Explanation
Box 1: Microsoft legacy Cryptographic Service Provider (CSP) key provider Give an app access to Azure Stack Hub resources An application that deploys or configures resources through Azure Resource Manager must be represented by its own identity, known as a security principal. Just as a user is represented by a user principal, an app is represented by a service principal.
Create an app registration that uses a certificate credential
When creating a certificate credential, the following requirements must be met:
* The cryptographic provider must be specified as a Microsoft legacy Cryptographic Service Provider (CSP) key provider.
* The certificate format must be in PFX file, as both the public and private keys are required. Windows servers use .pfx files that contain the public key file (TLS/SSL certificate file) and the associated private key file.
* Etc.
Box 2: .pfx
Reference: https://learn.microsoft.com/en-us/azure-stack/operator/give-app-access-to-resources