Private/Checksum/Get-SecretSharingChecksumCustomizationString.ps1

function Get-SecretSharingChecksumCustomizationString {
    <#
    .SYNOPSIS
        Returns the ASCII customization string mixed into every RS1024 checksum.
    .DESCRIPTION
        SLIP-0039 uses "shamir" for non-extendable backups and "shamir_extendable" for
        extendable ones, binding a checksum (and therefore a mnemonic) to the backup
        type it was created for - mixing shares from one type into a combine attempt
        for the other fails checksum verification.
    #>

    [CmdletBinding()]
    [OutputType([byte[]], [System.Object[]])]
    param(
        [switch]$Extendable
    )

    if ($Extendable) {
        return , [System.Text.Encoding]::ASCII.GetBytes('shamir_extendable')
    }

    return , [System.Text.Encoding]::ASCII.GetBytes('shamir')
}