tofu: proper implementation

BOARD.txt

@@ -13,13 +13,12 @@ TODO                                    DONE
                                         downloads
                                         configuration
                                         help page
+                                        TOFU
 open last download
-actual TOFU
 home page
 media files
 view history
 identity management
-help page for keybinds
 --------------------------------------------------------------------------------
 BACKLOG
 click on links to open them

README.md

@@ -29,15 +29,13 @@ Why use Bebop instead of something else?
 
 ### Lightweight
 
-It only uses a single dependency, [asn1crypto][asn1crypto], to delegate
+It does not use any external dependencies. Everything including NCurses or TLS
-parsing certificates. Everything else including NCurses or TLS is done using
+is done using Python's standard library.
-Python's standard library.
-
-[asn1crypto]: https://github.com/wbond/asn1crypto
 
 ### Nice keybinds
 
-A lot of keybinds are defined. Find them in the help page by pressing `?`.
+A lot of keybinds are defined, and Vim users should get quickly familiar with
+them. Find them in the help page by pressing `?`.
 
 ### Fun
 

bebop/__main__.py

@@ -2,8 +2,8 @@ import argparse
 
 from bebop.browser.browser import Browser
 from bebop.config import load_config
-from bebop.fs import get_config_path, get_user_data_path
+from bebop.fs import ensure_bebop_files_exist, get_config_path
-from bebop.tofu import load_cert_stash, save_cert_stash
+from bebop.tofu import get_cert_stash_path, load_cert_stash, save_cert_stash
 
 
 def main():
@@ -19,11 +19,9 @@ def main():
     config_path = get_config_path()
     config = load_config(config_path)
 
-    user_data_path = get_user_data_path()
+    ensure_bebop_files_exist()
-    if not user_data_path.exists():
-        user_data_path.mkdir()
 
-    cert_stash_path = user_data_path / "known_hosts.txt"
+    cert_stash_path = get_cert_stash_path()
     cert_stash = load_cert_stash(cert_stash_path) or {}
     try:
         Browser(config, cert_stash).run(start_url=start_url)

bebop/browser/browser.py

@@ -47,7 +47,7 @@ class Browser:
 
     def __init__(self, config, cert_stash):
         self.config = config
-        self.stash = cert_stash or {}
+        self.stash = cert_stash
         self.screen = None
         self.dim = (0, 0)
         self.page_pad = None
@@ -272,6 +272,9 @@ class Browser:
             return
         if command in ("o", "open"):
             self.open_url(words[1], assume_absolute=True)
+        elif command == "forget-certificate":
+            from bebop.browser.gemini import forget_certificate
+            forget_certificate(self, words[1])
 
     def open_url(self, url, base_url=None, redirects=0, assume_absolute=False,
                  history=True, use_cache=True):

bebop/browser/gemini.py

@@ -8,6 +8,7 @@ from bebop.fs import get_downloads_path
 from bebop.navigation import set_parameter
 from bebop.page import Page
 from bebop.protocol import Request, Response
+from bebop.tofu import trust_fingerprint, untrust_fingerprint, WRONG_FP_ALERT
 
 
 MAX_URL_LEN = 1024
@@ -17,10 +18,30 @@ def open_gemini_url(browser: Browser, url, redirects=0, history=True,
                     use_cache=True):
     """Open a Gemini URL and set the formatted response as content.
 
