async def get_old_timestamp(signature, timestamp_url=None):
"""Retrieve an old style timestamp countersignature.
Args:
signature (str): the signature to get a counter signature for. This is
usally the encryptedDigest of our file's signerInfo section.
timestamp_url (str): what service to use to fetch the timestamp countersignature from.
defaults to 'http://timestamp.digicert.com'.
Returns:
SignedData object
"""
req = OldTimeStampReq()
req["type"] = univ.ObjectIdentifier("1.3.6.1.4.1.311.3.2.1")
req["blob"]["signature"] = signature
req["blob"]["type"] = univ.ObjectIdentifier("1.2.840.113549.1.7.1")
encoded_req = der_encode(req)
b64_req = base64.b64encode(encoded_req)
url = timestamp_url or "http://timestamp.digicert.com"
async with aiohttp.request(
"POST", url, data=b64_req, headers={"Content-Type": "application/octet-stream"}
) as resp:
# Uncomment below to capture a real response
# open('old-ts.dat', 'wb').write(resp.content)
ci, _ = der_decode(base64.b64decode(await resp.read()), ContentInfo())
ts, _ = der_decode(ci["content"], SignedData())
return ts
def verify_pefile_rfc3161_timestamp(f, pe):
"""Verifies that the timestamp in this PE file is valid."""
certificates = pe.certificates
if not certificates:
return True, "No certificates present"
messages = []
x509_certs_by_serial = get_x509_certificates(pe)
passed = True
for pe_cert in certificates:
content_info, _ = der_decode(pe_cert.data, ContentInfo())
signed_data, _ = der_decode(content_info["content"], SignedData())
for info in signed_data["signerInfos"]:
# Check if there are any timestamps in unauthenticatedAttributes
for a in info["unauthenticatedAttributes"]:
if a["type"] == id_timestampSignature: # RFC3161 timestamps
# Calculate the hash of our signature
# TODO: don't hardcode the digest algorithm
digest_algo = DIGEST_NAME_BY_OID[info["digestAlgorithm"]
["algorithm"]]
signature_digest = hashlib.new(
digest_algo,
info["encryptedDigest"].asOctets()).digest()
counter_sig = der_decode(a["values"][0], ContentInfo())[0]
counter_sig = der_decode(counter_sig["content"],
CMSSignedData())[0]
tst_info = der_decode(
counter_sig["encapContentInfo"]["eContent"],
TSTInfo())[0]
message_digest = tst_info["messageImprint"][
"hashedMessage"].asOctets()
if message_digest != signature_digest:
passed = False
messages.append(
f"counter signature is over the wrong data (hash: {hexlify(signature_digest)})"
)
t_passed, message = verify_signed_data(
counter_sig, x509_certs_by_serial)
passed = passed and t_passed
messages.append(message)
return passed, "\n".join(messages)
async def make_authenticode_signeddata(
cert,
signer,
authenticode_digest,
digest_algo,
timestamp=None,
opus_info=None,
opus_url=None,
):
"""Creates a SignedData object containing the signature for a PE file.
Arguments:
cert (X509): public certificate used for signing
signer (function): signing function
authenticode_digest (bytes): Authenticode digest of PE file to sign
NB. This is not simply the hash of the file!
digest_algo (str): digest algorithm to use. e.g. 'sha256'
timestamp (UTCTime): optional. timestamp to include in the signature.
If not provided, the current time is used.
opus_info (string): Additional information to include in the signature
opus_url (string): URL to include in the signature
Returns:
A ContentInfo ASN1 object
"""
if not timestamp:
timestamp = useful.UTCTime.fromDateTime(datetime.now())
asn_digest_algo = ASN_DIGEST_ALGO_MAP[digest_algo]
spc = make_spc(digest_algo, authenticode_digest)
encoded_spc = der_encode(spc)
pkcs7_cert = x509_to_pkcs7(cert)
signer_info = make_signer_info(pkcs7_cert, digest_algo, timestamp,
calc_spc_digest(encoded_spc, digest_algo))
signer_digest = calc_signerinfo_digest(signer_info, digest_algo)
signer_info["encryptedDigest"] = await signer(signer_digest, digest_algo)
sig = SignedData()
sig["version"] = 1
sig["digestAlgorithms"][0] = asn_digest_algo
sig["certificates"][0]["certificate"] = pkcs7_cert
sig["contentInfo"]["contentType"] = id_spcIndirectDataContext
sig["contentInfo"]["content"] = encoded_spc
sig["signerInfos"][0] = signer_info
ci = ContentInfo()
ci["contentType"] = id_signedData
ci["content"] = sig
return ci
def get_signatures_from_certificates(certificates):
"""Retrieve the signatures from a list of certificates."""
retval = []
for cert in certificates:
ci, _ = der_decode(cert["data"], ContentInfo())
signed_data, _ = der_decode(ci["content"], SignedData())
spc, _ = der_decode(signed_data["contentInfo"]["content"],
SpcIndirectDataContent())
signed_data["contentInfo"]["content"] = spc
retval.append(signed_data)
return retval
def get_x509_certificates(pe):
"""Returns a mapping of (issuer, serial) to x509 certificates."""
certificates = pe.certificates
x509_certs_by_serial = {}
for pe_cert in certificates:
content_info, _ = der_decode(pe_cert.data, ContentInfo())
signed_data, _ = der_decode(content_info["content"], SignedData())
for cert in signed_data["certificates"]:
cert = der_encode(cert["certificate"])
x509_cert = x509.load_der_x509_certificate(cert, default_backend())
x509_certs_by_serial[x509_cert.issuer,
x509_cert.serial_number] = x509_cert
return x509_certs_by_serial
async def resign(old_sig, certs, signer):
"""Resigns an old signature with a new certificate.
