we're sigsum nowv0.2.1

1 files changed, 505 insertions, 0 deletions

diff --git a/sigsum-witness.py b/sigsum-witness.py
new file mode 100755
index 0000000..730c6b8
--- /dev/null
+++ b/sigsum-witness.py
@@ -0,0 +1,505 @@
+#! /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
+
+BASE_URL_DEFAULT = 'http://poc.sigsum.org:4780/'
+CONFIG_DIR_DEFAULT = os.path.expanduser('~/.config/sigsum-witness/')
+
+ERR_OK                         = 0
+ERR_USAGE                      = 1
+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 serialise(self, pubkey):
+        data = struct.pack('!QQ', self.timestamp, self.tree_size)
+        data += unhexlify(self._text['root_hash'])
+        data += sha256(pubkey.encode()).digest()
+        assert(len(data) == 8 + 8 + 32 + 32)
+        return data
+
+    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.serialise(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)
+        if ts_sec < now - 12 * 3600:
+            return (ERR_OK,
+                    "WARNING: Tree head timestamp too old: {} ({})".format(ts_sec, ts_asc))
+        if ts_sec > now + 12 * 3600:
+            return (ERR_OK,
+                    "WARNING: Tree head timestamp too new: {} ({})".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):
+    req = requests.get(g_args.base_url + 'sigsum/v0/get-tree-head-to-sign')
+    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):
+    post_data = 'old_size={}\n'.format(first)
+    post_data += 'new_size={}\n'.format(second)
+    req = requests.post(g_args.base_url + 'sigsum/v0/get-consistency-proof', post_data)
+    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.serialise(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())
+
+    req = requests.post(g_args.base_url + 'sigsum/v0/add-cosignature', post_data)
+    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):
+    s = os.stat(fn, follow_symlinks=False)
+    if not S_ISREG(s.st_mode):
+        return None, (ERR_SIGKEYFILE,
+                      "ERROR: Signing key file {} must be a regular file".format(fn))
+    if S_IMODE(s.st_mode) & 0o077 != 0:
+        return None, (ERR_SIGKEYFILE,
+                      "ERROR: Signing key file {} permissions too lax: {:04o}".format(fn, S_IMODE(s.st_mode)))
+
+    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
+
+
+# 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])