diff --git a/docs/api/client/proof_of_possession.rst b/docs/api/client/proof_of_possession.rst
new file mode 100644
index 000000000..9f1ea0793
--- /dev/null
+++ b/docs/api/client/proof_of_possession.rst
@@ -0,0 +1,5 @@
+:mod:`letsencrypt.client.proof_of_possession`
+--------------------------------------------------
+
+.. automodule:: letsencrypt.client.proof_of_possession
+   :members:
diff --git a/letsencrypt/client/client.py b/letsencrypt/client/client.py
index 48573f922..49942e299 100644
--- a/letsencrypt/client/client.py
+++ b/letsencrypt/client/client.py
@@ -72,7 +72,8 @@ class Client(object):
         # standalone (then default is False, otherwise default is True)
 
         if dv_auth is not None:
-            cont_auth = continuity_auth.ContinuityAuthenticator(config)
+            cont_auth = continuity_auth.ContinuityAuthenticator(config,
+                                                                installer)
             self.auth_handler = auth_handler.AuthHandler(
                 dv_auth, cont_auth, self.network, self.account)
         else:
diff --git a/letsencrypt/client/continuity_auth.py b/letsencrypt/client/continuity_auth.py
index 063d3d408..c6926d952 100644
--- a/letsencrypt/client/continuity_auth.py
+++ b/letsencrypt/client/continuity_auth.py
@@ -6,6 +6,7 @@ from letsencrypt.acme import challenges
 from letsencrypt.client import achallenges
 from letsencrypt.client import errors
 from letsencrypt.client import interfaces
+from letsencrypt.client import proof_of_possession
 from letsencrypt.client import recovery_token
 
 
@@ -13,32 +14,42 @@ class ContinuityAuthenticator(object):
     """IAuthenticator for
     :const:`~letsencrypt.acme.challenges.ContinuityChallenge` class challenges.
 
-    :ivar rec_token: Performs "recoveryToken" challenges
+    :ivar rec_token: Performs "recoveryToken" challenges.
     :type rec_token: :class:`letsencrypt.client.recovery_token.RecoveryToken`
 
+    :ivar proof_of_pos: Performs "proofOfPossession" challenges.
+    :type proof_of_pos:
+        :class:`letsencrypt.client.proof_of_possession.Proof_of_Possession`
+
     """
     zope.interface.implements(interfaces.IAuthenticator)
 
     # This will have an installer soon for get_key/cert purposes
-    def __init__(self, config):
+    def __init__(self, config, installer):
         """Initialize Client Authenticator.
 
         :param config: Configuration.
         :type config: :class:`letsencrypt.client.interfaces.IConfig`
 
+        :param installer: Let's Encrypt Installer.
+        :type installer: :class:`letsencrypt.client.interfaces.IInstaller`
+
         """
         self.rec_token = recovery_token.RecoveryToken(
             config.server, config.rec_token_dir)
+        self.proof_of_pos = proof_of_possession.ProofOfPossession(installer)
 
     def get_chall_pref(self, unused_domain):  # pylint: disable=no-self-use
         """Return list of challenge preferences."""
-        return [challenges.RecoveryToken]
+        return [challenges.ProofOfPossession, challenges.RecoveryToken]
 
