diff --git a/lib/diaspora_federation.rb b/lib/diaspora_federation.rb
index 8ccd903..ebbb1ef 100644
--- a/lib/diaspora_federation.rb
+++ b/lib/diaspora_federation.rb
@@ -3,6 +3,7 @@ require "diaspora_federation/logging"
 require "diaspora_federation/callbacks"
 require "diaspora_federation/properties_dsl"
 require "diaspora_federation/entity"
+require "diaspora_federation/validators"
 
 require "diaspora_federation/fetcher"
 
diff --git a/lib/diaspora_federation/validators.rb b/lib/diaspora_federation/validators.rb
new file mode 100644
index 0000000..a5f0e86
--- /dev/null
+++ b/lib/diaspora_federation/validators.rb
@@ -0,0 +1,32 @@
+require "validation"
+require "validation/rule/not_empty"
+require "validation/rule/email"
+require "validation/rule/regular_expression"
+require "validation/rule/uri"
+
+# +valid+ gem namespace
+module Validation
+  # This module contains custom validation rules for various data field types.
+  # That includes types for which there are no provided rules by the +valid+ gem
+  # or types that are very specific to Diaspora* federation and need special handling.
+  # The rules are used inside the {DiasporaFederation::Validators validator classes}
+  # to perform basic sanity-checks on {DiasporaFederation::Entities federation entities}.
+  module Rule
+  end
+end
+
+require "diaspora_federation/validators/rules/birthday"
+require "diaspora_federation/validators/rules/boolean"
+require "diaspora_federation/validators/rules/guid"
+require "diaspora_federation/validators/rules/not_nil"
+require "diaspora_federation/validators/rules/public_key"
+require "diaspora_federation/validators/rules/tag_count"
+
+module DiasporaFederation
+  # Validators to perform basic sanity-checks on {DiasporaFederation::Entities federation entities}.
+  module Validators
+  end
+end
+
+require "diaspora_federation/validators/person_validator"
+require "diaspora_federation/validators/profile_validator"
diff --git a/lib/diaspora_federation/validators/person_validator.rb b/lib/diaspora_federation/validators/person_validator.rb
new file mode 100644
index 0000000..1e58117
--- /dev/null
+++ b/lib/diaspora_federation/validators/person_validator.rb
@@ -0,0 +1,17 @@
+module DiasporaFederation
+  module Validators
+    class PersonValidator < Validation::Validator
+      include Validation
+
+      rule :guid, :guid
+
+      rule :diaspora_handle, %i(not_empty email)
+
+      rule :url, :u_r_i # WTF? :uri -> Uri -> "uninitialized constant Uri", :u_r_i -> URI -> \o/
+
+      rule :profile, :not_nil
+
+      rule :exported_key, :public_key
+    end
+  end
+end
diff --git a/lib/diaspora_federation/validators/profile_validator.rb b/lib/diaspora_federation/validators/profile_validator.rb
new file mode 100644
index 0000000..1e6cfa8
--- /dev/null
+++ b/lib/diaspora_federation/validators/profile_validator.rb
@@ -0,0 +1,22 @@
+module DiasporaFederation
+  module Validators
+    class ProfileValidator < Validation::Validator
+      include Validation
+
+      rule :diaspora_handle, %i(not_empty email)
+
+      # the name must not contain a semicolon because of mentions
+      # @{<name> ; <handle>}
+      rule :first_name, regular_expression: {regex: /\A[^;]{,32}\z/}
+      rule :last_name, regular_expression: {regex: /\A[^;]{,32}\z/}
+
+      rule :tag_string, tag_count: {maximum: 5}
+
+      rule :birthday, :birthday
+
+      rule :searchable, :boolean
+
+      rule :nsfw, :boolean
+    end
+  end
+end
diff --git a/lib/diaspora_federation/validators/rules/birthday.rb b/lib/diaspora_federation/validators/rules/birthday.rb
new file mode 100644
index 0000000..3562922
--- /dev/null
+++ b/lib/diaspora_federation/validators/rules/birthday.rb
@@ -0,0 +1,30 @@
+require "date"
+
+module Validation
+  module Rule
+    class Birthday
+      attr_reader :params
+
+      # no parameters
+      def initialize
+        @params = {}
+      end
+
+      def error_key
+        :birthday
+      end
+
+      def valid_value?(value)
+        return true if value.nil? || (value.is_a?(String) && value.empty?)
