CODEWITHSUNDEEP
c#.netencryptionpkcs#7azure-keyvault
Frode
Frode

Reputation: 3455

EnvelopedCms decryption does not work with Azure Key Vault

I've been struggeling with this for days now and RFC 2315 is a bit hard to understand.

I'm trying to implement my own version of EnvelopedCms.Decrypt(), so that I can use the certificate operations of Azure Key Vault to UnwrapKey and/or Decrypt a PKCS#7 message (CMS Object) in a correct way. I use EnevelopedCms in .Net to Decode the message, then I try to Decrypt the EnvelopedCms.ContentInfo.Content.

This is what I try to do;

public static async Task<byte[]> DecryptCustom(string certificateId, string encryptedBase64Content)
{
    var bytes = Convert.FromBase64String(encryptedBase64Content);
    var contentInfo = new ContentInfo(bytes);
    var envelopedCms = new EnvelopedCms(contentInfo);
    envelopedCms.Decode(bytes);
    // envelopedCms.Decrypt()  <-- no go. Can't extract certificate from Key Vault

    // My (naive) attempt to decrypt CMS content using Azure Key Vault certificates
    byte[] decryptedContent;
    using (var client = new KeyVaultClient(GetKeyVaultToken))
    {
        var decryptionresult = await client.DecryptAsync(GetKeyUrl(certificateId), "RSA1_5", envelopedCms.ContentInfo.Content);
        decryptedContent = decryptionresult.Result;
    }
    return decryptedContent;
}

I was hoping it could be that easy, but it gives me the following error;

Unable to decrypt specified value with this key.

I read something about octets in RFC 2315, so maybe the stream (byte-array) needs some reordering before I decrypt. Do I need to unwrap some symmetric key to decrypt the real payload? I'm on thin ice here.

I'm not a cryptography professional so I might have missed something obvious, too. I was hoping someone knew what to do in this case as I really want to keep my certificates inside the Key Vault (HSM)

Upvotes: 3

Views: 773

Answers (1)

EventHorizon
EventHorizon

Reputation: 2986

CMS envelope contents are encrypted using a session key, and this key is encrypted with each recipients (there can be many) public key before transmission.

What you need is to extract your recipient's encrypted session key, and unwrap it with the private key stored in key vault. I'm not near Visual Studio right now, but here is the pseudocode:

// Extract the first (and often only) receiver's encrypted session key
var key = envelopedCms.Receivers[0].EncryptionKey; 
// Unwrap the sessionKey using the receiver's private key stored in key vault:
var sessionKey = (await keyVaultClient.Unwrap(uri, "certificatename", key)).Result;

Finally, using the sessionKey, you can decrypt the envelope contents (ContentInfo.Content). The encryption type is specified in the envelope's encryption algorithm-property.

Upvotes: 4

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