-    After initiating the connection, TODO
+    While the specification is not set in stone, every client takes a slightly
+    different approach to enforcing TOFU. Read the `Request.connect` docs to
+    find about cases where connection is aborted without asking the user. What
+    interests us here is what happens when the user should decide herself? This
+    happens in several cases, matching the request possible states. Here is
+    what Bebop do (or want to do):
+
+    - STATE_INVALID_CERT: the certificate has non-fatal issues; we may
+      present the user the problems found and let her decide whether to trust
+      temporarily the certificate or not BUT we currently do not parse the
+      certificate's fields, so this state is never used.
+    - STATE_UNKNOWN_CERT: the certificate is valid but has not been seen before;
+      as we're doing TOFU here, we could automatically trust it or let the user
+      choose. For simplicity, we always trust it permanently.
+
+    Attributes:
+    - browser: Browser object making the request.
+    - url: a valid URL with Gemini scheme to open.
+    - redirects: current amount of redirections done to open the initial URL.
+    - history: if true, save the final URL to history.
+    - use_cache: if true, look up if the page is cached before requesting it.
     """
     if len(url) >= MAX_URL_LEN:
-        browser.set_status_error(f"Request URL too long.")
+        browser.set_status_error("Request URL too long.")
         return
 
     browser.set_status(f"Loading {url}")
@@ -40,8 +61,8 @@ def open_gemini_url(browser: Browser, url, redirects=0, history=True,
         if req.state == Request.STATE_ERROR_CERT:
             error = f"Certificate was missing or corrupt ({url})."
         elif req.state == Request.STATE_UNTRUSTED_CERT:
+            _handle_untrusted_cert(browser, req)
             error = f"Certificate has been changed ({url})."
-            # TODO propose the user ways to handle this.
         elif req.state == Request.STATE_CONNECTION_FAILED:
             error_details = ": " + req.error if req.error else "."
             error = f"Connection failed ({url})" + error_details
@@ -51,13 +72,18 @@ def open_gemini_url(browser: Browser, url, redirects=0, history=True,
         return
 
     if req.state == Request.STATE_INVALID_CERT:
-        # TODO propose abort / temp trust
         pass
     elif req.state == Request.STATE_UNKNOWN_CERT:
-        # TODO propose abort / temp trust / perm trust
+        # Certificate is valid but unknown: trust it permanently.
-        pass
+        hostname = req.hostname
-    else:
+        fingerprint = req.cert_validation["hash"]
-        pass # TODO
+        trust_fingerprint(
+            browser.stash,
+            hostname,
+            "SHA-512",
+            fingerprint,
+            trust_always=True
+        )
 
     data = req.proceed()
     if not data:
@@ -71,6 +97,26 @@ def open_gemini_url(browser: Browser, url, redirects=0, history=True,
     _handle_response(browser, response, url, redirects, history)
 
 
+def _handle_untrusted_cert(browser: Browser, request: Request):
+    """Handle a mismatch between known & server fingerprints.
+
+    This function formats an alert page to explain to the user what the hell is
+    going on and displays it.
+    """
+    remote_fp = request.cert_validation["hash"]
+    local_fp = request.cert_validation["saved_hash"]
+    alert_page_source = WRONG_FP_ALERT.format(
+        hostname=request.hostname,
+        local_fp=local_fp,
+        remote_fp=remote_fp,
+    )
+    alert_page = Page.from_gemtext(
+        alert_page_source,
+        browser.config["text_width"]
+    )
+    browser.load_page(alert_page)
+
+
 def _handle_response(browser: Browser, response: Response, url: str,
                      redirects: int, history: bool):
     """Handle a response from a Gemini server."""
@@ -167,3 +213,15 @@ def _handle_input_request(browser: Browser, from_url: str, message: str =None):
         return
     url = set_parameter(from_url, user_input)
     open_gemini_url(browser, url)
+
+
+def forget_certificate(browser: Browser, hostname: str):
+    """Remove the fingerprint associated to this hostname for the cert stash."""
+    key = browser.prompt(f"Remove fingerprint from {hostname}? [y/N]", "ynN")
+    if key != "y":
+        browser.reset_status()
+        return
+    if untrust_fingerprint(browser.stash, hostname):
+        browser.set_status(f"Known certificate for {hostname} removed.")
+    else:
+        browser.set_status_error(f"Known certificate for {hostname} not found.")

