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"""
<Module Name>
dsa.py
<Author>
Santiago Torres-Arias <santiago@nyu.edu>
<Started>
Nov 15, 2017
<Copyright>
See LICENSE for licensing information.
<Purpose>
DSA-specific handling routines for signature verification and key parsing
"""
import binascii
CRYPTO = True
NO_CRYPTO_MSG = "DSA 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 dsa
from cryptography.hazmat.primitives.asymmetric import utils as dsautils
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 a DSAPublicKey object from the passed pubkey_info
using pyca/cryptography.
<Arguments>
pubkey_info:
The DSA pubkey dict.
<Exceptions>
securesystemslib.exceptions.UnsupportedLibraryError if
the cryptography module is not available
<Returns>
A cryptography.hazmat.primitives.asymmetric.dsa.DSAPublicKey based on the
passed pubkey_info.
"""
if not CRYPTO: # pragma: no cover
raise exceptions.UnsupportedLibraryError(NO_CRYPTO_MSG)
y = int(pubkey_info["keyval"]["public"]["y"], 16)
g = int(pubkey_info["keyval"]["public"]["g"], 16)
p = int(pubkey_info["keyval"]["public"]["p"], 16)
q = int(pubkey_info["keyval"]["public"]["q"], 16)
parameter_numbers = dsa.DSAParameterNumbers(p, q, g)
pubkey = dsa.DSAPublicNumbers(y, parameter_numbers).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>
A DSA public key dict.
"""
ptr = 0
prime_p_length = gpg_util.get_mpi_length(data[ptr : ptr + 2])
ptr += 2
prime_p = data[ptr : ptr + prime_p_length]
if len(prime_p) != prime_p_length: # pragma: no cover
raise PacketParsingError("This MPI was truncated!")
ptr += prime_p_length
group_order_q_length = gpg_util.get_mpi_length(data[ptr : ptr + 2])
ptr += 2
group_order_q = data[ptr : ptr + group_order_q_length]
if len(group_order_q) != group_order_q_length: # pragma: no cover
raise PacketParsingError("This MPI has been truncated!")
ptr += group_order_q_length
generator_length = gpg_util.get_mpi_length(data[ptr : ptr + 2])
ptr += 2
generator = data[ptr : ptr + generator_length]
if len(generator) != generator_length: # pragma: no cover
raise PacketParsingError("This MPI has been truncated!")
ptr += generator_length
value_y_length = gpg_util.get_mpi_length(data[ptr : ptr + 2])
ptr += 2
value_y = data[ptr : ptr + value_y_length]
if len(value_y) != value_y_length: # pragma: no cover
raise PacketParsingError("This MPI has been truncated!")
return {
"y": binascii.hexlify(value_y).decode("ascii"),
"p": binascii.hexlify(prime_p).decode("ascii"),
"g": binascii.hexlify(generator).decode("ascii"),
"q": binascii.hexlify(group_order_q).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 fourth paragraph of section 5.2.2
<Exceptions>
securesystemslib._gpg.exceptions.PacketParsingError:
if the public key parameters are malformed
securesystemslib.exceptions.UnsupportedLibraryError:
if the cryptography module is not available
<Side Effects>
None.
<Returns>
The decoded signature buffer
"""
if not CRYPTO: # pragma: no cover
return exceptions.UnsupportedLibraryError(NO_CRYPTO_MSG)
ptr = 0
r_length = gpg_util.get_mpi_length(data[ptr : ptr + 2])
ptr += 2
r = data[ptr : ptr + r_length]
if len(r) != r_length: # pragma: no cover
raise PacketParsingError("r-value truncated in signature")
ptr += r_length
s_length = gpg_util.get_mpi_length(data[ptr : ptr + 2])
ptr += 2
s = data[ptr : ptr + s_length]
if len(s) != s_length: # pragma: no cover
raise PacketParsingError("s-value truncated in signature")
s = int(binascii.hexlify(s), 16)
r = int(binascii.hexlify(r), 16)
signature = dsautils.encode_dss_signature(r, s)
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
DSA public key using pyca/cryptography.
<Arguments>
signature_object:
A signature dict.
pubkey_info:
The DSA public key dict.
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.
content:
The signed bytes against which the signature is verified
<Exceptions>
securesystemslib.exceptions.UnsupportedLibraryError if:
the cryptography module is not available
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)
digest = gpg_util.hash_object(
binascii.unhexlify(signature_object["other_headers"]), hasher(), content
)
try:
pubkey_object.verify(
binascii.unhexlify(signature_object["signature"]),
digest,
dsautils.Prehashed(hasher()),
)
return True
except InvalidSignature:
return False
|
