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Jenny Ferries edited this page Sep 22, 2020 · 30 revisions

Using certificates with Microsoft.Identity.Web

Microsoft.Identity.Web uses certificates in two situations:

  • In web apps and web APIs, to prove the identity of the application, instead of using a client secret.
  • In web APIs, to decrypt tokens if the web API opted to get encrypted tokens.

This article explains both usages, as well as describes the certificates to use.

Table of contents:

Client certificates

Web apps and web APIs are confidential client applications.

They can prove their identity to Azure AD or Azure AD B2C by 3 means:

Method Supported in Microsoft.Identity.Web
Client secrets Yes
Client certificates Yes
Client assertions Not yet

Microsoft.Identity.Web supports specifying client certificates. The configuration property to specify the client certificates is ClientCertificates. It is an array of certificate descriptions. There are several ways of describing certificates. see Specifying certificates below.

Describing client certificates to use by configuration

You can express the client certificates in the ClientCertificates property. ClientCertificates and ClientSecret are mutually exclusive.

{
  "AzureAd": {
    "Instance": "https://login.microsoftonline.com/",
    "Domain": "msidentitysamplestesting.onmicrosoft.com",
    "TenantId": "7f58f645-c190-4ce5-9de4-e2b7acd2a6ab",
    "ClientId": "86699d80-dd21-476a-bcd1-7c1a3d471f75",

    "ClientCertificates": [
      {
        "SourceType": "KeyVault",
        "KeyVaultUrl": "https://msidentitywebsamples.vault.azure.net",
        "KeyVaultCertificateName": "MicrosoftIdentitySamplesCert"
      }
     ]
  }
}

See Specifying certificates below for all the ways to describe certificates.

Describing client certificates to use programmatically

You can also specify the certificate description programmatically. For this you add CertificateDescription instances to the ClientCertificates property of MicrosoftIdentityOptions. You can then use some of the overloads of AddMicrosoftIdentityWebApp, EnableTokenAcquisitionToCallDownstreamApi using delegates to set the MicrosoftIdentityOptions.

MicrosoftIdentityOptions options = new MicrosoftIdentityOptions();
options.ClientCertificates = new CertificateDescription[] {
 CertificateDescription.FromKeyVault("https://msidentitywebsamples.vault.azure.net",
                                     "MicrosoftIdentitySamplesCert")
};

See Specifying certificates below for all the ways to describe certificates.

Decryption certificates

Web APIs can request token encryption (for privacy reasons). This is even compulsory for first-party (Microsoft) web APIs that access MSA identities. The configuration property to specify the client certificates is TokenDecryptionCertificates. It is an array of descriptions of certificates.

Describing decryption certificates to use by configuration

You can express the decryption certificates in the TokenDecryptionCertificates property.

{
  "AzureAd": {
    "Instance": "https://login.microsoftonline.com/",
    "Domain": "msidentitysamplestesting.onmicrosoft.com",
    "TenantId": "7f58f645-c190-4ce5-9de4-e2b7acd2a6ab",
    "ClientId": "86699d80-dd21-476a-bcd1-7c1a3d471f75",

    "TokenDecryptionCertificates": [
      {
        "SourceType": "KeyVault",
        "KeyVaultUrl": "https://msidentitywebsamples.vault.azure.net",
        "KeyVaultCertificateName": "MicrosoftIdentitySamplesCert"
      }
     ]
  }
}

See Specifying certificates below for all the ways to describe certificates.

Describing decryption certificates to use programmatically

You can also specify the certificate description programmatically:

MicrosoftIdentityOptions options = new MicrosoftIdentityOptions();
options.TokenDecryptionCertificates = new CertificateDescription[] {
 CertificateDescription.FromKeyVault("https://msidentitywebsamples.vault.azure.net",
                                     "MicrosoftIdentitySamplesCert")
};

See Specifying certificates below for all the ways to describe certificates.

Helping certificate rotation by sending x5c

It's possible to specify if the x5c claim (public key of the certificate) should be sent to the STS each time the web app or web API calls Azure AD. Sending the x5c enables application developers to achieve easy certificate rollover in Azure AD: this method will send the public certificate to Azure AD along with the token request, so that Azure AD can use it to validate the subject name based on a trusted issuer policy. This saves the application admin from the need to explicitly manage the certificate rollover (either via portal or PowerShell/CLI operation). For details see https://aka.ms/msal-net-sni.

To specify to send the x5c claim, set the boolean SendX5C property of the options to true either by configuration or programmatically.

{
  "AzureAd": {
    "Instance": "https://login.microsoftonline.com/",
    "Domain": "msidentitysamplestesting.onmicrosoft.com",
    "TenantId": "7f58f645-c190-4ce5-9de4-e2b7acd2a6ab",
    "ClientId": "86699d80-dd21-476a-bcd1-7c1a3d471f75",

    "TokenDecryptionCertificates": [
      {
        "SourceType": "KeyVault",
        "KeyVaultUrl": "https://msidentitywebsamples.vault.azure.net",
        "KeyVaultCertificateName": "MicrosoftIdentitySamplesCert"
      }
     ],
     "SendX5C": "true"
  }
}

Specifying certificates

You can describe the certificates to load, either by configuration, or programmatically:

  • from the certificate store (Windows) and a thumbprint ("440A5BE6C4BE2FF02A0ADBED1AAA43D6CF12E269"),
  • from the certificate store (Windows) and a distinguished name ("CN=TestCert"),
  • from a path on the disk and optionally a password (probably only for debugging locally),
  • directly from a Base64 representation of the certificate,
  • from Azure Key Vault,
  • directly providing it (programmatically only).

