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metadata.py
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metadata.py
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"""
<Program Name>
metadata.py
<Author>
Lukas Puehringer <lukas.puehringer@nyu.edu>
Santiago Torres <santiago@nyu.edu>
<Started>
Oct 23, 2017
<Copyright>
See LICENSE for licensing information.
<Purpose>
Provides a container class `Metablock` for signed metadata and
functions for signing, signature verification, de-serialization and
serialization from and to JSON.
"""
import attr
import json
import securesystemslib.keys
import securesystemslib.formats
import securesystemslib.exceptions
import securesystemslib.gpg.functions
from in_toto.models.common import ValidationMixin
from in_toto.models.link import Link
from in_toto.models.layout import Layout
from in_toto.exceptions import SignatureVerificationError
@attr.s(repr=False, init=False)
class Metablock(ValidationMixin):
"""A container for signed in-toto metadata.
Provides methods for metadata JSON (de-)serialization, reading from and
writing to disk, creating and verifying signatures, and self-validation.
Attributes:
signed: A subclass of Signable which has the actual metadata payload,
usually a Link or Layout object.
signatures: A list of signatures over the canonical JSON representation
of the value of the signed attribute.
compact_json: A boolean indicating if the dump method should write a
compact JSON string representation of the metadata.
"""
signatures = attr.ib()
signed = attr.ib()
def __init__(self, **kwargs):
self.signatures = kwargs.get("signatures", [])
self.signed = kwargs.get("signed")
self.compact_json = kwargs.get("compact_json", False)
self.validate()
def __repr__(self):
"""Returns the JSON string representation. """
indent = None if self.compact_json else 1
separators = (',', ':') if self.compact_json else (',', ': ')
return json.dumps(
{
"signatures": self.signatures,
"signed": attr.asdict(self.signed)
},
indent=indent,
separators=separators,
sort_keys=True
)
def dump(self, path):
"""Writes the JSON string representation of the instance to disk.
Arguments:
path: The path to write the file to.
Raises:
IOError: File cannot be written.
"""
with open(path, "wb") as fp:
fp.write("{}".format(self).encode("utf-8"))
@staticmethod
def load(path):
"""Loads the JSON string representation of in-toto metadata from disk.
Arguments:
path: The path to read the file from.
Raises:
IOError: The file cannot be written.
securesystemslib.exceptions.FormatError: Metadata format is invalid.
Returns:
A Metablock object whose signable attribute is either a Link or a Layout
object.
"""
with open(path, "r") as fp:
data = json.load(fp)
signatures = data.get("signatures", [])
signed_data = data.get("signed", {})
signed_type = signed_data.get("_type")
if signed_type == "link":
signed = Link.read(signed_data)
elif signed_type == "layout":
signed = Layout.read(signed_data)
else:
raise securesystemslib.exceptions.FormatError("Invalid Metadata format")
return Metablock(signatures=signatures, signed=signed)
@property
def type_(self):
"""A shortcut to the `type_` attribute of the object on the signable
attribute (should be one of "link" or "layout"). """
# NOTE: Trailing underscore is used by convention (pep8) to avoid conflict
# with Python's type keyword.
return self.signed.type_
def sign(self, key):
"""Creates signature over signable with key and adds it to signatures.
Uses the UTF-8 encoded canonical JSON byte representation of the signable
attribute to create signatures deterministically.
Attributes:
key: A signing key. The format is securesystemslib.formats.KEY_SCHEMA.
Raises:
securesystemslib.exceptions.FormatError: Key argument is malformed.
securesystemslib.exceptions.CryptoError, \
securesystemslib.exceptions.UnsupportedAlgorithmError:
Signing errors.
Returns:
The signature. Format is securesystemslib.formats.SIGNATURE_SCHEMA.
