Add agent RSA-SHA2 support, also tweak changelog w/ more tickets

paramiko/agent.py

@@ -42,6 +42,18 @@
 cSSH2_AGENTC_SIGN_REQUEST = byte_chr(13)
 SSH2_AGENT_SIGN_RESPONSE = 14
 
+SSH_AGENT_RSA_SHA2_256 = 2
+SSH_AGENT_RSA_SHA2_512 = 4
+# NOTE: RFC mildly confusing; while these flags are OR'd together, OpenSSH at
+# least really treats them like "AND"s, in the sense that if it finds the
+# SHA256 flag set it won't continue looking at the SHA512 one; it
+# short-circuits right away.
+# Thus, we never want to eg submit 6 to say "either's good".
+ALGORITHM_FLAG_MAP = {
+    "rsa-sha2-256": SSH_AGENT_RSA_SHA2_256,
+    "rsa-sha2-512": SSH_AGENT_RSA_SHA2_512,
+}
+
 
 class AgentSSH(object):
     def __init__(self):
@@ -416,7 +428,7 @@ def sign_ssh_data(self, data, algorithm=None):
         msg.add_byte(cSSH2_AGENTC_SIGN_REQUEST)
         msg.add_string(self.blob)
         msg.add_string(data)
-        msg.add_int(0)
+        msg.add_int(ALGORITHM_FLAG_MAP.get(algorithm, 0))
         ptype, result = self.agent._send_message(msg)
         if ptype != SSH2_AGENT_SIGN_RESPONSE:
             raise SSHException("key cannot be used for signing")

sites/www/changelog.rst

@@ -2,10 +2,11 @@
 Changelog
 =========
 
-- :feature:`1643` Add support for SHA-2 variants of RSA key verification
-  algorithms (as described in :rfc:`8332`) as well as limited SSH extension
-  negotiation (:rfc:`8308`). How SSH servers/clients decide when and how to use
-  this functionality can be complicated; Paramiko's support is as follows:
+- :feature:`1643` (also :issue:`1925`, :issue:`1644`, :issue:`1326`) Add
+  support for SHA-2 variants of RSA key verification algorithms (as described
+  in :rfc:`8332`) as well as limited SSH extension negotiation (:rfc:`8308`).
+  How SSH servers/clients decide when and how to use this functionality can be
+  complicated; Paramiko's support is as follows:
 
   - Client verification of server host key during key exchange will now prefer
     ``rsa-sha2-512``, ``rsa-sha2-256``, and legacy ``ssh-rsa`` algorithms, in
@@ -35,6 +36,9 @@ Changelog
       supported by both ends is used, or if there is none, it falls back to the
       previous behavior.
 
+  - SSH agent support grew the ability to specify algorithm flags when
+    requesting private key signatures; this is now used to forward SHA2
+    algorithms when appropriate.
   - Server mode is now capable of pubkey auth involving SHA-2 signatures from
     clients, provided one's server implementation actually provides for doing
     so.

tests/test_agent.py

@@ -0,0 +1,50 @@
+import unittest
+
+from paramiko.message import Message
+from paramiko.agent import (
+    SSH2_AGENT_SIGN_RESPONSE,
+    cSSH2_AGENTC_SIGN_REQUEST,
+    SSH_AGENT_RSA_SHA2_256,
+    SSH_AGENT_RSA_SHA2_512,
+    AgentKey,
+)
+from paramiko.py3compat import b
+
+
+class ChaosAgent:
+    def _send_message(self, msg):
+        self._sent_message = msg
+        sig = Message()
+        sig.add_string(b("lol"))
+        sig.rewind()
+        return SSH2_AGENT_SIGN_RESPONSE, sig
+
+
+class AgentTests(unittest.TestCase):
+    def _sign_with_agent(self, kwargs, expectation):
+        agent = ChaosAgent()
+        key = AgentKey(agent, b("secret!!!"))
+        result = key.sign_ssh_data(b("token"), **kwargs)
+        assert result == b("lol")
+        msg = agent._sent_message
+        msg.rewind()
+        assert msg.get_byte() == cSSH2_AGENTC_SIGN_REQUEST
+        assert msg.get_string() == b("secret!!!")
+        assert msg.get_string() == b("token")
+        assert msg.get_int() == expectation
+
+    def test_agent_signing_defaults_to_0_for_flags_field(self):
+        # No algorithm kwarg at all
+        self._sign_with_agent(kwargs=dict(), expectation=0)
+
+    def test_agent_signing_is_2_for_SHA256(self):
+        self._sign_with_agent(
+            kwargs=dict(algorithm="rsa-sha2-256"),
+            expectation=SSH_AGENT_RSA_SHA2_256,
+        )
+
+    def test_agent_signing_is_2_for_SHA512(self):
+        self._sign_with_agent(
+            kwargs=dict(algorithm="rsa-sha2-512"),
+            expectation=SSH_AGENT_RSA_SHA2_512,
+        )