+        return true if value.is_a? Date
+
+        if value =~ /[0-9]{4}\-[0-9]{2}\-[0-9]{2}/
+          date_field = value.split("-").map(&:to_i)
+          return Date.valid_civil?(date_field[0], date_field[1], date_field[2])
+        end
+
+        false
+      end
+    end
+  end
+end
diff --git a/lib/diaspora_federation/validators/rules/boolean.rb b/lib/diaspora_federation/validators/rules/boolean.rb
new file mode 100644
index 0000000..f4498df
--- /dev/null
+++ b/lib/diaspora_federation/validators/rules/boolean.rb
@@ -0,0 +1,30 @@
+module Validation
+  module Rule
+    class Boolean
+      attr_reader :params
+
+      # no parameters
+      def initialize
+        @params = {}
+      end
+
+      def error_key
+        :numeric
+      end
+
+      def valid_value?(value)
+        return false if value.nil?
+
+        if value.is_a?(String)
+          true if value =~ /\A(true|false|t|f|yes|no|y|n|1|0)\z/i
+        elsif value.is_a?(Fixnum)
+          true if value == 1 || value == 0
+        elsif value.is_a?(TrueClass) || value.is_a?(FalseClass)
+          true
+        else
+          false
+        end
+      end
+    end
+  end
+end
diff --git a/lib/diaspora_federation/validators/rules/guid.rb b/lib/diaspora_federation/validators/rules/guid.rb
new file mode 100644
index 0000000..7aca5d8
--- /dev/null
+++ b/lib/diaspora_federation/validators/rules/guid.rb
@@ -0,0 +1,20 @@
+module Validation
+  module Rule
+    class Guid
+      attr_reader :params
+
+      # no parameters
+      def initialize
+        @params = {}
+      end
+
+      def error_key
+        :guid
+      end
+
+      def valid_value?(value)
+        value.is_a?(String) && value.downcase =~ /\A[0-9a-z\-_@.:]{16,}\z/
+      end
+    end
+  end
+end
diff --git a/lib/diaspora_federation/validators/rules/not_nil.rb b/lib/diaspora_federation/validators/rules/not_nil.rb
new file mode 100644
index 0000000..834384f
--- /dev/null
+++ b/lib/diaspora_federation/validators/rules/not_nil.rb
@@ -0,0 +1,20 @@
+module Validation
+  module Rule
+    class NotNil
+      attr_reader :params
+
+      # no parameters
+      def initialize
+        @params = {}
+      end
+
+      def error_key
+        :not_nil
+      end
+
+      def valid_value?(value)
+        !value.nil?
+      end
+    end
+  end
+end
diff --git a/lib/diaspora_federation/validators/rules/public_key.rb b/lib/diaspora_federation/validators/rules/public_key.rb
new file mode 100644
index 0000000..cc35e07
--- /dev/null
+++ b/lib/diaspora_federation/validators/rules/public_key.rb
@@ -0,0 +1,24 @@
+module Validation
+  module Rule
+    class PublicKey
+      attr_reader :params
+
+      # no parameters
+      def initialize
+        @params = {}
+      end
+
+      def error_key
+        :public_key
+      end
+
+      # allow both "PUBLIC KEY" and "RSA PUBLIC KEY"
+      def valid_value?(value)
+        (value.strip.start_with?("-----BEGIN PUBLIC KEY-----") &&
+         value.strip.end_with?("-----END PUBLIC KEY-----")) ||
+        (value.strip.start_with?("-----BEGIN RSA PUBLIC KEY-----") &&
+          value.strip.end_with?("-----END RSA PUBLIC KEY-----"))
+      end
+    end
+  end
+end
diff --git a/lib/diaspora_federation/validators/rules/tag_count.rb b/lib/diaspora_federation/validators/rules/tag_count.rb
new file mode 100644
index 0000000..8f48082
--- /dev/null
+++ b/lib/diaspora_federation/validators/rules/tag_count.rb
@@ -0,0 +1,27 @@
+module Validation
+  module Rule
+    # Rule for validating the number of tags in a string.
+    # Only the "#" characters will be counted.
