This is the script to generate the BEK file for unlocking the encrypted disk

GenerateBEKFileForDiskUnlockV2.ps1

Param(
  [Parameter(Mandatory = $true,
             HelpMessage="URL of the secret stored in the keyvault")]
  [ValidateNotNullOrEmpty()]
  [string]$secretUrl,
  
  [Parameter(Mandatory = $true,
             HelpMessage="Resource group of keyvault")]
  [ValidateNotNullOrEmpty()]
  [string]$keyVaultResourceGroup,

  [Parameter(Mandatory = $true,
             HelpMessage="URL of the KEK")]
  [ValidateNotNullOrEmpty()]
  [string]$kekUrl,

  [Parameter(Mandatory = $true,
             HelpMessage="Location where the retrieved secret should be written to")]
  [ValidateNotNullOrEmpty()]
  [string]$secretFilePath
)

Login-AzAccount;

#Install Active directory module
Install-Module -Name MSOnline;

#Get current logged in user and active directory tenant details
$ctx = Get-AzContext;
$adTenant = $ctx.Tenant.Id;
$currentUser = $ctx.Account.Id

#Parse the secret URL
$secretUri = [System.Uri] $secretUrl;

#Retrieve keyvault name, secret name and secret version from secret URL
$keyVaultName = $secretUri.Host.Split('.')[0];
$secretName = $secretUri.Segments[2].TrimEnd('/');
$secretVersion = $secretUri.Segments[3].TrimEnd('/');

#Set permissions for the current user to unwrap keys and retrieve secrets from KeyVault
$KeyVault = Get-AzKeyVault -VaultName $keyVaultName -ResourceGroupName $keyVaultResourceGroup;
$acl = $KeyVault.AccessPolicies;
$currentacl = $acl | Where-Object { $_.DisplayName -match $currentUser };
$aclp2k = $currentacl.PermissionsToKeys + "unwrapKey"
$aclp2s = $currentacl.PermissionsToSecrets + "get"
Set-AzKeyVaultAccessPolicy -VaultName $keyVaultName -PermissionsToKeys $aclp2k -PermissionsToSecrets $aclp2s -UserPrincipalName $currentUser;

#Retrieve secret from KeyVault secretUrl
$keyVaultSecret = Get-AzKeyVaultSecret -VaultName $keyVaultName -Name $secretName -Version $secretVersion;
$secretBase64 = $keyVaultSecret.SecretValue;
$bstr = [Runtime.InteropServices.Marshal]::SecureStringToBSTR($secretBase64)
$secretBase64 = [Runtime.InteropServices.Marshal]::PtrToStringAuto($bstr)

#Unwrap secret if the secret is wrapped with KEK
if($kekUrl)
{

    ########################################################################################################################
    # Initialize ADAL libraries and get authentication context required to make REST API called against KeyVault REST APIs. 
    ########################################################################################################################
	
	# Install the ADAL Module
	Install-Module -Name Az.Accounts -Scope AllUsers -Repository PSGallery -Force -AllowClobber

    $Token = "Bearer {0}" -f (Get-AzAccessToken -Resource "https://vault.azure.net").Token
    
    $headers = @{
    'Authorization' = $token
    "x-ms-version"  = '2014-08-01'
}

	# Place wrapped BEK in JSON object to send to KeyVault REST API

    ########################################################################################################################
    # 1. Retrieve the secret from KeyVault
    # 2. If Kek is not NULL, unwrap the secret with Kek by making KeyVault REST API call
    # 3. Convert Base64 string to bytes and write to the BEK file
    ########################################################################################################################

    #Call KeyVault REST API to Unwrap 
    $jsonObject = @"
    {
        "alg": "RSA-OAEP",
        "value" : "$secretBase64"
    }
"@

    $unwrapKeyRequestUrl = $kekUrl+ "/unwrapkey?api-version=2015-06-01";
    $result = Invoke-RestMethod -Method POST -Uri $unwrapKeyRequestUrl -Headers $headers -Body $jsonObject -ContentType "application/json";

    #Convert Base64Url string returned by KeyVault unwrap to Base64 string
    $secretBase64 = $result.value;
}

$secretBase64 = $secretBase64.Replace('-', '+');
$secretBase64 = $secretBase64.Replace('_', '/');
if($secretBase64.Length %4 -eq 2)
{
    $secretBase64+= '==';
}
elseif($secretBase64.Length %4 -eq 3)
{
    $secretBase64+= '=';
}

if($secretFilePath)
{
    $bekFileBytes = [System.Convert]::FromBase64String($secretBase64);
    [System.IO.File]::WriteAllBytes($secretFilePath,$bekFileBytes);
}

#Delete the key from the memory
[Runtime.InteropServices.Marshal]::ZeroFreeBSTR($bstr)
clear-variable -name secretBase64

Get rid of ADAL library (which incurred extra complexity and made lot of errors) and use the new method to get the token (Get-AzAccessToken)