bebop/command_line.py

@@ -208,9 +208,13 @@ class CommandLine:
         """Handle input chars and raise a terminate interrupt on a valid key."""
         # Handle common keys.
         ch = self._validate_common_input(ch)
-        char = chr(ch)
+        try:
-        if char in keys:
+            char = chr(ch)
-            raise TerminateCommandInterrupt(char)
+        except ValueError:
+            pass
+        else:
+            if char in keys:
+                raise TerminateCommandInterrupt(char)
         return 0
 
 

bebop/fs.py

@@ -45,3 +45,11 @@ def get_downloads_path() -> Path:
     if download_path:
         return Path(download_path)
     return Path.home()
+
+
+def ensure_bebop_files_exist():
+    """Ensure various Bebop's files or directories are present."""
+    # Ensure the user data directory exists.
+    user_data_path = get_user_data_path()
+    if not user_data_path.exists():
+        user_data_path.mkdir(parents=True)

bebop/protocol.py

@@ -8,19 +8,13 @@ from enum import IntEnum
 from typing import Optional
 
 from bebop.mime import DEFAULT_MIME_TYPE, MimeType
-from bebop.tofu import CertStatus, CERT_STATUS_INVALID, validate_cert
+from bebop.tofu import CertStatus, validate_cert
 
 
 GEMINI_URL_RE = re.compile(r"gemini://(?P<host>[^/]+)(?P<path>.*)")
 LINE_TERM = b"\r\n"
 
 
-def parse_gemini_url(url):
-    """Return a dict containing the hostname and the request path, or None."""
-    match = GEMINI_URL_RE.match(url)
-    return match.groupdict() if match else None
-
-
 class Request:
     """A Gemini request.
 
@@ -30,8 +24,21 @@ class Request:
     sending the request header and receiving the response:
 
     1. Instantiate a Request.
-    2. `connect` opens the connection, leaves the caller free to check stuff.
+    2. `connect` opens the connection and aborts it or leaves the caller free to
+       check stuff.
     3. `proceed` or `abort` can be called.
+
+    Attributes:
+    - url: URL to open.
+    - cert_stash: certificate stash to use an possibly update.
+    - state: request state.
+    - hostname: hostname derived from url, stored when `connect` is called.
+    - payload: bytes object of the payload request; build during `connect`, used
+      during `proceed`.
+    - ssock: TLS-wrapped socket.
+    - cert_validation: validation results dict, set after certificate has been
+      reviewed.
+    - error: human-readable connection error, may be set during `connect`.
     """
 
     # Initial state, connection is not established yet.
@@ -55,28 +62,69 @@ class Request:
         self.url = url
         self.cert_stash = cert_stash
         self.state = Request.STATE_INIT
+        self.hostname = ""
         self.payload = b""
         self.ssock = None
-        self.cert = None
+        self.cert_validation = None
-        self.cert_status = None
         self.error = ""
 
-    def connect(self, timeout):
+    def connect(self, timeout: int) -> bool:
         """Connect to a Gemini server and return a RequestEventType.
 
         Return True if the connection is established. The caller has to verify
         the request state and propose appropriate choices to the user if the
         certificate status is not CertStatus.VALID (Request.STATE_OK).
 
