Functions/ConvertFrom-Base64UrlEncodedString.ps1
|
function ConvertFrom-Base64UrlEncodedString { <# .SYNOPSIS Decodes a base 64 URL encoded string. .DESCRIPTION Decodes a base 64 URL encoded string such as a JWT header or payload. .PARAMETER InputString The string to be base64 URL decoded. .PARAMETER AsBytes Instructions this function to return the result as a byte array as opposed to a default string. .EXAMPLE "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9" | ConvertFrom-Base64UrlEncodedString Decodes a JWT header. .INPUTS System.String A string is received by the InputString parameter. .OUTPUTS System.String Returns a base 64 URL decoded string for the given input. .LINK https://tools.ietf.org/html/rfc4648#section-5 #> [CmdletBinding()] [OutputType([System.String], [System.Byte[]])] [Alias('b64d', 'Decode')] param ( [Parameter(Position=0,Mandatory=$true,ValueFromPipeline=$true,ValueFromPipelineByPropertyName=$true)] [string]$InputString, [Parameter(Position=1,Mandatory=$false,ValueFromPipeline=$false,ValueFromPipelineByPropertyName=$false)] [switch]$AsBytes ) BEGIN { $argumentExceptionMessage = "The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters." $ArgumentException = New-Object -TypeName System.ArgumentException -ArgumentList $argumentExceptionMessage } PROCESS { try { $output = $InputString $output = $output.Replace('-', '+') # 62nd char of encoding $output = $output.Replace('_', '/') # 63rd char of encoding switch ($output.Length % 4) # Pad with trailing '='s { 0 { break }# No pad chars in this case 2 { $output += "=="; break } # Two pad chars 3 { $output += "="; break } # One pad char default { Write-Error -Exception ([ArgumentException]::new("Illegal base64url string!")) -Category InvalidArgument -ErrorAction Stop } } # Byte array conversion: [byte[]]$convertedBytes = [Convert]::FromBase64String($output) if ($PSBoundParameters.ContainsKey("AsBytes")) { return $convertedBytes } else { # String to be returned: $decodedString = [System.Text.Encoding]::ASCII.GetString($convertedBytes) return $decodedString } } catch { Write-Error -Exception $ArgumentException -Category InvalidArgument -ErrorAction Stop } } } |