A sample implementation for mapping 10 digit TOTP tokens to 5 digit tokens that are accepted by Steam.

SteamTOTPConverter.scala

object SteamTOTPConverter {

  private val base26Chars = "0123456789ABCDEFGHIJKLMNOP"
  private val steamChars = "23456789BCDFGHJKMNPQRTVWXY"
  private val charMap = (base26Chars zip steamChars) toMap
  private def toBase26(number: Int) = Integer.toString(number, 26).toUpperCase

  def convertFrom(totpToken: Int) = {
    toBase26(totpToken)
      .takeRight(5)
      .map(char => charMap.getOrElse(char, '?'))
      .padTo(5, '2')
      .reverse
  }
}