+    class TagCount
+      attr_reader :params
+
+      # @param [Hash] params
+      # @option params [Fixnum] :maximum maximum allowed tag count
+      def initialize(params)
+        unless params.include?(:maximum) && params[:maximum].is_a?(Fixnum)
+          raise "A number has to be specified for :maximum"
+        end
+
+        @params = params
+      end
+
+      def error_key
+        :tag_count
+      end
+
+      def valid_value?(value)
+        value.count("#") <= params[:maximum]
+      end
+    end
+  end
+end
diff --git a/spec/factories.rb b/spec/factories.rb
index 88173c4..79f2b69 100644
--- a/spec/factories.rb
+++ b/spec/factories.rb
@@ -5,12 +5,36 @@ def r_str
 end
 
 FactoryGirl.define do
+  sequence(:diaspora_handle) {|n| "person-#{n}-#{r_str}@localhost:3000" }
+  sequence(:public_key) { OpenSSL::PKey::RSA.generate(1024).public_key.export }
+
   factory :person do
-    sequence(:diaspora_handle) {|n| "person-#{n}-#{r_str}@localhost:3000" }
+    diaspora_handle
     url "http://localhost:3000/"
-    serialized_public_key OpenSSL::PKey::RSA.generate(1024).public_key.export
+    serialized_public_key { generate(:public_key) }
     after(:create) do |u|
       u.save
     end
   end
+
+  factory :person_entity, class: DiasporaFederation::Entities::Person do
+    guid UUID.generate :compact
+    diaspora_handle "testing@example.com"
+    url "http://localhost:3000/"
+    exported_key { generate(:public_key) }
+    profile {
+      DiasporaFederation::Entities::Profile.new(
+        FactoryGirl.attributes_for(:profile_entity, diaspora_handle: diaspora_handle))
+    }
+  end
+
+  factory :profile_entity, class: DiasporaFederation::Entities::Profile do
+    diaspora_handle "testing@example.com"
+    first_name "my_name"
+    last_name nil
+    tag_string "#i #love #tags"
+    birthday "1988-07-15"
+    searchable true
+    nsfw false
+  end
 end
diff --git a/spec/lib/diaspora_federation/validators/person_validator_spec.rb b/spec/lib/diaspora_federation/validators/person_validator_spec.rb
new file mode 100644
index 0000000..f954d36
--- /dev/null
+++ b/spec/lib/diaspora_federation/validators/person_validator_spec.rb
@@ -0,0 +1,41 @@
+module DiasporaFederation
+  describe Validators::PersonValidator do
+    it "validates a well-formed instance" do
+      instance = OpenStruct.new(FactoryGirl.attributes_for(:person_entity))
+      validator = Validators::PersonValidator.new(instance)
+
+      expect(validator).to be_valid
+      expect(validator.errors).to be_empty
+    end
+
+    it_behaves_like "a diaspora_handle validator" do
+      let(:entity) { :person_entity }
+      let(:validator_class) { Validators::PersonValidator }
+      let(:property) { :diaspora_handle }
+    end
+
+    it_behaves_like "a guid validator" do
+      let(:entity) { :person_entity }
+      let(:validator_class) { Validators::PersonValidator }
+      let(:property) { :guid }
+    end
+
+    context "#exported_key" do
+      it "fails for malformed rsa key" do
+        instance = OpenStruct.new(FactoryGirl.attributes_for(:person_entity, exported_key: "ASDF"))
+        validator = Validators::PersonValidator.new(instance)
+
+        expect(validator).not_to be_valid
+        expect(validator.errors).to include(:exported_key)
+      end
+
+      it "must not be empty" do
+        instance = OpenStruct.new(FactoryGirl.attributes_for(:person_entity, exported_key: ""))
+        validator = Validators::PersonValidator.new(instance)
+
+        expect(validator).not_to be_valid
+        expect(validator.errors).to include(:exported_key)
+      end
+    end
+  end
+end
diff --git a/spec/lib/diaspora_federation/validators/profile_validator_spec.rb b/spec/lib/diaspora_federation/validators/profile_validator_spec.rb
new file mode 100644
index 0000000..ef11474
--- /dev/null
+++ b/spec/lib/diaspora_federation/validators/profile_validator_spec.rb
@@ -0,0 +1,94 @@
+module DiasporaFederation
+  describe Validators::ProfileValidator do
