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"""
<Module Name>
rsa.py
<Author>
Santiago Torres-Arias <santiago@nyu.edu>
<Started>
Nov 15, 2017
<Copyright>
See LICENSE for licensing information.
<Purpose>
RSA-specific handling routines for signature verification and key parsing
"""
import binascii
CRYPTO = True
NO_CRYPTO_MSG = "RSA key support for GPG requires the cryptography library"
try:
from cryptography.exceptions import InvalidSignature
from cryptography.hazmat import backends
from cryptography.hazmat.primitives.asymmetric import padding, rsa, utils
except ImportError:
CRYPTO = False
# ruff: noqa: E402
from securesystemslib import exceptions
from securesystemslib._gpg import util as gpg_util
from securesystemslib._gpg.exceptions import PacketParsingError
def create_pubkey(pubkey_info):
"""
<Purpose>
Create and return an RSAPublicKey object from the passed pubkey_info
using pyca/cryptography.
<Arguments>
pubkey_info:
An RSA pubkey dict.
<Exceptions>
securesystemslib.exceptions.UnsupportedLibraryError if
the cryptography module is unavailable
<Returns>
A cryptography.hazmat.primitives.asymmetric.rsa.RSAPublicKey based on the
passed pubkey_info.
"""
if not CRYPTO: # pragma: no cover
raise exceptions.UnsupportedLibraryError(NO_CRYPTO_MSG)
e = int(pubkey_info["keyval"]["public"]["e"], 16)
n = int(pubkey_info["keyval"]["public"]["n"], 16)
pubkey = rsa.RSAPublicNumbers(e, n).public_key(backends.default_backend())
return pubkey
def get_pubkey_params(data):
"""
<Purpose>
Parse the public key parameters as multi-precision-integers.
<Arguments>
data:
the RFC4880-encoded public key parameters data buffer as described
in the fifth paragraph of section 5.5.2.
<Exceptions>
securesystemslib._gpg.exceptions.PacketParsingError:
if the public key parameters are malformed
<Side Effects>
None.
<Returns>
An RSA public key dict.
"""
ptr = 0
modulus_length = gpg_util.get_mpi_length(data[ptr : ptr + 2])
ptr += 2
modulus = data[ptr : ptr + modulus_length]
if len(modulus) != modulus_length: # pragma: no cover
raise PacketParsingError("This modulus MPI was truncated!")
ptr += modulus_length
exponent_e_length = gpg_util.get_mpi_length(data[ptr : ptr + 2])
ptr += 2
exponent_e = data[ptr : ptr + exponent_e_length]
if len(exponent_e) != exponent_e_length: # pragma: no cover
raise PacketParsingError("This e MPI has been truncated!")
return {
"e": binascii.hexlify(exponent_e).decode("ascii"),
"n": binascii.hexlify(modulus).decode("ascii"),
}
def get_signature_params(data):
"""
<Purpose>
Parse the signature parameters as multi-precision-integers.
<Arguments>
data:
the RFC4880-encoded signature data buffer as described
in the third paragraph of section 5.2.2.
<Exceptions>
securesystemslib._gpg.exceptions.PacketParsingError:
if the public key parameters are malformed
<Side Effects>
None.
<Returns>
The decoded signature buffer
"""
ptr = 0
signature_length = gpg_util.get_mpi_length(data[ptr : ptr + 2])
ptr += 2
signature = data[ptr : ptr + signature_length]
if len(signature) != signature_length: # pragma: no cover
raise PacketParsingError("This signature was truncated!")
return signature
def verify_signature(signature_object, pubkey_info, content, hash_algorithm_id):
"""
<Purpose>
Verify the passed signature against the passed content with the passed
RSA public key using pyca/cryptography.
<Arguments>
signature_object:
A signature dict.
pubkey_info:
The RSA public key dict.
content:
The signed bytes against which the signature is verified
hash_algorithm_id:
one of SHA1, SHA256, SHA512 (see securesystemslib._gpg.constants)
used to verify the signature
NOTE: Overrides any hash algorithm specification in "pubkey_info"'s
"hashes" or "method" fields.
<Exceptions>
securesystemslib.exceptions.UnsupportedLibraryError if:
the cryptography module is unavailable
ValueError:
if the passed hash_algorithm_id is not supported (see
securesystemslib._gpg.util.get_hashing_class)
<Returns>
True if signature verification passes and False otherwise
"""
if not CRYPTO: # pragma: no cover
raise exceptions.UnsupportedLibraryError(NO_CRYPTO_MSG)
hasher = gpg_util.get_hashing_class(hash_algorithm_id)
pubkey_object = create_pubkey(pubkey_info)
# zero-pad the signature due to a discrepancy between the openssl backend
# and the gnupg interpretation of PKCSv1.5. Read more at:
# https://github.com/in-toto/in-toto/issues/171#issuecomment-440039256
# we are skipping this if on the tests because well, how would one test this
# deterministically.
pubkey_length = len(pubkey_info["keyval"]["public"]["n"])
signature_length = len(signature_object["signature"])
if pubkey_length != signature_length: # pragma: no cover
zero_pad = "0" * (pubkey_length - signature_length)
signature_object["signature"] = "{}{}".format(
zero_pad, signature_object["signature"]
)
digest = gpg_util.hash_object(
binascii.unhexlify(signature_object["other_headers"]), hasher(), content
)
try:
pubkey_object.verify(
binascii.unhexlify(signature_object["signature"]),
digest,
padding.PKCS1v15(),
utils.Prehashed(hasher()),
)
return True
except InvalidSignature:
return False
|
