Powershell code to implement Client Credential authentication using Self-Signed Certificate - azure

Question

Can anyone help me out troubleshooting this script as to why doesn't return the $TokenResponse?
I was able to use secret key and user auth but can't find a way to auth using the self-signed cert which is the most secure option of all.
In Azure, permissions are assigned to Application and work fine with said secret key
#Define Client Variables Here
########################
$TenantName = "contoso.com"
$AppId = "11111-2222-3333-4444-5555"
$Certificate = Get-Item Cert:LocalMachine\My\1234567890987654321
$Scope = "https://graph.microsoft.com/.default"
#Create base64 hash of certificate
##################################
$CertificateBase64Hash = [System.Convert]::ToBase64String($Certificate.GetCertHash())
#Create JWT timestamp for expiration
####################################
$StartDate = (Get-Date "1970-01-01T00:00:00Z" ).ToUniversalTime()
$JWTExpirationTimeSpan = (New-TimeSpan -Start $StartDate -End (Get-Date).ToUniversalTime().AddMinutes(2)).TotalSeconds
$JWTExpiration = [math]::Round($JWTExpirationTimeSpan,0)
#Create JWT validity start timestamp
####################################
$NotBeforeExpirationTimeSpan = (New-TimeSpan -Start $StartDate -End ((Get-Date).ToUniversalTime())).TotalSeconds
$NotBefore = [math]::Round($NotBeforeExpirationTimeSpan,0)
#Create JWT header
#################
$JWTHeader = #{
alg = "RS256"
typ = "JWT"
x5t = $CertificateBase64Hash -replace '\+','-' -replace '/','_' -replace '='
}
#Create JWT payload
#################
$JWTPayLoad = #{
aud = "https://login.microsoftonline.com/$TenantName/oauth2/token"
exp = $JWTExpiration
iss = $AppId
jti = [guid]::NewGuid()
nbf = $NotBefore
sub = $AppId
}
# Convert header and payload to base64
#################################
$JWTHeaderToByte = [System.Text.Encoding]::UTF8.GetBytes(($JWTHeader | ConvertTo-Json))
$EncodedHeader = [System.Convert]::ToBase64String($JWTHeaderToByte)
$JWTPayLoadToByte = [System.Text.Encoding]::UTF8.GetBytes(($JWTPayload | ConvertTo-Json))
$EncodedPayload = [System.Convert]::ToBase64String($JWTPayLoadToByte)
# Join header and Payload with "." to create a valid (unsigned) JWT
######################################################
$JWT = $EncodedHeader + "." + $EncodedPayload
# Get the private key object of your certificate
######################################
$PrivateKey = $Certificate.PrivateKey
# Define RSA signature and hashing algorithm
#####################################
$RSAPadding = [Security.Cryptography.RSASignaturePadding]::Pkcs1
$HashAlgorithm = [Security.Cryptography.HashAlgorithmName]::SHA256
# Create a signature of the JWT
#########################
$Signature = [Convert]::ToBase64String(
$PrivateKey.SignData([System.Text.Encoding]::UTF8.GetBytes($JWT),$HashAlgorithm,$RSAPadding)
) -replace '\+','-' -replace '/','_' -replace '='
# Join the signature to the JWT with "."
###############################
$JWT = $JWT + "." + $Signature
# Use the self-generated JWT as Authorization to get the Access Token
##########################################################
$Header = #{
Authorization = "Bearer $JWT"
}
$Body = #{
client_id = $AppId
client_assertion = $JWT
client_assertion_type = "urn:ietf:params:oauth:client-assertion-type:jwt-bearer"
scope = $Scope
grant_type = "client_credentials"
}
#$authUri = "https://login.microsoftonline.com/common/oauth2/token"
$authUri = "https://login.microsoftonline.com/common/oauth2/nativeclient"
$TokenResponse = Invoke-RestMethod -Header $Header -Uri $authUri -Method POST -Body $Body
$TokenResponse
When I run it as Admin, I simply get an empty $TokenResponse
Any help will be highly appreciated.
Thanks!

Answer 1

Sorted by replacing the AUD and payload token with this URL:
https://login.microsoftonline.com/$TenantName/oauth2/v2.0/token