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How to get a certificate from a PKCS7 file

In the last period of time I was forced to play a lot with certificates from .NET code (in my case was C#). The good part when you are using .NET is that you have an API that can be used to manage system certificates.
It is pretty simple to load a certificate from certificates store, file or an array of bytes.
X509Certificate2 cert = 
    new X509Certificate2(bytes);
In the above example I am loading a certificate from an array of bytes. The code is pretty simple and works great.
There are situations when you receive a certificate from an external system. In this cases the first step that you need to do is to save the certificate local. When you are a 3rd party and you are receiving a certificate from a server you should be aware of the format of the certificate.
In my case I received a certificate signed in PKCS7 byte array without knowing this and I assumed that it is enough to load the certificate using the constructor. The funny thing is that you will be able to load a signed PKCS7 file using the certificate constructor without any kind of problems, but you will not load the certificate that you expect.
This is happening because PKCS7 signed filed can have more than one certificate and X509Certificate2 constructor will load the certificate that was used to signed the store rather than certificates that can be found in the rawData of PKCS7.
To be able to access and load a certificate from the rawData of PKCS7 file you will need to use SignedCms. This class give you the possibility to access a message in PKCS7 format. This is the best and simple way to access all the certificates from a signed file.
SignedCms cms = new SignedCms();
x509certificate2Collection certs = cms.Certificates;
The Certificates property is a X509CertificateCollection that contains all the certificates from the signed file.


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