-        If connect returns False, the secure socket is aborted before return. If
+        If connect returns False, the secure socket is aborted before return so
-        connect returns True, it is up to the caller to decide whether to
+        there is no need to call `abort`. If connect returns True, it is up to the
-        continue (call proceed) the connection or abort it (call abort).
+        caller to decide whether to continue (call `proceed`) the connection or
+        abort it (call `abort`).
+
+        The request `state` is updated to reflect the connection state after the
+        function returns. The following list describes states related to
+        connection failure (False returned):
+
+        - STATE_INVALID_URL: URL is not valid.
+        - STATE_CONNECTION_FAILED: connection failed, either TCP timeout or
+          local TLS failure. Additionally, the request `error` attribute is set
+          to an error string describing the issue.
+
+        For all request states from now on, the `cert_validation` attribute is
+        updated with the result of the certificate validation.
+
+        The following list describes states related to validation failure (False
+        returned):
+
+        - STATE_ERROR_CERT: server certificate could not be validated at all.
+        - STATE_UNTRUSTED_CERT: server certificate mismatched the known
+          certificate for that hostname. The user should be presented with
+          options to solve the matter.
+
+        For other states, the connection is not aborted (True returned):
+
+        - STATE_INVALID_CERT: the certificate has one or more issues, e.g.
+          mismatching hostname or it is expired.
+        - STATE_UNKNOWN_CERT: the certificate is valid but unknown.
+        - STATE_OK: the certificate is valid and matches the known certificate
+          of that hostname.
+
+        After this function returns, the request state cannot be STATE_INIT.
+
+        Additional notes:
+
+        - The DER hash is compared against the fingerprint for this hostname
+          *and port*; the specification does not tell much about that, but we
+          are slightly more restrictive here by adding the port in the equation.
+        - The state STATE_INVALID_CERT is actually never used in Bebop because
+          of the current tendency to ignore any certificate fields and only
+          check the whole cert fingerprint. Here it is considered the same as a
+          valid certificate.
         """
-        url_parts = parse_gemini_url(self.url)
+        url_parts = GEMINI_URL_RE.match(self.url)
         if not url_parts:
             self.state = Request.STATE_INVALID_URL
             return False
-        hostname = url_parts["host"]
+        hostname = url_parts.groupdict()["host"]
         if ":" in hostname:
             hostname, port = hostname.split(":", maxsplit=1)
             try:
@@ -86,6 +134,7 @@ class Request:
                 return False
         else:
             port = 1965
+        self.hostname = hostname
 
         try:
             self.payload = self.url.encode()
@@ -105,27 +154,26 @@ class Request:
         try:
             self.ssock = context.wrap_socket(sock, server_hostname=hostname)
         except OSError as exc:
+            sock.close()
             self.state = Request.STATE_CONNECTION_FAILED
             self.error = exc.strerror
             return False
 
         der = self.ssock.getpeercert(binary_form=True)
-        self.cert_status, self.cert = \
+        self.cert_validation = validate_cert(der, hostname, self.cert_stash)
-            validate_cert(der, hostname, self.cert_stash)
+        cert_status = self.cert_validation["status"]
-        if self.cert_status == CertStatus.ERROR:
+        if cert_status == CertStatus.ERROR:
             self.abort()
             self.state = Request.STATE_ERROR_CERT
             return False
-        if self.cert_status == CertStatus.WRONG_FINGERPRINT:
+        if cert_status == CertStatus.WRONG_FINGERPRINT:
             self.abort()
             self.state = Request.STATE_UNTRUSTED_CERT
             return False
 
-        if self.cert_status in CERT_STATUS_INVALID:
+        if cert_status == CertStatus.VALID_NEW:
-            self.state = Request.STATE_INVALID_CERT
-        elif self.cert_status == CertStatus.VALID_NEW:
             self.state = Request.STATE_UNKNOWN_CERT
-        else:  # self.cert_status == CertStatus.VALID
+        else:  # self.cert_status in (VALID, VALID_NEW, INVALID_CERT)
             self.state = Request.STATE_OK
         return True
 
@@ -232,6 +280,6 @@ class Response:
         return response
 
     @staticmethod
-    def get_generic_code(code) -> int:
+    def get_generic_code(code: int) -> int:
         """Return the generic version (x0) of this code."""
         return code - (code % 10)

bebop/tests/test_metalines.py

@@ -1,9 +1,9 @@
 import unittest
 
-from ..rendering import _explode_words, _find_next_sep, wrap_words
+from ..metalines import _explode_words, _find_next_sep, wrap_words
 