     def perform(self, achalls):
         """Perform client specific challenges for IAuthenticator"""
         responses = []
         for achall in achalls:
-            if isinstance(achall, achallenges.RecoveryToken):
+            if isinstance(achall, achallenges.ProofOfPossession):
+                responses.append(self.proof_of_pos.perform(achall))
+            elif isinstance(achall, achallenges.RecoveryToken):
                 responses.append(self.rec_token.perform(achall))
             else:
                 raise errors.LetsEncryptContAuthError("Unexpected Challenge")
@@ -49,5 +60,5 @@ class ContinuityAuthenticator(object):
         for achall in achalls:
             if isinstance(achall, achallenges.RecoveryToken):
                 self.rec_token.cleanup(achall)
-            else:
+            elif not isinstance(achall, achallenges.ProofOfPossession):
                 raise errors.LetsEncryptContAuthError("Unexpected Challenge")
diff --git a/letsencrypt/client/proof_of_possession.py b/letsencrypt/client/proof_of_possession.py
new file mode 100644
index 000000000..8f11f56c8
--- /dev/null
+++ b/letsencrypt/client/proof_of_possession.py
@@ -0,0 +1,85 @@
+"""Proof of Possession Identifier Validation Challenge."""
+import M2Crypto
+import os
+import zope.component
+
+from letsencrypt.acme import challenges
+from letsencrypt.acme import jose
+from letsencrypt.acme import other
+from letsencrypt.client import interfaces
+from letsencrypt.client.display import util as display_util
+
+
+class ProofOfPossession(object): # pylint: disable=too-few-public-methods
+    """Proof of Possession Identifier Validation Challenge.
+
+    Based on draft-barnes-acme, section 6.5.
+
+    :ivar installer: Installer object
+    :type installer: :class:`~letsencrypt.client.interfaces.IInstaller`
+
+    """
+    def __init__(self, installer):
+        self.installer = installer
+
+    def perform(self, achall):
+        """Perform the Proof of Possession Challenge.
+
+        :param achall: Proof of Possession Challenge
+        :type achall: :class:`letsencrypt.client.achallenges.ProofOfPossession`
+
+        :returns: Response or None/False if the challenge cannot be completed
+        :rtype: :class:`letsencrypt.acme.challenges.ProofOfPossessionResponse`
+            or False
+
+        """
+        if (achall.alg in [jose.HS256, jose.HS384, jose.HS512] or
+                not isinstance(achall.hints.jwk, achall.alg.kty)):
+            return None
+
+        for cert, key, _ in self.installer.get_all_certs_keys():
+            der_cert_key = M2Crypto.X509.load_cert(cert).get_pubkey().as_der()
+            try:
+                cert_key = achall.alg.kty.load(der_cert_key)
+            # If JWKES.load raises other exceptions, they should be caught here
+            except (IndexError, ValueError, TypeError):
+                continue
+            if cert_key == achall.hints.jwk:
+                return self._gen_response(achall, key)
+
+        # Is there are different prompt we should give the user?
+        code, key = zope.component.getUtility(
+            interfaces.IDisplay).input(
+                "Path to private key for identifier: %s " % achall.domain)
+        if code != display_util.CANCEL:
+            return self._gen_response(achall, key)
+
+        # If we get here, the key wasn't found
+        return False
+
+    def _gen_response(self, achall, key_path): # pylint: disable=no-self-use
+        """Create the response to the Proof of Possession Challenge.
+
+        :param achall: Proof of Possession Challenge
+        :type achall: :class:`letsencrypt.client.achallenges.ProofOfPossession`
+
+        :param str key_path: Path to the key corresponding to the hinted to
+            public key.
+
+        :returns: Response or False if the challenge cannot be completed
+        :rtype: :class:`letsencrypt.acme.challenges.ProofOfPossessionResponse`
+            or False
+
+        """
+        if os.path.isfile(key_path):
+            with open(key_path, 'rb') as key:
+                try:
+                    # Needs to be changed if JWKES doesn't have a key attribute
+                    jwk = achall.alg.kty.load(key.read())
+                    sig = other.Signature.from_msg(achall.nonce, jwk.key,
+                                                   alg=achall.alg)
+                except (IndexError, ValueError, TypeError, jose.errors.Error):
+                    return False
+            return challenges.ProofOfPossessionResponse(nonce=achall.nonce,
+                                                        signature=sig)
+        return False
diff --git a/letsencrypt/client/tests/continuity_auth_test.py b/letsencrypt/client/tests/continuity_auth_test.py
index 96813fc62..6aeb8ec8f 100644
--- a/letsencrypt/client/tests/continuity_auth_test.py
+++ b/letsencrypt/client/tests/continuity_auth_test.py
@@ -16,9 +16,11 @@ class PerformTest(unittest.TestCase):
         from letsencrypt.client.continuity_auth import ContinuityAuthenticator
 
         self.auth = ContinuityAuthenticator(
-            mock.MagicMock(server="demo_server.org"))
+            mock.MagicMock(server="demo_server.org"), None)
         self.auth.rec_token.perform = mock.MagicMock(
             name="rec_token_perform", side_effect=gen_client_resp)
+        self.auth.proof_of_pos.perform = mock.MagicMock(
+            name="proof_of_pos_perform", side_effect=gen_client_resp)
 
     def test_rec_token1(self):
         token = achallenges.RecoveryToken(challb=None, domain="0")
@@ -36,6 +38,24 @@ class PerformTest(unittest.TestCase):
         for i in xrange(5):
             self.assertEqual(responses[i], "RecoveryToken%d" % i)
 
