Multiply two huge numbers together without Ruby's help

multiply.rb

first_string = gets.chomp
second_string = gets.chomp
to_find = gets.chomp

def string_times_int(string, int)

  product = ""
  carry = 0

  loop = string.length - 1

  while loop >= 0 or carry != 0

    if loop >= 0
      from_string = string[loop]
    else
      from_string = 0
    end

    this_number = from_string.to_i * int + carry

    carry = this_number / 10

    product += (this_number % 10).to_s;

    loop = loop - 1

  end

  product.reverse

end

def string_add_string(a, b)

  if a.length > b.length
    first = a
    second = b
  else
    first = b
    second = a
  end

  sum = ""
  carry = 0

  loop = first.length - 1

  while loop >= 0 or carry != 0

    if loop >= 0
      first_from_string = first[loop]
    else
      first_from_string = 0
    end

    second_index = loop - (first.length - second.length)

    if second_index >= 0
        second_from_string = second[second_index]
    else
      second_from_string = 0
    end

    this_number = first_from_string.to_i + second_from_string.to_i + carry

    carry = this_number / 10

    sum += (this_number % 10).to_s

    loop -= 1

  end

  sum.reverse

end

total_sum = "0"
loop = first_string.length - 1

while loop >= 0

  reverse_index = first_string.length - 1 - loop

  the_number = first_string[loop].to_i

  addition = string_times_int(second_string, the_number)

  reverse_index.times do
    addition += "0"
  end

  total_sum = string_add_string(total_sum, addition)

  loop -= 1

end

found_count = 0

total_sum.each_char do |char|
  found_count += 1 if char == to_find
end

puts "Product was #{total_sum}"
puts "Found #{found_count} instances of #{to_find}"