Private/Share/ConvertFrom-SecretSharingWordIndex.ps1

function ConvertFrom-SecretSharingWordIndex {
    <#
    .SYNOPSIS
        Combines a most-significant-digit-first array of base-2^RadixBit digits back
        into a single non-negative big integer.
    #>

    [CmdletBinding()]
    [OutputType([System.Numerics.BigInteger])]
    param(
        [Parameter(Mandatory)]
        [int[]]$Index,

        [Parameter(Mandatory)]
        [ValidateRange(1, 31)]
        [int]$RadixBit
    )

    $radix = [System.Numerics.BigInteger]::Pow(2, $RadixBit)
    $value = [System.Numerics.BigInteger]::Zero

    foreach ($d in $Index) {
        $value = ($value * $radix) + $d
    }

    return $value
}