Replaces the encrypted signature digest in the given signature with new one
generated with the given signer function.
Args:
old_sig (SignedData): the original signature as a SignedData object
certs (list of x509 certificates): certificates to attach to the new signature
signer (function): function to call to generate the new encrypted
digest. The function is passed two arguments: (signer_digest,
digest_algo)
Returns:
ContentInfo object with the new signature embedded
"""
new_sig = SignedData()
new_sig["version"] = old_sig["version"]
new_sig["contentInfo"] = old_sig["contentInfo"]
new_sig["digestAlgorithms"] = old_sig["digestAlgorithms"]
for i, cert in enumerate(certs):
pkcs7_cert = x509_to_pkcs7(cert)
new_sig["certificates"][i]["certificate"] = pkcs7_cert
new_si = copy_signer_info(old_sig["signerInfos"][0],
new_sig["certificates"][0]["certificate"])
if new_si["digestAlgorithm"]["algorithm"] == id_sha1:
digest_algo = "sha1"
elif new_si["digestAlgorithm"]["algorithm"] == id_sha256:
digest_algo = "sha256"
signer_digest = calc_signerinfo_digest(new_si, digest_algo)
log.debug("Digest to sign is: %s", hexlify(signer_digest))
new_si["encryptedDigest"] = await signer(signer_digest, digest_algo)
new_sig["signerInfos"][0] = new_si
ci = ContentInfo()
ci["contentType"] = id_signedData
ci["content"] = new_sig
sig = der_encode(ci)
return sig
def verify_pefile_signature(f, pe):
"""Verifies that the signature in this PE file is valid."""
# TODO: Check that the message being signed refers to something.
# e.g. the authenticatedAttributes' digest is our hash
certificates = pe.certificates
if not certificates:
return True, "No certificates present"
messages = []
x509_certs_by_serial = get_x509_certificates(pe)
passed = True
for pe_cert in certificates:
content_info, _ = der_decode(pe_cert.data, ContentInfo())
signed_data, _ = der_decode(content_info["content"], SignedData())
spc = der_decode(signed_data["contentInfo"]["content"],
SpcIndirectDataContent())[0]
digest_algo_oid = spc["messageDigest"]["digestAlgorithm"]["algorithm"]
digest_algo = DIGEST_NAME_BY_OID[digest_algo_oid]
a_digest = calc_authenticode_digest(f, digest_algo)
e_spc = der_encode(make_spc(digest_algo, a_digest))
spc_digest = calc_spc_digest(e_spc, digest_algo)
for info in signed_data["signerInfos"]:
# Check that the signature is on the right hash
info_digest = der_decode(
get_attribute(info["authenticatedAttributes"],
id_messageDigest)[0])[0].asOctets()
if info_digest != spc_digest:
passed = False
messages.append(
f"Wrong digest: {hexlify(info_digest)} != {hexlify(spc_digest)}"
)
continue
t_passed, message = verify_signer_info(info, x509_certs_by_serial)
passed = passed and t_passed
messages.append(message)
return passed, "\n".join(messages)
def verify_pefile_old_timestamp(f, pe):
"""Verifies that the timestamp in this PE file is valid."""
certificates = pe.certificates
if not certificates:
return True, "No certificates present"
messages = []
x509_certs_by_serial = get_x509_certificates(pe)
passed = True
for pe_cert in certificates:
content_info, _ = der_decode(pe_cert.data, ContentInfo())
signed_data, _ = der_decode(content_info["content"], SignedData())
for info in signed_data["signerInfos"]:
# Check if there are any timestamps in unauthenticatedAttributes
for a in info["unauthenticatedAttributes"]:
if a["type"] == id_counterSignature: # Old timestamps
# Calculate the hash of our signature
# These timestamps are always sha1
signature_digest = hashlib.new(
"sha1", info["encryptedDigest"].asOctets()).digest()
counter_sig = der_decode(a["values"][0], SignerInfo())[0]
counter_sig_digest = der_decode(
get_attribute(counter_sig["authenticatedAttributes"],
id_messageDigest)[0])[0].asOctets()
# Check that the counter signature is of the right data
if counter_sig_digest != signature_digest:
passed = False
messages.append(
f"counter signature is over the wrong data (hash: {hexlify(signature_digest)})"
)
# Check that the timestamp signature itself is valid
t_passed, message = verify_signer_info(
counter_sig, x509_certs_by_serial)
passed = passed and t_passed
messages.append(message)
return passed, "\n".join(messages)
def verify_pefile_digest(f, pe):
"""Verifies that the authenticode digest in this PE file is valid."""
certificates = pe.certificates
if not certificates:
return True, "No certificates present"
for pe_cert in certificates:
content_info, _ = der_decode(pe_cert.data, ContentInfo())
signed_data, _ = der_decode(content_info["content"], SignedData())
spc = der_decode(signed_data["contentInfo"]["content"],
SpcIndirectDataContent())[0]
file_digest = spc["messageDigest"]["digest"].asOctets()
digest_algo_oid = spc["messageDigest"]["digestAlgorithm"]["algorithm"]
a_digest = calc_authenticode_digest(
f, DIGEST_NAME_BY_OID[digest_algo_oid])
if file_digest != a_digest:
return (
False,
f"authenticode digests don't match: {hexlify(file_digest)} != {hexlify(a_digest)}",
)
return True, "authenticode digests match"
def get_signeddata(s):
"""Gets the SignedData from an encoded ContentInfo object."""
ci = der_decode(s, ContentInfo())[0]
sd = der_decode(ci["content"], SignedData())[0]
return sd