+    def profile_stub(data={})
+      OpenStruct.new(FactoryGirl.attributes_for(:profile_entity).merge(data))
+    end
+
+    it "validates a well-formed instance" do
+      validator = Validators::ProfileValidator.new(profile_stub)
+
+      expect(validator).to be_valid
+      expect(validator.errors).to be_empty
+    end
+
+    it_behaves_like "a diaspora_handle validator" do
+      let(:entity) { :profile_entity }
+      let(:validator_class) { Validators::ProfileValidator }
+      let(:property) { :diaspora_handle }
+    end
+
+    %i(first_name last_name).each do |prop|
+      describe "##{prop}" do
+        it "allowed to contain special chars" do
+          validator = Validators::ProfileValidator.new(profile_stub(prop => "cool name ©"))
+
+          expect(validator).to be_valid
+          expect(validator.errors).to be_empty
+        end
+
+        it "must not exceed 32 chars" do
+          validator = Validators::ProfileValidator.new(profile_stub(prop => "abcdefghijklmnopqrstuvwxyz_aaaaaaaaaa"))
+
+          expect(validator).not_to be_valid
+          expect(validator.errors).to include(prop)
+        end
+
+        it "must not contain semicolons" do
+          validator = Validators::ProfileValidator.new(profile_stub(prop => "asdf;qwer;yxcv"))
+
+          expect(validator).not_to be_valid
+          expect(validator.errors).to include(prop)
+        end
+      end
+    end
+
+    describe "#tag_string" do
+      it "must not contain more than 5 tags" do
+        validator = Validators::ProfileValidator.new(
+          profile_stub(tag_string: "#i #have #too #many #tags #in #my #profile"))
+
+        expect(validator).not_to be_valid
+        expect(validator.errors).to include(:tag_string)
+      end
+    end
+
+    describe "#birthday" do
+      it "may be empty or nil" do
+        [nil, ""].each do |val|
+          validator = Validators::ProfileValidator.new(profile_stub(birthday: val))
+
+          expect(validator).to be_valid
+          expect(validator.errors).to be_empty
+        end
+      end
+
+      it "may be a Date or date string" do
+        [Date.parse("2013-06-29"), "2013-06-29"].each do |val|
+          validator = Validators::ProfileValidator.new(profile_stub(birthday: val))
+
+          expect(validator).to be_valid
+          expect(validator.errors).to be_empty
+        end
+      end
+
+      it "must not be an arbitrary string or other object" do
+        ["asdf asdf", true, 1234].each do |val|
+          validator = Validators::ProfileValidator.new(profile_stub(birthday: val))
+
+          expect(validator).not_to be_valid
+          expect(validator.errors).to include(:birthday)
+        end
+      end
+    end
+
+    %i(searchable nsfw).each do |prop|
+      describe "##{prop}" do
+        it_behaves_like "a boolean validator" do
+          let(:entity) { :profile_entity }
+          let(:validator_class) { Validators::ProfileValidator }
+          let(:property) { prop }
+        end
+      end
+    end
+  end
+end
diff --git a/spec/lib/diaspora_federation/validators/rules/birthday_spec.rb b/spec/lib/diaspora_federation/validators/rules/birthday_spec.rb
new file mode 100644
index 0000000..e4811cc
--- /dev/null
+++ b/spec/lib/diaspora_federation/validators/rules/birthday_spec.rb
@@ -0,0 +1,50 @@
+describe Validation::Rule::Birthday do
+  it "will not accept parameters" do
+    validator = Validation::Validator.new({})
+    expect {
+      validator.rule(:birthday, birthday: {param: true})
+    }.to raise_error ArgumentError
+  end
+
+  context "validation" do
+    it "validates a date object" do
+      validator = Validation::Validator.new(OpenStruct.new(birthday: Date.new))
+      validator.rule(:birthday, :birthday)
+
+      expect(validator).to be_valid
+      expect(validator.errors).to be_empty
+    end
+
+    it "validates a string" do
+      validator = Validation::Validator.new(OpenStruct.new(birthday: "2015-07-19"))
