GetHmacSha256.ps1

param(
    [Parameter(Mandatory=$true, Position=0, ValueFromPipeline=$false)]
    [string]
    $secret,

    [Parameter(Mandatory=$true, Position=1, ValueFromPipeline=$false)]
    [string]
    $message
)

# When this logic is called from Function and returned, the hash differs...
[Byte[]] $key = [System.Text.Encoding]::UTF8.GetBytes($secret)
[Byte[]] $messageBytes = [Text.Encoding]::ASCII.GetBytes($message)

#$hmacSha = [System.Security.Cryptography.HMACSHA256]::Create()
$hmacSha = New-Object System.Security.Cryptography.HMACSHA256
$hmacSha.Key = $key
$signature = $hmacSha.ComputeHash($messageBytes)

# Hexidecimal string
[string] $hash = [System.BitConverter]::ToString($signature)
$hash = $hash.Replace('-', '').ToLowerInvariant()

$hash