add signature_scheme to Session and connection files

also change default hash to sha256 instead of md5, since md5 is bad.
(this is the main motivation)

IPython/consoleapp.py

@@ -265,6 +265,8 b' class IPythonConsoleApp(Configurable):'
                 setattr(self, name, cfg[name])
         if 'key' in cfg:
             self.config.Session.key = str_to_bytes(cfg['key'])
+        if 'signature_scheme' in cfg:
+            self.config.Session.signature_scheme = cfg['signature_scheme']
     
     def init_ssh(self):
         """set up ssh tunnels, if needed."""

IPython/kernel/connect.py

@@ -50,7 +50,9 b' from IPython.utils.traitlets import ('
 #-----------------------------------------------------------------------------
 
 def write_connection_file(fname=None, shell_port=0, iopub_port=0, stdin_port=0, hb_port=0,
-                         control_port=0, ip=LOCALHOST, key=b'', transport='tcp'):
+                         control_port=0, ip=LOCALHOST, key=b'', transport='tcp',
+                         signature_scheme='hmac-sha256',
+                         ):
     """Generates a JSON config file, including the selection of random ports.
     
     Parameters
@@ -78,7 +80,15 b' def write_connection_file(fname=None, shell_port=0, iopub_port=0, stdin_port=0, '
         The ip address the kernel will bind to.
 
     key : str, optional
-        The Session key used for HMAC authentication.
+        The Session key used for message authentication.
+    
+    signature_scheme : str, optional
+        The scheme used for message authentication.
+        This has the form 'digest-hash', where 'digest'
+        is the scheme used for digests, and 'hash' is the name of the hash function
+        used by the digest scheme.
+        Currently, 'hmac' is the only supported digest scheme,
+        and 'sha256' is the default hash function.
 
     """
     # default to temporary connector file
@@ -129,6 +139,7 b' def write_connection_file(fname=None, shell_port=0, iopub_port=0, stdin_port=0, '
     cfg['ip'] = ip
     cfg['key'] = bytes_to_str(key)
     cfg['transport'] = transport
+    cfg['signature_scheme'] = signature_scheme
     
     with open(fname, 'w') as f:
         f.write(json.dumps(cfg, indent=2))
@@ -380,6 +391,7 b' class ConnectionFileMixin(HasTraits):'
     _connection_file_written = Bool(False)
 
     transport = CaselessStrEnum(['tcp', 'ipc'], default_value='tcp', config=True)
+    signature_scheme = Unicode('')
 
     ip = Unicode(LOCALHOST, config=True,
         help="""Set the kernel\'s IP address [default localhost].
@@ -427,6 +439,7 b' class ConnectionFileMixin(HasTraits):'
             stdin_port=self.stdin_port,
             hb_port=self.hb_port,
             control_port=self.control_port,
+            signature_scheme=self.signature_scheme,
         )
 
     def cleanup_connection_file(self):
@@ -463,6 +476,7 b' class ConnectionFileMixin(HasTraits):'
             stdin_port=self.stdin_port, iopub_port=self.iopub_port,
             shell_port=self.shell_port, hb_port=self.hb_port,
             control_port=self.control_port,
+            signature_scheme=self.signature_scheme,
         )
         # write_connection_file also sets default ports:
         for name in port_names:
@@ -479,7 +493,10 b' class ConnectionFileMixin(HasTraits):'
         self.ip = cfg['ip']
         for name in port_names:
             setattr(self, name, cfg[name])
-        self.session.key = str_to_bytes(cfg['key'])
+        if 'key' in cfg:
+            self.session.key = str_to_bytes(cfg['key'])
+        if cfg.get('signature_scheme'):
+            self.session.signature_scheme = cfg['signature_scheme']
 
     #--------------------------------------------------------------------------
     # Creating connected sockets

IPython/kernel/zmq/session.py

@@ -24,6 +24,7 b' Authors:'
 # Imports
 #-----------------------------------------------------------------------------
 
+import hashlib
 import hmac
 import logging
 import os
@@ -50,7 +51,9 b' from IPython.utils.importstring import import_item'
 from IPython.utils.jsonutil import extract_dates, squash_dates, date_default
 from IPython.utils.py3compat import str_to_bytes, str_to_unicode
 from IPython.utils.traitlets import (CBytes, Unicode, Bool, Any, Instance, Set,
-                                        DottedObjectName, CUnicode, Dict, Integer)
+                                        DottedObjectName, CUnicode, Dict, Integer,
+                                        TraitError,
+)
 from IPython.kernel.zmq.serialize import MAX_ITEMS, MAX_BYTES
 
 #-----------------------------------------------------------------------------
@@ -308,10 +311,26 b' class Session(Configurable):'
         help="""execution key, for extra authentication.""")
     def _key_changed(self, name, old, new):
         if new:
-            self.auth = hmac.HMAC(new)
+            self.auth = hmac.HMAC(new, digestmod=self.digest_mod)
         else:
             self.auth = None
     
+    signature_scheme = Unicode('hmac-sha256', config=True,
+        help="""The digest scheme used to construct the message signatures.
+        Must have the form 'hmac-HASH'.""")
+    def _signature_scheme_changed(self, name, old, new):
+        if not new.startswith('hmac-'):
+            raise TraitError("signature_scheme must start with 'hmac-', got %r" % new)
+        hash_name = new.split('-', 1)[1]
+        try:
+            self.digest_mod = getattr(hashlib, hash_name)
+        except AttributeError:
+            raise TraitError("hashlib has no such attribute: %s" % hash_name)
+    
+    digest_mod = Any()
+    def _digest_mod_default(self):
+        return hashlib.sha256
+    
     auth = Instance(hmac.HMAC)
     
     digest_history = Set()
@@ -387,6 +406,11 b' class Session(Configurable):'
         key : bytes
             The key used to initialize an HMAC signature.  If unset, messages
             will not be signed or checked.
+        signature_scheme : str
+            The message digest scheme. Currently must be of the form 'hmac-HASH',
+            where 'HASH' is a hashing function available in Python's hashlib.
+            The default is 'hmac-sha256'.
+            This is ignored if 'key' is empty.
         keyfile : filepath
             The file containing a key.  If this is set, `key` will be
             initialized to the contents of the file.