 
-class TestRenderer(unittest.TestCase):
+class TestMetalines(unittest.TestCase):
 
     def test_wrap_words(self):
         t = "wrap me wrap me youcantwrapthisonewithoutforce bla bla bla bla"

bebop/tests/test_protocol.py

@@ -1,10 +0,0 @@
-import unittest
-
-from ..protocol import parse_gemini_url
-
-
-class TestGemini(unittest.TestCase):
-
-    def test_parse_url(self):
-        r1 = parse_gemini_url("gemini://dece.space")
-        self.assertDictEqual(r1, {"host": "dece.space", "path": ""})

bebop/tofu.py

@@ -4,19 +4,70 @@ As of writing there is still some debate around it, so it is quite messy and
 requires more clarity both in specification and in our own implementation.
 """
 
-import datetime
 import hashlib
 import re
 from enum import Enum
 from pathlib import Path
+from typing import Any, Dict, Optional
 
-import asn1crypto.x509
+from bebop.fs import get_user_data_path
 
 
-STASH_LINE_RE = re.compile(r"(\S+) (\S+) (\S+) (\d+)")
+STASH_LINE_RE = re.compile(r"(\S+) (\S+) (\S+)")
+
+WRONG_FP_ALERT = """\
+The request could not complete because the certificate presented by the server \
+does not match the certificate stored in the local stash.
+
+``` details of the fingerprint mismatch
+Hostname:           {hostname}
+Local fingerprint:  {local_fp}
+Server fingerprint: {remote_fp}
+```
+
+If you are sure this new certificate can be trusted, press ":" and type the \
+following command to remove the previous certificate from the local stash, \
+then retry your request:
+
+``` command to use to forget about the previous certificate
+forget-certificate {hostname}
+```
+
+You can also manually remove the certificate line from the known hosts file in \
+your user data directory.
+
+## FAQ
+
+### What is this mismatch about?
+
+Gemini uses TOFU (Trust On First Use) to verify the identity of the server you \
+are visiting. It means that the first time you visited this capsule, it showed \
+you its unique ID, but this time the ID is different, so the trust is broken.
+
+Capsule owners often tell in advance when they are about the use a new \
+certificate, but they may have forgotten or you may have missed it. Maybe the \
+old certificate expired and/or has been replaced for another reason (e.g. \
+using a far away expiration time, borking certificates during a migration, …)
+
+### Am I being hacked?
+
+Probably not, but if you are visiting a sensitive capsule, make sure you're \
+confident enough before trusting this new certificate.
+
+### How to ensure this new certificate can be trusted?
+
+Can you join the owner through mail or instant messaging? This is the simplest \
+way for you to make sure that the server is fine, and maybe alert the owner on \
+a problem on his server she did not notice.
+"""
 
 
-def load_cert_stash(stash_path: Path):
+def get_cert_stash_path() -> Path:
+    """Return the default certificate stash path."""
+    return get_user_data_path() / "known_hosts.txt"
+
+
+def load_cert_stash(stash_path: Path) -> Optional[Dict]:
     """Load the certificate stash from the file, or None on error.
 
     The stash is a dict with host names as keys and tuples as values. Tuples
@@ -36,8 +87,8 @@ def load_cert_stash(stash_path: Path):
                 match = STASH_LINE_RE.match(line)
                 if not match:
                     continue
-                name, algo, fingerprint, timestamp = match.groups()
+                name, algo, fingerprint = match.groups()
-                stash[name] = (algo, fingerprint, timestamp, True)
+                stash[name] = (algo, fingerprint, True)
     except (OSError, ValueError):
         return None
     return stash
@@ -47,71 +98,67 @@ def save_cert_stash(stash: dict, stash_path: Path):
     """Save the certificate stash."""
     try:
         with open(stash_path, "wt") as stash_file:
-            for name, entry in stash.values():
+            for name, entry in stash.items():
-                algo, fingerprint, timestamp, is_permanent = entry
+                algo, fingerprint, is_permanent = entry
                 if not is_permanent:
                     continue
-                entry_line = f"{name} {algo} {fingerprint} {timestamp}\n"
+                entry_line = f"{name} {algo} {fingerprint}\n"
                 stash_file.write(entry_line)
-    except (OSError, ValueError):
+    except (OSError, ValueError) as exc:
-        pass
+        print(f"Failed to save certificate stash '{stash_path}': {exc}")
 
