diff --git a/sshagent/protocol.py b/sshagent/protocol.py
index e65a8fe..f72200c 100644
--- a/sshagent/protocol.py
+++ b/sshagent/protocol.py
@@ -20,6 +20,18 @@ SSH2_AGENTC_REMOVE_IDENTITY = 18
 SSH2_AGENTC_REMOVE_ALL_IDENTITIES = 19
 
 
+class Error(Exception):
+    pass
+
+
+class BadSignature(Error):
+    pass
+
+
+class MissingKey(Error):
+    pass
+
+
 class Handler(object):
 
     def __init__(self, keys, signer):
@@ -76,18 +88,20 @@ class Handler(object):
                 key = k
                 break
         else:
-            raise ValueError('key not found')
+            raise MissingKey('key not found')
 
         log.debug('signing %d-byte blob', len(blob))
         r, s = self.signer(label=key['name'], blob=blob)
         signature = (r, s)
         log.debug('signature: %s', signature)
 
-        success = key['verifying_key'].verify(signature=signature, data=blob,
-                                              sigdecode=lambda sig, _: sig)
-        log.info('signature status: %s', 'OK' if success else 'ERROR')
-        if not success:
-            raise ValueError('invalid signature')
+        try:
+            key['verifying_key'].verify(signature=signature, data=blob,
+                                        sigdecode=lambda sig, _: sig)
+            log.info('signature status: OK')
+        except formats.ecdsa.BadSignatureError:
+            log.exception('signature status: ERROR')
+            raise BadSignature('invalid ECDSA signature')
 
