Add control over session key enctype negotiation

Adds a principal string attribute named "session_enctypes" which can
specify what enctypes the principal supports for session keys.  (For
what it's worth, this actually allows one to list des-cbc-md5 as a
supported session key enctype, though obviously this hardly matters
now.)

Add a [realms] section parameter for specifying whether to assume that
principals (which lack the session_enctypes attribute) support
des-cbc-crc for session keys.  This allows those who still need to use
allow_weak_crypto=true, for whatever reason, to start reducing the
number of tickets issued with des-cbc-crc session keys to clients
which still give des-cbc-crc preference in their default_tgs_enctypes
list.

[ghudson@mit.edu: Miscellaneous edits, cleanups, and fixes; refactored
test script; documented session_enctypes attribute]

1 files changed, 83 insertions, 0 deletions

diff --git a/src/tests/t_sesskeynego.py b/src/tests/t_sesskeynego.py
new file mode 100644
index 0000000000..9239e12544
--- /dev/null
+++ b/src/tests/t_sesskeynego.py
@@ -0,0 +1,83 @@
+#!/usr/bin/python
+from k5test import *
+import re
+
+# Run "kvno server" with a fresh set of client tickets, then check that the
+# enctypes in the service ticket match the expected values.
+etypes_re = re.compile(r'server@[^\n]+\n\tEtype \(skey, tkt\): '
+                       '([^,]+), ([^\s]+)')
+def test_kvno(realm, expected_skey, expected_tkt):
+    realm.kinit(realm.user_princ, password('user'))
+    realm.run_as_client([kvno, 'server'])
+    output = realm.run_as_client([klist, '-e'])
+    m = etypes_re.search(output)
+    if not m:
+        fail('could not parse etypes from klist -e output')
+    skey, tkt = m.groups()
+    if skey != expected_skey:
+        fail('got session key type %s, expected %s' % (skey, expected_skey))
+    if tkt != expected_tkt:
+        fail('got ticket key type %s, expected %s' % (tkt, expected_tkt))
+
+krb5_conf1 = {'all': {'libdefaults': {
+            'default_tgs_enctypes': 'aes128-cts,aes256-cts'}}}
+
+krb5_conf2 = {'all': {'libdefaults': {
+            'default_tgs_enctypes': 'aes256-cts,aes128-cts'}}}
+
+krb5_conf3 = {'all': {'libdefaults': {
+            'allow_weak_crypto': 'true',
+            'default_tkt_enctypes': 'aes128-cts',
+            'default_tgs_enctypes': 'rc4-hmac,aes128-cts,des-cbc-crc'}}}
+
+krb5_conf4 = {'all' :{
+        'libdefaults': {
+            'allow_weak_crypto': 'true',
+            'default_tkt_enctypes': 'aes256-cts',
+            'default_tgs_enctypes': 'des-cbc-crc,rc4-hmac,aes256-cts'
+        },
+        'realms': {'$realm': {
+                'des_crc_session_supported' : 'false'}}}}
+
+# Test with client request and session_enctypes preferring aes128, but
+# aes256 long-term key.
+realm = K5Realm(krb5_conf=krb5_conf1, create_host=False, get_creds=False)
+realm.run_kadminl('addprinc -randkey -e aes256-cts:normal server')
+realm.run_kadminl('setstr server session_enctypes aes128-cts,aes256-cts')
+test_kvno(realm, 'aes128-cts-hmac-sha1-96', 'aes256-cts-hmac-sha1-96')
+realm.stop()
+
+# Second go, almost same as first, but resulting session key must be aes256
+# because of the difference in default_tgs_enctypes order.  This tests that
+# session_enctypes doesn't change the order in which we negotiate.
+realm = K5Realm(krb5_conf=krb5_conf2, create_host=False, get_creds=False)
+realm.run_kadminl('addprinc -randkey -e aes256-cts:normal server')
+realm.run_kadminl('setstr server session_enctypes aes128-cts,aes256-cts')
+test_kvno(realm, 'aes256-cts-hmac-sha1-96', 'aes256-cts-hmac-sha1-96')
+realm.stop()
+
+# Next we use krb5_conf3 and try various things.
+realm = K5Realm(krb5_conf=krb5_conf3, create_host=False, get_creds=False)
+realm.run_kadminl('addprinc -randkey -e aes256-cts:normal server')
+
+# 3a: Negotiate aes128 session key when principal only has aes256 long-term.
+realm.run_kadminl('setstr server session_enctypes aes128-cts,aes256-cts')
+test_kvno(realm, 'aes128-cts-hmac-sha1-96', 'aes256-cts-hmac-sha1-96')
+
+# 3b: Negotiate rc4-hmac session key when principal only has aes256 long-term.
+realm.run_kadminl('setstr server session_enctypes '
+                  'rc4-hmac,aes128-cts,aes256-cts')
+test_kvno(realm, 'arcfour-hmac', 'aes256-cts-hmac-sha1-96')
+
+# 3c: Test des-cbc-crc default assumption.
+realm.run_kadminl('delstr server session_enctypes')
+test_kvno(realm, 'des-cbc-crc', 'aes256-cts-hmac-sha1-96')
+realm.stop()
+
+# Last go: test that we can disable the des-cbc-crc assumption
+realm = K5Realm(krb5_conf=krb5_conf4, get_creds=False)
+realm.run_kadminl('addprinc -randkey -e aes256-cts:normal server')
+test_kvno(realm, 'aes256-cts-hmac-sha1-96', 'aes256-cts-hmac-sha1-96')
+realm.stop()
+
+success('sesskeynego')