 
 class CertStatus(Enum):
     """Value returned by validate_cert."""
     # Cert is valid: proceed.
     VALID = 0      # Known and valid.
-    VALID_NEW = 7  # New and valid.
+    VALID_NEW = 1  # New and valid.
     # Cert is unusable or wrong: abort.
-    ERROR = 1              # General error.
+    ERROR = 2              # General error.
-    WRONG_FINGERPRINT = 2  # Fingerprint in the stash is different.
+    WRONG_FINGERPRINT = 3  # Fingerprint in the stash is different.
-    # Cert has some issues: ask to proceed.
-    NOT_VALID_YET = 3  # not-before date invalid.
-    EXPIRED = 4        # not-after date invalid.
-    BAD_DOMAIN = 5     # Host name is not in cert's valid domains.
 
 
-CERT_STATUS_INVALID = (
+def validate_cert(der, hostname, cert_stash) -> Dict[str, Any]:
-    CertStatus.NOT_VALID_YET,
+    """Return a dict containing validation info for this certificate.
-    CertStatus.EXPIRED,
-    CertStatus.BAD_DOMAIN,
-)
 
-
+    Returns:
-def validate_cert(der, hostname, cert_stash):
+    The validation dict can contain two keys:
-    """Return a tuple (CertStatus, Certificate) for this certificate."""
+    - status: CertStatus, always present.
+    - hash: DER hash to be used as certificate fingerprint, present if status is
+      not CertStatus.ERROR.
+    - saved_hash: fingerprint for this hostname in the local stash, present if
+      status is CertStatus.WRONG_FINGERPRINT.
+    """
     if der is None:
-        return CertStatus.ERROR, None
+        return {"status": CertStatus.ERROR}
-    try:
-        cert = asn1crypto.x509.Certificate.load(der)
-    except ValueError:
-        return CertStatus.ERROR, None
     
-    # Check for sane parameters.
+    known = False
-    now = datetime.datetime.now(tz=datetime.timezone.utc)
-    if now < cert.not_valid_before:
-        return CertStatus.NOT_VALID_YET, cert
-    if now > cert.not_valid_after:
-        return CertStatus.EXPIRED, cert
-    if hostname not in cert.valid_domains:
-        return CertStatus.BAD_DOMAIN, cert
 
     # Check the entire certificate fingerprint.
     cert_hash = hashlib.sha512(der).hexdigest()
+    result = {"hash": cert_hash}  # type: Dict[str, Any]
     if hostname in cert_stash:
-        _, fingerprint, timestamp, _ = cert_stash[hostname]
+        _, fingerprint, _ = cert_stash[hostname]
-        if timestamp >= now.timestamp():
+        if cert_hash != fingerprint:
-            if cert_hash != fingerprint:
+            result.update(
-                return CertStatus.WRONG_FINGERPRINT, cert
+                status=CertStatus.WRONG_FINGERPRINT,
-        else:
+                saved_hash=fingerprint
-            # Disregard expired fingerprints.
+            )
-            pass
+            return result
-        return CertStatus.VALID, cert
+        known = True
 
-    # The certificate is unknown and valid.
+    result.update(status=CertStatus.VALID if known else CertStatus.VALID_NEW)
-    return CertStatus.VALID_NEW, cert
+    return result
 
 
-def trust(cert_stash, hostname, algo, fingerprint, timestamp,
+def trust_fingerprint(stash, hostname, algo, fingerprint, trust_always=False):
-          trust_always=False):
+    """Add a fingerprint entry to this stash."""
-    cert_stash[hostname] = (algo, fingerprint, timestamp, trust_always)
+    stash[hostname] = (algo, fingerprint, trust_always)
+
+
+def untrust_fingerprint(stash, hostname):
+    """Remove a fingerprint entry from this stash; return True on deletion."""
+    if hostname in stash:
+        del stash[hostname]
+        return True
+    return False

requirements.txt

@@ -1 +0,0 @@
-asn1crypto