+      validator.rule(:birthday, :birthday)
+
+      expect(validator).to be_valid
+      expect(validator.errors).to be_empty
+    end
+
+    it "validates an empty string" do
+      validator = Validation::Validator.new(OpenStruct.new(birthday: ""))
+      validator.rule(:birthday, :birthday)
+
+      expect(validator).to be_valid
+      expect(validator.errors).to be_empty
+    end
+
+    it "validates nil" do
+      validator = Validation::Validator.new(OpenStruct.new(birthday: nil))
+      validator.rule(:birthday, :birthday)
+
+      expect(validator).to be_valid
+      expect(validator.errors).to be_empty
+    end
+
+    it "fails for invalid date string" do
+      validator = Validation::Validator.new(OpenStruct.new(birthday: "i'm no date"))
+      validator.rule(:birthday, :birthday)
+
+      expect(validator).not_to be_valid
+      expect(validator.errors).to include(:birthday)
+    end
+  end
+end
diff --git a/spec/lib/diaspora_federation/validators/rules/boolean_spec.rb b/spec/lib/diaspora_federation/validators/rules/boolean_spec.rb
new file mode 100644
index 0000000..dc32f9a
--- /dev/null
+++ b/spec/lib/diaspora_federation/validators/rules/boolean_spec.rb
@@ -0,0 +1,70 @@
+describe Validation::Rule::Boolean do
+  it "will not accept parameters" do
+    validator = Validation::Validator.new({})
+    expect {
+      validator.rule(:number, numeric: {param: true})
+    }.to raise_error ArgumentError
+  end
+
+  context "validation" do
+    context "strings" do
+      it "validates boolean-esque strings" do
+        %w(true false yes no t f y n 1 0).each do |str|
+          validator = Validation::Validator.new(OpenStruct.new(boolean: str))
+          validator.rule(:boolean, :boolean)
+
+          expect(validator).to be_valid
+          expect(validator.errors).to be_empty
+        end
+      end
+
+      it "fails for non-boolean-esque strings" do
+        validator = Validation::Validator.new(OpenStruct.new(boolean: "asdf"))
+        validator.rule(:boolean, :boolean)
+
+        expect(validator).not_to be_valid
+        expect(validator.errors).to include(:boolean)
+      end
+    end
+
+    context "numbers" do
+      it "validates 0 and 1 to boolean" do
+        [0, 1].each do |num|
+          validator = Validation::Validator.new(OpenStruct.new(boolean: num))
+          validator.rule(:boolean, :boolean)
+
+          expect(validator).to be_valid
+          expect(validator.errors).to be_empty
+        end
+      end
+
+      it "fails for all other numbers" do
+        validator = Validation::Validator.new(OpenStruct.new(boolean: 1234))
+        validator.rule(:boolean, :boolean)
+
+        expect(validator).not_to be_valid
+        expect(validator.errors).to include(:boolean)
+      end
+    end
+
+    context "boolean types" do
+      it "validates true and false" do
+        [true, false].each do |bln|
+          validator = Validation::Validator.new(OpenStruct.new(boolean: bln))
+          validator.rule(:boolean, :boolean)
+
+          expect(validator).to be_valid
+          expect(validator.errors).to be_empty
+        end
+      end
+    end
+
+    it "fails for nil" do
+      validator = Validation::Validator.new(OpenStruct.new(boolean: nil))
+      validator.rule(:boolean, :boolean)
+
+      expect(validator).not_to be_valid
+      expect(validator.errors).to include(:boolean)
+    end
+  end
+end
diff --git a/spec/lib/diaspora_federation/validators/rules/guid_spec.rb b/spec/lib/diaspora_federation/validators/rules/guid_spec.rb
new file mode 100644
index 0000000..5cd0b82
--- /dev/null
+++ b/spec/lib/diaspora_federation/validators/rules/guid_spec.rb
@@ -0,0 +1,59 @@
+describe Validation::Rule::Guid do
+  it "will not accept parameters" do
+    validator = Validation::Validator.new({})
+    expect {
+      validator.rule(:guid, guid: {param: true})
+    }.to raise_error ArgumentError
+  end
+
+  context "validation" do
+    it "validates a string at least 16 chars long, consisting of [0-9a-f] (diaspora)" do
