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