A python module for validating detached PKCS7 signatures.
This can be used to verify AWS EC2 Instance Identity documents as described at https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/instance-identity-documents.html
Python pkcs7 detached signature verify
License: MIT License
A python module for validating detached PKCS7 signatures.
This can be used to verify AWS EC2 Instance Identity documents as described at https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/instance-identity-documents.html
Hi. I used your library as a guide on how to do a pkcs7 verification, but didn't directly use your lib. Instead I derived this function, that uses bytes
and cryptography.x509.Certificate
parameters instead. I did this because I already have the things like that in memory, so serializing it just to deserializing it seemed a bit roundabout to me. Also passing the document as str
instead of bytes
seems fragile to me anyway. Who knows how the document was encoded (UTF-8? ISO-8859-1?) or if it even was string data or something random binary.
I use this derived function in another MIT licensed Python script (verify_ehc.py), but maybe you want to include this (or something similar) in your library:
from cryptography import x509
from cryptography.hazmat.backends.openssl.backend import backend # type: ignore
from cryptography.hazmat.bindings._openssl import ffi, lib # type: ignore
from cryptography.hazmat.bindings.openssl import binding
def verify_detached_signature_binary(document: bytes, signature: bytes, certificate: x509.Certificate) -> bool:
# Load the string into a bio
bio = backend._bytes_to_bio(signature) # pylint: disable=W0212
# Create the pkcs7 object
pkcs7_object = lib.d2i_PKCS7_bio(bio.bio, ffi.NULL)
if pkcs7_object == ffi.NULL:
binding._consume_errors(lib)
raise ValueError("Unable to parse PKCS7 data")
# Load the specified certificate
stack = lib.sk_X509_new_null()
res = lib.sk_X509_push(stack, certificate._x509) # type: ignore
binding._openssl_assert(lib, res >= 1)
# We need a CA store, even though we don't use it
store = lib.X509_STORE_new()
binding._openssl_assert(lib, store != ffi.NULL)
# Load the document into a bio
content = backend._bytes_to_bio(document) # pylint: disable=W0212
# If PKCS7_NOVERIFY is set the signer's certificates are not chain verified.
flags = lib.PKCS7_NOVERIFY
# https://www.openssl.org/docs/man1.0.2/crypto/PKCS7_verify.html
return lib.PKCS7_verify(pkcs7_object, stack, store, content.bio, ffi.NULL, flags) == 1
I don't know how to use OpenSSL via ffi (or at all). Are you sure this isn't leaking memory and there is no manual free required after lib.X509_STORE_new()
?
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