+      validator = Validation::Validator.new(OpenStruct.new(guid: "abcdef0123456789"))
+      validator.rule(:guid, :guid)
+
+      expect(validator).to be_valid
+      expect(validator.errors).to be_empty
+    end
+
+    it "validates a long string with random characters and [-_@.:] (redmatrix)" do
+      validator = Validation::Validator.new(
+        OpenStruct.new(guid: "1234567890ABCDefgh_ijkl-mnopqrSTUVwxyz@example.com:3000"))
+      validator.rule(:guid, :guid)
+
+      expect(validator).to be_valid
+      expect(validator.errors).to be_empty
+    end
+
+    it "fails if the string is too short" do
+      validator = Validation::Validator.new(OpenStruct.new(guid: "012345"))
+      validator.rule(:guid, :guid)
+
+      expect(validator).not_to be_valid
+      expect(validator.errors).to include(:guid)
+    end
+
+    it "fails if the string contains invalid chars" do
+      validator = Validation::Validator.new(OpenStruct.new(guid: "ghijklmnopqrstuvwxyz++"))
+      validator.rule(:guid, :guid)
+
+      expect(validator).not_to be_valid
+      expect(validator.errors).to include(:guid)
+    end
+
+    it "fails if the string is empty" do
+      validator = Validation::Validator.new(OpenStruct.new(guid: ""))
+      validator.rule(:guid, :guid)
+
+      expect(validator).not_to be_valid
+      expect(validator.errors).to include(:guid)
+    end
+
+    it "fails if the string is nil" do
+      validator = Validation::Validator.new(OpenStruct.new(guid: nil))
+      validator.rule(:guid, :guid)
+
+      expect(validator).not_to be_valid
+      expect(validator.errors).to include(:guid)
+    end
+  end
+end
diff --git a/spec/lib/diaspora_federation/validators/rules/public_key_spec.rb b/spec/lib/diaspora_federation/validators/rules/public_key_spec.rb
new file mode 100644
index 0000000..5d1ada0
--- /dev/null
+++ b/spec/lib/diaspora_federation/validators/rules/public_key_spec.rb
@@ -0,0 +1,51 @@
+describe Validation::Rule::Guid do
+  it "will not accept parameters" do
+    validator = Validation::Validator.new({})
+    expect {
+      validator.rule(:key, public_key: {param: true})
+    }.to raise_error ArgumentError
+  end
+
+  context "validation" do
+    ["PUBLIC KEY", "RSA PUBLIC KEY"].each do |key_type|
+      context key_type do
+        let(:prefix) { "-----BEGIN #{key_type}-----" }
+        let(:suffix) { "-----END #{key_type}-----" }
+
+        let(:key) { "#{prefix}\nAAAAAA==\n#{suffix}\n" }
+
+        it "validates an exported RSA key" do
+          validator = Validation::Validator.new(OpenStruct.new(key: key))
+          validator.rule(:key, :public_key)
+
+          expect(validator).to be_valid
+          expect(validator.errors).to be_empty
+        end
+
+        it "strips whitespace" do
+          validator = Validation::Validator.new(OpenStruct.new(key: "  \n   #{key}\n \n  "))
+          validator.rule(:key, :public_key)
+
+          expect(validator).to be_valid
+          expect(validator.errors).to be_empty
+        end
+
+        it "fails if the prefix is missing" do
+          validator = Validation::Validator.new(OpenStruct.new(key: "\nAAAAAA==\n#{suffix}\n"))
+          validator.rule(:key, :public_key)
+
+          expect(validator).not_to be_valid
+          expect(validator.errors).to include(:key)
+        end
+
+        it "fails if the suffix is missing" do
+          validator = Validation::Validator.new(OpenStruct.new(key: "#{prefix}\nAAAAAA==\n\n"))
+          validator.rule(:key, :public_key)
+
+          expect(validator).not_to be_valid
+          expect(validator.errors).to include(:key)
+        end
+      end
+    end
+  end
+end
diff --git a/spec/lib/diaspora_federation/validators/rules/tag_count_spec.rb b/spec/lib/diaspora_federation/validators/rules/tag_count_spec.rb
new file mode 100644
index 0000000..32c1aa5
--- /dev/null
+++ b/spec/lib/diaspora_federation/validators/rules/tag_count_spec.rb
@@ -0,0 +1,36 @@
