diff --git a/library/random/formatter/alphanumeric_spec.rb b/library/random/formatter/alphanumeric_spec.rb
index 8e3141302..bb50c6bb0 100644
--- a/library/random/formatter/alphanumeric_spec.rb
+++ b/library/random/formatter/alphanumeric_spec.rb
@@ -1,4 +1,5 @@
 require_relative '../../../spec_helper'
+require_relative 'shared/interface'
 
 ruby_version_is "3.1" do
   require 'random/formatter'
@@ -58,4 +59,6 @@ def @object.bytes(n)
       @object.alphanumeric(1, chars: [to_s, to_s]).should == "[mock to_s]"
     end
   end
+
+  it_behaves_like :random_formatter_interface, :alphanumeric
 end
diff --git a/library/random/formatter/shared/interface.rb b/library/random/formatter/shared/interface.rb
new file mode 100644
index 000000000..6b4738f6a
--- /dev/null
+++ b/library/random/formatter/shared/interface.rb
@@ -0,0 +1,33 @@
+require_relative '../../../../spec_helper'
+
+describe :random_formatter_interface, shared: true do
+  it "calls the method #bytes for 4 blocks of 4 bytes" do
+    bytes = mock('bytes')
+    bytes.extend(Random::Formatter)
+    bytes.should_receive(:bytes).with(4).exactly(4).and_return("\x00".b * 4)
+    bytes.send(@method)
+  end
+
+  it "has the same output if the random byte streams are the same" do
+    bytes = Object.new
+    bytes.extend(Random::Formatter)
+    def bytes.bytes(n)
+      "\x00".b * n
+    end
+    bytes.send(@method).should == bytes.send(@method)
+  end
+
+  it "has different output if the random byte streams are not the same" do
+    lhs = Object.new
+    lhs.extend(Random::Formatter)
+    def lhs.bytes(n)
+      "\x00".b * n
+    end
+    rhs = Object.new
+    rhs.extend(Random::Formatter)
+    def rhs.bytes(n)
+      "\x01".b * n
+    end
+    lhs.send(@method).should != rhs.send(@method)
+  end
+end