Stephan van Rooij

Software architect with a passion for home automation.

Protect against certificate extraction - encryption

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You’re encrypting sensitive data with RSA encryption, great start! But how do you protect your certificates from being extracted? You get hacked and they steal your encrypted data, no problem, it’s encrypted! If they hack you, are you sure they can’t also steal the certificate? This series covers several risks of Certificate Extraction and what you can do about it.

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Certificate extraction series

Certificates in application might leak

No problem, we store all certificates in an Azure Key Vault and only the application has access using a managed identity.

Yes you’re mostly right, the app has access to the certificate using a managed identity. But it downloads it before using it for decrypting. Which means the certificate is available on the server where your app is running. And if you get hacked they might also be able to change the code of the application and extract the certificate to use it for decrypting all the encrypted data.

IF the certificate is available to the application, it MIGHT be extracted and send to some other service.

sequenceDiagram; participant c as Client application participant s as Storage participant kv as Key Vault participant ad as Azure AD participant m as Malicious actor rect rgb(191,227,180) note right of c: Managed identity c->>ad: Access token for KeyVault? ad->>c: Here you go end c->>kv: Get certificate kv->>c: Here is the certificate rect rgb(244,113,116) note right of c: Cert available in memory! c-->>m: Upload certificate to remote server m-->>m: Decrypt all extracted data remotely end c->>s: Get encrypted data with ID s->>c: Encrypted data c->>c: Decrypt data with local certificate

Data encryption with certificates

You can use certificates to encrypt data, which makes it a lot harder for an attacker to do something malicious with it. To encrypt data using a certificate you’ll need the public key of the certificate. Which means you can ask others to encrypt data for you, without them knowing the private key of your certificate.

We don’t have to go in much details on how to do the encryption, but you can find enough samples on stackoverflow.

public static byte[] EncryptDataOaepSha1(X509Certificate2 cert, byte[] data)
{
    using (RSA rsa = cert.GetRSAPublicKey())
    {
        return rsa.Encrypt(data, RSAEncryptionPadding.OaepSHA1);
    }
}

The main thing to understand here is that you only need the public key to encrypt data.

Data decryption with certificates

To decrypt data you’ll need the private key of the certificate, this key is meant to kept secret, because it can be used to decrypt all the data. You can use code similar to this to decrypt the data:

public static byte[] DecryptDataOaepSha1(X509Certificate2 cert, byte[] data)
{
    using (RSA rsa = cert.GetRSAPrivateKey())
    {
        return rsa.Decrypt(data, RSAEncryptionPadding.OaepSHA1);
    }
}

Protecting against certificate extraction

To protect against attackers being able to get their hands on the certificate and being able to decrypt all the encrypted data, we are going to make sure that no application or user/admin has access to the private key of the certificate.

Upon certificate generation in the Azure Key Vault, you have the option to set Exportable Private Key to No under Advanced Policy Configuration. Which means the private key, will NEVER be exported.

Key vault generate not exportable certificate

It’s possible to create a local backup including this private key, but the local backup can only be restored in a key vault in the same Azure subscription. see Key Vault backup.

Decrypting data in the cloud

Now that we have this certificate in the Key Vault with private key marked as not exportable, how do we decrypt the data? The KeyVault has a Decrypt operation, that means we are able to decrypt any data directly in the Key Vault without being able to extract the certificate.

sequenceDiagram; participant c as Client application participant s as Storage participant kv as Key Vault participant ad as Azure AD c->>s: Get encrypted data by id s->>c: Here is the encrypted data rect rgb(225,225,225) note right of c: Managed identity c->>kv: Decrypt data with certificate x? kv-->>ad: Is this managed identity allowed to decrypt? ad-->>kv: Yes / No kv->>c: Here is the decrypted data end c->>c: Convert decrypted bytes to needed format

Encrypting data in the cloud

For encrypting data, you don’t need the private key, so you can still download the certificate and use that for encryption. Because you marked the private key as not exportable, it will not be included in the certificate. And you’ll only receive the public part of the certificate.

But for convenience the KeyVault also has an Encrypt endpoint.

sequenceDiagram; participant c as Client application participant s as Storage participant kv as Key Vault participant ad as Azure AD c->>c: Convert data to byte[] rect rgb(225,225,225) note right of c: Managed identity c->>kv: Encrypt data with certificate kv-->>ad: Is this managed identity allowed to encrypt? ad-->>kv: Yes / No kv->>c: Here is the encrypted data end c->>s: Save encrypted data

Know about certificate extraction

It’s most import for companies to be aware of this issue, so please share this series to as much developers/architects as possible.

As long as you’re aware of the issue you might get to solve it before you’re attacked. There are a lot off things you can do with certificates, and most of them have a cloud alternative.

Use managed identities as much as possible

A lot of services in Azure support managed identities, you should use them as much as possible. Keeping secrets is hard, so why don’t you let Microsoft do the hard part, that is one thing less to worry.

In this sample we only use the managed identity to access the key vault, but you can also use managed identities to access cosmos db if you use that for storage.

Protect against certificate extraction - Client credentials

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You have this multi-tenant application that is protected with Azure AD, great! How about the certificate you’re using as a client credential? If you followed the Microsoft samples it’s probably stored in the Azure Key Vault. This seems really secure, but there is one thing no one thought about Certificate Extraction.

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Securing multi-tenant app with Managed Identity

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Keeping your secrets secure, can be a huge challenge. And keeping secrets becomes a huge responsibility, especially if you’re in the business of building multi-tenant applications. Microsoft created managed identities to ease this responsibility, but according to the faq, you cannot use them to secure resources in other tenants. And that is exactly where we could use extra help is securing secrets.

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