         sig_bytes = io.BytesIO()
         for x in signature:
diff --git a/sshagent/tests/test_protocol.py b/sshagent/tests/test_protocol.py
new file mode 100644
index 0000000..8ca4adc
--- /dev/null
+++ b/sshagent/tests/test_protocol.py
@@ -0,0 +1,56 @@
+from .. import protocol
+from .. import formats
+
+import pytest
+
+# pylint: disable=line-too-long
+
+KEY = 'ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBEUksojS/qRlTKBKLQO7CBX7a7oqFkysuFn1nJ6gzlR3wNuQXEgd7qb2bjmiiBHsjNxyWvH5SxVi3+fghrqODWo= ssh://localhost'  # nopep8
+BLOB = b'\x00\x00\x00 !S^\xe7\xf8\x1cKN\xde\xcbo\x0c\x83\x9e\xc48\r\xac\xeb,]"\xc1\x9bA\x0eit\xc1\x81\xd4E2\x00\x00\x00\x05roman\x00\x00\x00\x0essh-connection\x00\x00\x00\tpublickey\x01\x00\x00\x00\x13ecdsa-sha2-nistp256\x00\x00\x00h\x00\x00\x00\x13ecdsa-sha2-nistp256\x00\x00\x00\x08nistp256\x00\x00\x00A\x04E$\xb2\x88\xd2\xfe\xa4eL\xa0J-\x03\xbb\x08\x15\xfbk\xba*\x16L\xac\xb8Y\xf5\x9c\x9e\xa0\xceTw\xc0\xdb\x90\\H\x1d\xee\xa6\xf6n9\xa2\x88\x11\xec\x8c\xdcrZ\xf1\xf9K\x15b\xdf\xe7\xe0\x86\xba\x8e\rj'  # nopep8
+SIG = (61640221631134565789126560951398335114074531708367858563384221818711312348703, 51535548700089687831159696283235534298026173963719263249292887877395159425513)  # nopep8
+
+LIST_MSG = b'\x0b'
+LIST_REPLY = b'\x00\x00\x00\x84\x0c\x00\x00\x00\x01\x00\x00\x00h\x00\x00\x00\x13ecdsa-sha2-nistp256\x00\x00\x00\x08nistp256\x00\x00\x00A\x04E$\xb2\x88\xd2\xfe\xa4eL\xa0J-\x03\xbb\x08\x15\xfbk\xba*\x16L\xac\xb8Y\xf5\x9c\x9e\xa0\xceTw\xc0\xdb\x90\\H\x1d\xee\xa6\xf6n9\xa2\x88\x11\xec\x8c\xdcrZ\xf1\xf9K\x15b\xdf\xe7\xe0\x86\xba\x8e\rj\x00\x00\x00\x0fssh://localhost'  # nopep8
+
+SIGN_MSG = b'\r\x00\x00\x00h\x00\x00\x00\x13ecdsa-sha2-nistp256\x00\x00\x00\x08nistp256\x00\x00\x00A\x04E$\xb2\x88\xd2\xfe\xa4eL\xa0J-\x03\xbb\x08\x15\xfbk\xba*\x16L\xac\xb8Y\xf5\x9c\x9e\xa0\xceTw\xc0\xdb\x90\\H\x1d\xee\xa6\xf6n9\xa2\x88\x11\xec\x8c\xdcrZ\xf1\xf9K\x15b\xdf\xe7\xe0\x86\xba\x8e\rj\x00\x00\x00\xd1\x00\x00\x00 !S^\xe7\xf8\x1cKN\xde\xcbo\x0c\x83\x9e\xc48\r\xac\xeb,]"\xc1\x9bA\x0eit\xc1\x81\xd4E2\x00\x00\x00\x05roman\x00\x00\x00\x0essh-connection\x00\x00\x00\tpublickey\x01\x00\x00\x00\x13ecdsa-sha2-nistp256\x00\x00\x00h\x00\x00\x00\x13ecdsa-sha2-nistp256\x00\x00\x00\x08nistp256\x00\x00\x00A\x04E$\xb2\x88\xd2\xfe\xa4eL\xa0J-\x03\xbb\x08\x15\xfbk\xba*\x16L\xac\xb8Y\xf5\x9c\x9e\xa0\xceTw\xc0\xdb\x90\\H\x1d\xee\xa6\xf6n9\xa2\x88\x11\xec\x8c\xdcrZ\xf1\xf9K\x15b\xdf\xe7\xe0\x86\xba\x8e\rj\x00\x00\x00\x00'  # nopep8
+SIGN_REPLY = b'\x00\x00\x00j\x0e\x00\x00\x00e\x00\x00\x00\x13ecdsa-sha2-nistp256\x00\x00\x00J\x00\x00\x00!\x00\x88G!\x0c\n\x16:\xbeF\xbe\xb9\xd2\xa9&e\x89\xad\xc4}\x10\xf8\xbc\xdc\xef\x0e\x8d_\x8a6.\xb6\x1f\x00\x00\x00!\x00q\xf0\x16>,\x9a\xde\xe7(\xd6\xd7\x93\x1f\xed\xf9\x94ddw\xfe\xbdq\x13\xbb\xfc\xa9K\xea\x9dC\xa1\xe9'  # nopep8
+
+
+def test_list():
+    key = formats.import_public_key(KEY)
+    h = protocol.Handler(keys=[key], signer=None)
+    reply = h.handle(LIST_MSG)
+    assert reply == LIST_REPLY
+
+
+def signer(label, blob):
+    assert label == b'ssh://localhost'
+    assert blob == BLOB
+    return SIG
+
+
+def test_sign():
+    key = formats.import_public_key(KEY)
+    h = protocol.Handler(keys=[key], signer=signer)
+    reply = h.handle(SIGN_MSG)
+    assert reply == SIGN_REPLY
+
+
+def test_sign_missing():
+    h = protocol.Handler(keys=[], signer=signer)
+
+    with pytest.raises(protocol.MissingKey):
+        h.handle(SIGN_MSG)
+
+
+def test_sign_wrong():
+    def wrong_signature(label, blob):
+        assert label == b'ssh://localhost'
+        assert blob == BLOB
+        return (0, 0)
+
+    key = formats.import_public_key(KEY)
+    h = protocol.Handler(keys=[key], signer=wrong_signature)
+
+    with pytest.raises(protocol.BadSignature):
+        h.handle(SIGN_MSG)