function Get-CredentialFromSecureStore
{
    [cmdletbinding()]
    param
    (
        [Parameter(Mandatory=$true)][string]$TargetApplicationId
    )

    begin
    {
        [void][Reflection.Assembly]::LoadFile("C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.Office.SecureStoreService.Server.Security.dll")
    }
    process
    {
        $contextSite      = Get-SPSite -Limit 1 -WarningAction SilentlyContinue -Verbose:$false
        $serviceContext   = Get-SPServiceContext -Site $contextSite -Verbose:$false
        $provider         = New-Object Microsoft.Office.SecureStoreService.Server.SecureStoreProvider
        $provider.Context = $serviceContext

        $credentials = $provider.GetRestrictedCredentials($TargetApplicationId)

        $credentials | % {

            $credential = $_.Credential

            switch ($_.CredentialType)
            {
                "WindowsUserName"
                {
                    $marshal = [System.Runtime.InteropServices.Marshal]
                    $ptr = $marshal::SecureStringToBSTR($credential)
                    $plainTextUserName = [string]::Copy($marshal::PtrToStringBSTR($ptr))
                    $marshal::ZeroFreeBSTR($ptr)
                }

                "WindowsPassword"
                {
                    $marshal = [System.Runtime.InteropServices.Marshal]
                    $ptr = $marshal::SecureStringToBSTR($credential)
                    $unSecurePassword = [string]::Copy($marshal::PtrToStringBSTR($ptr))
                    $marshal::ZeroFreeBSTR($ptr)
                }

                "UserName"
                {
                    $marshal = [System.Runtime.InteropServices.Marshal]
                    $ptr = $marshal::SecureStringToBSTR($credential)
                    $unSecurePassword = [string]::Copy($marshal::PtrToStringBSTR($ptr))
                    $marshal::ZeroFreeBSTR($ptr)
                }

                "Password"
                {
                    $marshal = [System.Runtime.InteropServices.Marshal]
                    $ptr = $marshal::SecureStringToBSTR($credential)
                    $unSecurePassword = [string]::Copy($marshal::PtrToStringBSTR($ptr))
                    $marshal::ZeroFreeBSTR($ptr)
                }
            }
        }

        if($credentials)
        {
            $credentials.Dispose()
        }

        if( $plainTextUserName -and $unSecurePassword )
        {
            Write-Verbose "Username: $($plainTextUserName)"
            Write-Verbose "Password: $($unSecurePassword)"

            $secureString = New-Object System.Security.SecureString
            $unSecurePassword.ToCharArray() | % { $secureString.AppendChar($_) }

            $psCredential = New-Object PSCredential($plainTextUserName, $secureString)
            return $psCredential
        }


        return $null
    }
    end
    {
    }
}

Get-CredentialFromSecureStore -TargetApplicationId "Test" -Verbose