+describe Validation::Rule::TagCount do
+  it "requires a parameter" do
+    validator = Validation::Validator.new({})
+    expect {
+      validator.rule(:tags, :tag_count)
+    }.to raise_error ArgumentError
+  end
+
+  context "validation" do
+    let(:tag_str) { "#i #love #tags" }
+
+    it "validates less tags" do
+      validator = Validation::Validator.new(OpenStruct.new(tags: tag_str))
+      validator.rule(:tags, tag_count: {maximum: 5})
+
+      expect(validator).to be_valid
+      expect(validator.errors).to be_empty
+    end
+
+    it "validates exactly as many tags" do
+      validator = Validation::Validator.new(OpenStruct.new(tags: tag_str))
+      validator.rule(:tags, tag_count: {maximum: 3})
+
+      expect(validator).to be_valid
+      expect(validator.errors).to be_empty
+    end
+
+    it "fails for too many tags" do
+      validator = Validation::Validator.new(OpenStruct.new(tags: tag_str))
+      validator.rule(:tags, tag_count: {maximum: 1})
+
+      expect(validator).not_to be_valid
+      expect(validator.errors).to include(:tags)
+    end
+  end
+end
diff --git a/spec/support/shared_validator_specs.rb b/spec/support/shared_validator_specs.rb
new file mode 100644
index 0000000..7601335
--- /dev/null
+++ b/spec/support/shared_validator_specs.rb
@@ -0,0 +1,85 @@
+def entity_stub(entity, property, val=nil)
+  instance = OpenStruct.new(FactoryGirl.attributes_for(entity))
+  instance.public_send("#{property}=", val) unless val.nil?
+  instance
+end
+
+shared_examples "a diaspora_handle validator" do
+  it "validates a well-formed diaspora_handle" do
+    validator = validator_class.new(entity_stub(entity, property))
+
+    expect(validator).to be_valid
+    expect(validator.errors).to be_empty
+  end
+
+  it "must not be empty" do
+    validator = validator_class.new(entity_stub(entity, property, ""))
+
+    expect(validator).not_to be_valid
+    expect(validator.errors).to include(property)
+  end
+
+  it "must resemble an email address" do
+    validator = validator_class.new(entity_stub(entity, property, "i am a weird handle @@@ ### 12345"))
+
+    expect(validator).not_to be_valid
+    expect(validator.errors).to include(property)
+  end
+end
+
+shared_examples "a guid validator" do
+  it "validates a well-formed guid" do
+    validator = validator_class.new(entity_stub(entity, property))
+
+    expect(validator).to be_valid
+    expect(validator.errors).to be_empty
+  end
+
+  it "validates a well-formed guid from redmatrix" do
+    validator = validator_class.new(entity_stub(entity, property, "1234567890ABCDefgh_ijkl-mnopQR@example.com:3000"))
+
+    expect(validator).to be_valid
+    expect(validator.errors).to be_empty
+  end
+
+  it "must be at least 16 chars" do
+    validator = validator_class.new(entity_stub(entity, property, "aaaaaa"))
+
+    expect(validator).not_to be_valid
+    expect(validator.errors).to include(property)
+  end
+
+  it "must only contain [0-9a-z-_@.:]" do
+    validator = validator_class.new(entity_stub(entity, property, "zzz+-#*$$"))
+
+    expect(validator).not_to be_valid
+    expect(validator.errors).to include(property)
+  end
+
+  it "must not be empty" do
+    validator = validator_class.new(entity_stub(entity, property, ""))
+
+    expect(validator).not_to be_valid
+    expect(validator.errors).to include(property)
+  end
+end
+
+shared_examples "a boolean validator" do
+  it "validates a well-formed boolean" do
+    [true, "true", false, "false"].each do |val|
+      validator = validator_class.new(entity_stub(entity, property, val))
+
+      expect(validator).to be_valid
+      expect(validator.errors).to be_empty
+    end
+  end
+
+  it "must not be an arbitrary string or other object" do
+    ["asdf", Time.zone.today, 1234].each do |val|
+      validator = validator_class.new(entity_stub(entity, property, val))
+
+      expect(validator).not_to be_valid
+      expect(validator.errors).to include(property)
+    end
+  end
+end