"""
securesystemslib.formats.KEY_SCHEMA.check_match(key)
signature = securesystemslib.keys.create_signature(key,
self.signed.signable_bytes)
self.signatures.append(signature)
return signature
def sign_gpg(self, gpg_keyid=None, gpg_home=None):
"""Creates signature over signable with gpg and adds it to signatures.
Uses the UTF-8 encoded canonical JSON byte representation of the signable
attribute to create signatures deterministically.
Arguments:
gpg_keyid (optional): A keyid used to identify a local gpg signing key.
If omitted the default signing key is used.
gpg_home (optional): A path to the gpg home directory. If not set the
default gpg home directory is used.
Raises:
ValueError, OSError, securesystemslib.gpg.exceptions.CommandError, \
securesystemslib.gpg.exceptions.KeyNotFoundError:
gpg signing errors.
Side Effects:
Calls system gpg command in a subprocess.
Returns:
The signature. Format is securesystemslib.formats.GPG_SIGNATURE_SCHEMA.
"""
signature = securesystemslib.gpg.functions.create_signature(
self.signed.signable_bytes, gpg_keyid, gpg_home)
self.signatures.append(signature)
return signature
def verify_signature(self, verification_key):
"""Verifies a signature over signable in signatures with verification_key.
Uses the UTF-8 encoded canonical JSON byte representation of the signable
attribute to verify the signature deterministically.
NOTE: Only the first signature in the signatures attribute, whose keyid
matches the verification_key keyid, is verified. If the verification_key
format is securesystemslib.formats.GPG_PUBKEY_SCHEMA, subkey keyids are
considered too.
Arguments:
verification_key: A verification key. The format is
securesystemslib.formats.ANY_VERIFICATION_KEY_SCHEMA.
Raises:
securesystemslib.exceptions.FormatError: The passed key is malformed.
SignatureVerificationError: No signature keyid matches the verification
key keyid, or the matching signature is malformed, or the matching
signature is invalid.
securesystemslib.gpg.exceptions.KeyExpirationError: Passed verification
key is an expired gpg key.
"""
securesystemslib.formats.ANY_VERIFICATION_KEY_SCHEMA.check_match(
verification_key)
verification_keyid = verification_key["keyid"]
# Find a signature that corresponds to the keyid of the passed
# verification key or one of its subkeys
signature = None
for signature in self.signatures:
if signature["keyid"] == verification_keyid:
break
if signature["keyid"] in list(
verification_key.get("subkeys", {}).keys()):
break
else:
raise SignatureVerificationError("No signature found for key '{}'"
.format(verification_keyid))
if securesystemslib.formats.GPG_SIGNATURE_SCHEMA.matches(signature):
valid = securesystemslib.gpg.functions.verify_signature(signature,
verification_key, self.signed.signable_bytes)
elif securesystemslib.formats.SIGNATURE_SCHEMA.matches(signature):
valid = securesystemslib.keys.verify_signature(
verification_key, signature, self.signed.signable_bytes)
else:
valid = False
if not valid:
raise SignatureVerificationError("Invalid signature for keyid '{}'"
.format(verification_keyid))
def _validate_signed(self):
"""Private method to check if the 'signed' attribute contains a valid
Layout or Link object. """
if not (isinstance(self.signed, Layout) or isinstance(self.signed, Link)):
raise securesystemslib.exceptions.FormatError("The Metblock's 'signed'"
" property has has to be of type 'Link' or 'Layout'.")
# If the signed object is a Link or Layout object validate it.
self.signed.validate()
def _validate_signatures(self):
"""Private method to check that the 'signatures' attribute is a list of
signatures in the format 'securesystemslib.formats.ANY_SIGNATURE_SCHEMA'.
"""
if not isinstance(self.signatures, list):
raise securesystemslib.exceptions.FormatError("The Metablock's"
" 'signatures' property has to be of type 'list'.")
for signature in self.signatures:
securesystemslib.formats.ANY_SIGNATURE_SCHEMA.check_match(signature)