+    def test_pop_and_rec_token(self):
+        achalls = []
+        for i in xrange(4):
+            if i % 2 == 0:
+                achalls.append(achallenges.RecoveryToken(challb=None,
+                                                         domain=str(i)))
+            else:
+                achalls.append(achallenges.ProofOfPossession(challb=None,
+                                                             domain=str(i)))
+        responses = self.auth.perform(achalls)
+
+        self.assertEqual(len(responses), 4)
+        for i in xrange(4):
+            if i % 2 == 0:
+                self.assertEqual(responses[i], "RecoveryToken%d" % i)
+            else:
+                self.assertEqual(responses[i], "ProofOfPossession%d" % i)
+
     def test_unexpected(self):
         self.assertRaises(
             errors.LetsEncryptContAuthError, self.auth.perform, [
@@ -43,7 +63,8 @@ class PerformTest(unittest.TestCase):
 
     def test_chall_pref(self):
         self.assertEqual(
-            self.auth.get_chall_pref("example.com"), [challenges.RecoveryToken])
+            self.auth.get_chall_pref("example.com"),
+            [challenges.ProofOfPossession, challenges.RecoveryToken])
 
 
 class CleanupTest(unittest.TestCase):
@@ -53,7 +74,7 @@ class CleanupTest(unittest.TestCase):
         from letsencrypt.client.continuity_auth import ContinuityAuthenticator
 
         self.auth = ContinuityAuthenticator(
-            mock.MagicMock(server="demo_server.org"))
+            mock.MagicMock(server="demo_server.org"), None)
         self.mock_cleanup = mock.MagicMock(name="rec_token_cleanup")
         self.auth.rec_token.cleanup = self.mock_cleanup
 
