diff --git a/2020/ruby/day_25.rb b/2020/ruby/day_25.rb
new file mode 100644
index 0000000..9a18ffe
--- /dev/null
+++ b/2020/ruby/day_25.rb
@@ -0,0 +1,17 @@
+def transform(subject_number)
+  return enum_for(__method__, subject_number) unless block_given?
+
+  value = 1
+  (0..).each do |loop_size|
+    yield [loop_size, value]
+    value *= subject_number
+    value %= 20201227
+  end
+end
+
+public_keys = ARGF.read.scan(/\d+/).map(&:to_i)
+
+loop_size, _ = transform(7).find {|_,v| v == public_keys.first }
+_, encryption_key = transform(public_keys.last).find {|i,_| i == loop_size }
+
+p encryption_key