functions/azure/aad/Assert-AzureServicePrincipalForRbac.ps1

# <copyright file="Assert-AzureServicePrincipalForRbac.ps1" company="Endjin Limited">
# Copyright (c) Endjin Limited. All rights reserved.
# </copyright>

<#
.SYNOPSIS
Ensures that an Azure AD service principal exists, creating if necessary. Optionally storing the credential
in Azure Key Vault.

.DESCRIPTION
Ensures that a suitable Azure AD application & service principal exists. Optionally storing the credential
in Azure Key Vault.

.PARAMETER Name
The display name of the Azure AD service principal.

.PARAMETER KeyVaultName
The key vault where that service principal password will be stored.

.PARAMETER KeyVaultSecretName
The key vault secret name that service principal password will be stored in.

.PARAMETER RotateSecret
When specified, the service principal secret will be regenerated.

.OUTPUTS
Returns a tuple containing a hashtable representing the object describing the Azure AD service principal and
it's client secret. Where the client secret is not avilable (e.g. the service principal aleady exists) or the
Key Vault functionality is used, '$null' will be returned for this element.

e.g.
@(
    @{ <service-principal-definition> },
    "<client-secret>"
)

#>


function Assert-AzureServicePrincipalForRbac
{
    [CmdletBinding(SupportsShouldProcess)]
    param (
        [Parameter(Mandatory = $true)]
        [string] $Name,
        
        [Parameter(ParameterSetName = 'KeyVault',
                    Mandatory = $true)]
        [string] $KeyVaultName,

        [Parameter(ParameterSetName = 'KeyVault',
                    Mandatory = $true)]
        [string] $KeyVaultSecretName,

        [Parameter(ParameterSetName = 'KeyVault')]
        [switch] $RotateSecret
    )

    #region internal helper functions
    function _handleCredential {
        [CmdletBinding()]
        param (
            [Parameter(Mandatory=$true)]
            $ServicePrincipal
        )

        # Generate a service principal secret
        $spCred = $ServicePrincipal | New-AzADServicePrincipalCredential

        # get the secret from the credential object, based on which graph API we're using
        $spCredential = $useMsGraph `
                                ? ($spCred.SecretText | ConvertTo-SecureString -AsPlainText -Force) `
                                : $spCred.Secret

        if ($useKeyVault) {
            Write-Host "Storing service principal secret in key vault [VaultName=$KeyVaultName, SecretName=$KeyVaultSecretName]"
            Set-AzKeyVaultSecret -VaultName $KeyVaultName `
                                -Name $KeyVaultSecretName `
                                -SecretValue $spCredential `
                                -ContentType "text/plain" `
                | Out-Null

            return $null
        }
        else {
            return (ConvertFrom-SecureString $spCredential -AsPlainText)
        }
    }
    #endregion

    _EnsureAzureConnection -AzPowerShell | Out-Null

    $useKeyVault = ($PSCmdlet.ParameterSetName -eq "KeyVault")

    # Handle Azure Graph -> MS Graph transition
    # Breaking change to property names between v4 and v5 of the module
    $useMsGraph = $true
    $azResourcesModule = Import-Module Az.Resources -PassThru -Verbose:$false
    if ($azResourcesModule.Version.Major -gt 4) {
        Write-Verbose "Using Microsoft Graph"
        $appIdPropertyName = "AppId"
    }
    else {
        Write-Verbose "Using Azure Graph"
        $appIdPropertyName = "ApplicationId"
        $useMsGraph = $false
    }

    $spSecret = $null
    $existingSp = Get-AzADServicePrincipal -DisplayName $Name

    if (!$existingSp) {
        if ($PSCmdlet.ShouldProcess($Name, "Create Service Principal")) {

            # Create a new service principal
            $createParams = @{
                DisplayName = $Name
            }
            if (!$useMsGraph) {
                $createParams += @{ SkipAssignment = $true }
            }
            $newSp = New-AzADServicePrincipal @createParams
            Write-Host ("Created service principal [ObjectId={0}, ApplicationId={1}]" -f $newSp.Id, $newSp.$appIdPropertyName)

            $spSecret = _handleCredential $newSp
        }
    }
    else {
        Write-Host ("Service Principal '{0}' already exists [ObjectId={1}, ApplicationId={2}]" -f `
                            $Name,
                            $existingSp.Id,
                            $existingSp.$appIdPropertyName)

        if ($useKeyVault) {
            # if using key vault, check whether the specified secret is available
            $existingSecret = Get-AzKeyVaultSecret -VaultName $KeyVaultName -Name $KeyVaultSecretName
        }

        if (($useKeyVault -and !$existingSecret) -or $RotateSecret) {
            if ($PSCmdlet.ShouldProcess($Name, "Rotate Service Principal Secret")) {
                Write-Host "Rotating service principal credential [UseKeyVault=$useKeyVault, KeyVaultSecretMissing=$(!$existingSecret), RotateFlag=$RotateSecret]"
                $spSecret = _handleCredential $existingSp
            }
        }
    }

    return ($existingSp ? $existingSp : $newSp),$spSecret
}