diff --git a/letsencrypt/client/tests/proof_of_possession_test.py b/letsencrypt/client/tests/proof_of_possession_test.py
new file mode 100644
index 000000000..08f3f3797
--- /dev/null
+++ b/letsencrypt/client/tests/proof_of_possession_test.py
@@ -0,0 +1,86 @@
+"""Tests for proof_of_possession.py"""
+import Crypto.PublicKey.RSA
+import os
+import pkg_resources
+import unittest
+
+import mock
+
+from letsencrypt.acme import challenges
+from letsencrypt.acme import jose
+from letsencrypt.acme import messages2
+from letsencrypt.client import achallenges
+from letsencrypt.client import proof_of_possession
+from letsencrypt.client.display import util as display_util
+
+
+BASE_PACKAGE = "letsencrypt.client.tests"
+CERT0_PATH = pkg_resources.resource_filename(
+    BASE_PACKAGE, os.path.join("testdata", "cert.pem"))
+CERT1_PATH = pkg_resources.resource_filename(
+    BASE_PACKAGE, os.path.join("testdata", "cert-san.pem"))
+CERT2_PATH = pkg_resources.resource_filename(
+    BASE_PACKAGE, os.path.join("testdata", "dsa_cert.pem"))
+CERT2_KEY_PATH = pkg_resources.resource_filename(
+    BASE_PACKAGE, os.path.join("testdata", "dsa512_key.pem"))
+CERT3_PATH = pkg_resources.resource_filename(
+    BASE_PACKAGE, os.path.join("testdata", "matching_cert.pem"))
+CERT3_KEY_PATH = pkg_resources.resource_filename(
+    BASE_PACKAGE, os.path.join("testdata", "rsa512_key.pem"))
+CERT3_KEY = Crypto.PublicKey.RSA.importKey(pkg_resources.resource_string(
+    BASE_PACKAGE, os.path.join('testdata', 'rsa512_key.pem'))).publickey()
+
+
+class ProofOfPossessionTest(unittest.TestCase):
+    def setUp(self):
+        self.installer = mock.MagicMock()
+        certs = [CERT0_PATH, CERT1_PATH, CERT2_PATH, CERT3_PATH]
+        keys = [None, None, CERT2_KEY_PATH, CERT3_KEY_PATH]
+        self.installer.get_all_certs_keys.return_value = zip(
+            certs, keys, 4 * [None])
+        self.proof_of_pos = proof_of_possession.ProofOfPossession(
+            self.installer)
+
+        hints = challenges.ProofOfPossession.Hints(
+            jwk=jose.JWKRSA(key=CERT3_KEY), cert_fingerprints=(),
+            certs=(), serial_numbers=(), subject_key_identifiers=(),
+            issuers=(), authorized_for=())
+        chall = challenges.ProofOfPossession(
+            alg=jose.RS256, nonce='zczv4HMLVe_0kimJ25Juig', hints=hints)
+        challb = messages2.ChallengeBody(
+            chall=chall, uri="http://example", status=messages2.STATUS_PENDING)
+        self.achall = achallenges.ProofOfPossession(
+            challb=challb, domain="example.com")
+
+    def test_perform_bad_challenge(self):
+        hints = challenges.ProofOfPossession.Hints(
+            jwk=jose.jwk.JWKOct(key=CERT3_KEY), cert_fingerprints=(),
+            certs=(), serial_numbers=(), subject_key_identifiers=(),
+            issuers=(), authorized_for=())
+        chall = challenges.ProofOfPossession(
+            alg=jose.HS512, nonce='zczv4HMLVe_0kimJ25Juig', hints=hints)
+        challb = messages2.ChallengeBody(
+            chall=chall, uri="http://example", status=messages2.STATUS_PENDING)
+        self.achall = achallenges.ProofOfPossession(
+            challb=challb, domain="example.com")
+        self.assertEqual(self.proof_of_pos.perform(self.achall), None)
+
+    def test_perform_no_input(self):
+        self.assertTrue(self.proof_of_pos.perform(self.achall).verify())
+
+    @mock.patch("letsencrypt.client.recovery_token.zope.component.getUtility")
+    def test_perform_with_input(self, mock_input):
+        # Remove the matching certificate
+        self.installer.get_all_certs_keys.return_value.pop()
+        mock_input().input.side_effect = [(display_util.CANCEL, ""),
+                                          (display_util.OK, CERT0_PATH),
+                                          (display_util.OK, "imaginary_file"),
+                                          (display_util.OK, CERT3_KEY_PATH)]
+        self.assertFalse(self.proof_of_pos.perform(self.achall))
+        self.assertFalse(self.proof_of_pos.perform(self.achall))
+        self.assertFalse(self.proof_of_pos.perform(self.achall))
+        self.assertTrue(self.proof_of_pos.perform(self.achall).verify())
+
+
+if __name__ == "__main__":
+    unittest.main() # pragma: no cover
diff --git a/letsencrypt/client/tests/testdata/dsa512_key.pem b/letsencrypt/client/tests/testdata/dsa512_key.pem
new file mode 100644
index 000000000..78e164712
--- /dev/null
+++ b/letsencrypt/client/tests/testdata/dsa512_key.pem
@@ -0,0 +1,14 @@
+-----BEGIN DSA PARAMETERS-----
+MIGdAkEAwebEoGBfokKQeALHHnAZMQwYU35ILEBdV8oUmzv7qpSVUoHihyqfn6GC
+OixAKSP8EJYcTilIqPbFbfFyOPlbLwIVANoFHEDiQgknAvKrG78pHzAJdQSPAkEA
+qfka5Bnl+CeEMpzVZGrOVqZE/LFdZK9eT6YtWjzqtIkf3hwXUVxJsTnBG4xmrfvl
+41pgNJpgu99YOYqPpS0g7A==
+-----END DSA PARAMETERS-----
+-----BEGIN DSA PRIVATE KEY-----
+MIH5AgEAAkEAwebEoGBfokKQeALHHnAZMQwYU35ILEBdV8oUmzv7qpSVUoHihyqf
+n6GCOixAKSP8EJYcTilIqPbFbfFyOPlbLwIVANoFHEDiQgknAvKrG78pHzAJdQSP
+AkEAqfka5Bnl+CeEMpzVZGrOVqZE/LFdZK9eT6YtWjzqtIkf3hwXUVxJsTnBG4xm
+rfvl41pgNJpgu99YOYqPpS0g7AJATQ2LUzjGQSM6UljcPY5I2OD9THkUR9kH2tth
+zZd70UoI9btrVaTizgqYShuok94glSQNK0H92JgUk3scJPaAkAIVAMDn61h6vrCE
+mNv063So6E+eYaIN
+-----END DSA PRIVATE KEY-----
diff --git a/letsencrypt/client/tests/testdata/dsa_cert.pem b/letsencrypt/client/tests/testdata/dsa_cert.pem
new file mode 100644
index 000000000..ef94536e7
--- /dev/null
+++ b/letsencrypt/client/tests/testdata/dsa_cert.pem
@@ -0,0 +1,17 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----
diff --git a/letsencrypt/client/tests/testdata/matching_cert.pem b/letsencrypt/client/tests/testdata/matching_cert.pem
new file mode 100644
index 000000000..fda9cb1f4
--- /dev/null
+++ b/letsencrypt/client/tests/testdata/matching_cert.pem
@@ -0,0 +1,14 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----