add-cosignature', post_data)
    except requests.ConnectionError as err:
        return (ERR_COSIG_POST, f"ERROR: Unable to post signature to log: {err}")
    if req.status_code != 200:
        return (ERR_COSIG_POST,
                "ERROR: Unable to post signature to log: {} => {}: {}". format(req.url,
                                                                               req.status_code,
                                                                               req.text))
    # Store only when all else is done. Next invocation will treat a
    # stored tree head as having been verified.
    store_tree_head(tree_head)

def ensure_log_verification_key():
    if not g_args.log_verification_key:
        return None, (ERR_LOGKEY, "ERROR: missing log verification key")
    try:
        log_verification_key = nacl.signing.VerifyKey(g_args.log_verification_key, encoder=nacl.encoding.HexEncoder)
    except:
        return None, (ERR_LOGKEY_FORMAT,
                      "ERROR: invalid log verification key: {}".format(g_args.log_verification_key))

    assert(log_verification_key is not None)
    return log_verification_key, None

def generate_and_store_sigkey(fn):
    print("INFO: Generating signing key and writing it to {}".format(fn))
    signing_key = nacl.signing.SigningKey.generate()
    verify_key = signing_key.verify_key
    print("INFO: verification key: {}".format(verify_key.encode(nacl.encoding.HexEncoder).decode('ascii')))
    with open(fn, 'w') as f:
        os.chmod(f.fileno(), S_IRUSR)
        f.write(signing_key.encode(encoder=nacl.encoding.HexEncoder).decode('ascii'))

def read_sigkeyfile(fn):
    try:
        check_sigkeyfile(fn)
    except SigKeyFileError as err:
        return None, (ERR_SIGKEYFILE, str(err))
    with open(fn, 'r') as f:
        try:
            signing_key = nacl.signing.SigningKey(f.readline().strip(), nacl.encoding.HexEncoder)
        except:
            return None, (ERR_SIGKEY_FORMAT,
                          "ERROR: Invalid signing key in {}".format(fn))

    assert(signing_key is not None)
    return signing_key, None


def check_sigkeyfile(fn):
    try:
        s = os.stat(fn, follow_symlinks=True)
    except FileNotFoundError:
        raise SigKeyFileError(f"ERROR: File not found: {fn}")
    if not S_ISREG(s.st_mode):
        raise SigKeyFileError(f"ERROR: Signing key file {fn} must be a regular file")
    if S_IMODE(s.st_mode) & 0o077 != 0:
        raise SigKeyFileError(f"ERROR: Signing key file {fn} permissions too lax: {S_IMODE(s.st_mode):04o}")


class SigKeyFileError(Exception):
    pass


# Read signature key from file, or generate one and write it to file.
def ensure_sigkey(fn):
    try:
        os.stat(fn, follow_symlinks=False)
    except FileNotFoundError:
        if not g_args.generate_signing_key:
            return None, (ERR_SIGKEYFILE_MISSING,
                          "ERROR: Signing key file {} missing. "
                          "Use --generate-signing-key to create one.".format(fn))

        if not user_confirm("Really generate a new signing key and store it in {}?".format(fn)):
            return None, (ERR_SIGKEYFILE_MISSING,
                          "ERROR: Signing key file {} missing".format(fn))

        generate_and_store_sigkey(fn)
        return read_sigkeyfile(fn)

    if g_args.generate_signing_key:
        return None, (ERR_USAGE,
                      "ERROR: Signing key file {} already existing".format(fn))
    return read_sigkeyfile(fn)


def user_confirm(prompt):
    resp = input(prompt + ' y/n> ').lower()
    if resp and resp[0] == 'y':
        return True
    return False

def main(args):
    global g_args
    g_args = Parser()
    parse_args(args)            # get base_dir
    parse_config(str(PurePath(g_args.base_dir, 'sigsum-witness.conf')))
    parse_args(args)            # override config file options
    if g_args.save_config:
        # TODO write to config file
        return ERR_NYI, "ERROR: --save-config is not yet implemented"

    now = floor(time.time())
    consistency_verified = False
    ignore_consistency = False

    make_base_dir_maybe()

    log_verification_key, err = ensure_log_verification_key()
    if err: return err

    signing_key, err = ensure_sigkey(str(PurePath(g_args.base_dir, g_args.sigkey_file)))
    if err: return err

    cur_tree_head, err = read_tree_head_and_verify(log_verification_key)
    if err:
        new_tree_head, err2 = fetch_tree_head_and_verify(log_verification_key)
        if err2: return err2

        if not g_args.bootstrap_log:
            return err

        print("\nWARNING: We have only seen one single tree head from the\n"
              "log {},\n"
              "representing a tree of size {}. We are therefore unable to\n"
              "verify that the tree it represents is really a superset of an\n"
              "earlier version of the tree in this log.\n"
              "\nWe are effectively signing this tree head blindly.\n".format(g_args.base_url,
                                                                              new_tree_head.tree_size))
        if user_confirm("Really sign head for tree of size {} and upload "
                        "the signature?".format(new_tree_head.tree_size)):
            err3 = sign_send_store_tree_head(signing_key, log_verification_key, new_tree_head)
            if err3: return err3

        return 0, None
    else:
        if g_args.bootstrap_log:
            return (ERR_USAGE,
                    "ERROR: Valid tree head found: --bootstrap-log not allowed")

    new_tree_head, err = fetch_tree_head_and_verify(log_verification_key)
    if err: return err

    err = new_tree_head.timestamp_valid(now)
    if err: return err

    err = new_tree_head.history_valid(cur_tree_head)
    if err: return err

    if not cur_tree_head.signature_valid(log_verification_key):
        return ERR_TREEHEAD_SIGNATURE_INVALID, "ERROR: signature of current tree head invalid"

    err = sign_send_store_tree_head(signing_key, log_verification_key, new_tree_head)
    if err: return err

    return 0, None

if __name__ == '__main__':
    status = main(sys.argv)
    if status[1]:
        print(status[1])
    sys.exit(status[0])