Describing the certificate by configuration allows for just-in-time loading, rather than paying the startup cost. For instance for a web app that signs in a user, do not load the certificate until an access token is needed to call a web API.

When your certificate is in Key Vault, Microsoft.Identity.Web leverages Managed Identity, therefore enabling your application to have the same code when deployed (for instance on a VM or Azure app services), or locally on your developer box (using developer credentials).

The following sections show how to specify the client credential certificates, but the principle is the same for the decryption certificates. Just replace ClientCertificates with TokenDecryptionCertificates.

Getting certificates from Key Vault

Microsoft.Identity.Web leverages Managed Identity

To fetch certificates from KeyVault, Microsoft.Identity.Web leverages Managed Identity through the Azure SDK DefaultAzureCredential.

This works out of the box on the developer machine using the developer credentials, and also when deployed with Service fabric or App Services in Azure provided you've been using a System-assigned Managed identity.

However:

  • If you are using a User-assigned managed identity, you will need to set an environment variable AZURE_CLIENT_ID to be the user-assigned managed identity clientID. You can do that through the Azure portal:

    1. Go to Azure App Service -> Settings | Configuration -> Application Settings
    2. Add or update the AZURE_CLIENT_ID app setting to the user assigned managed identity ID.

  • When used on your developer machine, you have several accounts in Visual Studio, you'll need to specify which account to use, by setting another environement variable AZURE_USERNAME

Specifying client certificate from Key Vault by configuration
{
    "ClientCertificates": [
      {
        "SourceType": "KeyVault",
        "KeyVaultUrl": "https://msidentitywebsamples.vault.azure.net",
        "KeyVaultCertificateName": "MicrosoftIdentitySamplesCert"
      }
     ]
  }
}
Specifying client certificate from Key Vault programmatically
MicrosoftIdentityOptions options = new MicrosoftIdentityOptions();
options.ClientCertificates = new CertificateDescription[] {
 CertificateDescription.FromKeyVault("https://msidentitywebsamples.vault.azure.net",
                                     "MicrosoftIdentitySamplesCert")
};

Specifying certificates from a path

Specifying certificates from a path by configuration
{
    "ClientCertificates": [
      {
       "SourceType": "Path",
       "CertificateDiskPath": "c:\\temp\\WebAppCallingWebApiCert.pfx",
      "CertificatePassword": "password"
      }]
}
Specifying certificates from a path programmatically
MicrosoftIdentityOptions options = new MicrosoftIdentityOptions();
options.ClientCertificates = new CertificateDescription[] {
 CertificateDescription.FromPath(@"c:\temp\WebAppCallingWebApiCert.pfx",
                                     "password")
};

Specifying certificates from a certificate store by distinguished name

Specifying certificates from a certificate store by distinguished name by configuration
{
    "ClientCertificates": [
      {
      "SourceType": "StoreWithDistinguishedName",
      "CertificateStorePath": "CurrentUser/My",
      "CertificateDistinguishedName": "CN=WebAppCallingWebApiCert"
      }]
}
Specifying certificates from a certificate store by distinguished name programmatically
MicrosoftIdentityOptions options = new MicrosoftIdentityOptions();
options.ClientCertificates = new CertificateDescription[] {
 CertificateDescription.FromStoreWithDistinguishedName(StoreLocation.CurrentUser,
                                     StoreName.My,
                                     "CN=WebAppCallingWebApiCert")
};

Specifying certificates from a certificate store by thumbprint

Specifying certificates from a certificate store by thumbprint by configuration
{
    "ClientCertificates": [
      {
       "SourceType": "StoreWithThumbprint",
       "CertificateStorePath": "CurrentUser/My",
       "CertificateThumbprint": "962D129A859174EE8B5596985BD18EFEB6961684"
      }]
}
Specifying certificates from a certificate store by thumbprint programmatically
MicrosoftIdentityOptions options = new MicrosoftIdentityOptions();
options.ClientCertificates = new CertificateDescription[] {
 CertificateDescription.FromStoreWithThumbprint(StoreLocation.CurrentUser,
                                     StoreName.My,
                                     "962D129A859174EE8B5596985BD18EFEB6961684")
};

Specifying certificates from a certificate store by Base64 encoded value

Specifying certificates from a certificate store by Base64 encoded value by configuration
{
    "ClientCertificates": [
      {
       "SourceType": "Base64Encoded",
       "Base64EncodedValue": "MIIDHzCCAgegA.....r1n8Czew8TPfab4OG37BuEMNmBpqoRrRgFnDzVtItOnhuFTa0="
      }]
}
Specifying certificates from a certificate store by Base64 encoded value programmatically
MicrosoftIdentityOptions options = new MicrosoftIdentityOptions();
options.ClientCertificates = new CertificateDescription[] {
 CertificateDescription.FromBase64Encoded("MIIDHzCCAgegA.....r1n8Czew8TPfab4OG37BuEMNmBpqoRrRgFnDzVtItOnhuFTa0=")
};

Specifying certificates as an X509Certificate2

You can also directly specify the certificate description as an X509Certificate2 that would you have loaded. This is only possible programmatically

MicrosoftIdentityOptions options = new MicrosoftIdentityOptions();
options.ClientCertificates = new CertificateDescription[] {
 CertificateDescription.FromCertificate(x509certificate2)
};

Microsoft Identity Web classes used for certificate management

This is a class diagram showing how the classes involved in certificate management in Microsoft.Identity.Web are articulated:

image

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