Change in ...osmo-msc[master]: manual: adjust and fix auth and ciph docs
gerrit-no-reply at lists.osmocom.org
Thu Aug 1 16:23:43 UTC 2019
neels has uploaded this change for review. ( https://gerrit.osmocom.org/c/osmo-msc/+/15021
Change subject: manual: adjust and fix auth and ciph docs
manual: adjust and fix auth and ciph docs
1 file changed, 54 insertions(+), 14 deletions(-)
git pull ssh://gerrit.osmocom.org:29418/osmo-msc refs/changes/21/15021/1
diff --git a/doc/manuals/chapters/net.adoc b/doc/manuals/chapters/net.adoc
index 06be4ba..aa0a2f3 100644
@@ -101,32 +101,56 @@
-Authorized subscribers must be entered in the HLR database, see the _OsmoHLR
-reference manual_ <<userman-osmohlr>>. If authentication tokens (such as KI for
-2G, or K and OP/OPC for UMTS) are present in the HLR, OsmoMSC will only attach
-a subscriber after successful authentication.
+A subscriber's IMSI must be entered in the HLR database to be able to attach. A
+subscriber-create-on-demand feature is also available, see the _OsmoHLR
+reference manual_ <<userman-osmohlr>>.
-If no authentication keys are present in the HLR for a given subscriber,
-OsmoMSC will attach the subscriber _without_ authentication. You can reject
-subscribers that lack authentication info in the HLR with this setting:
+A known IMSI in the HLR may or may not have authentication keys associated,
+which profoundly affects the ability to attach and the algorithms used to
+negotiate authentication, as the following sections explain for 2G and 3G.
+==== Authentication on 2G
+If authentication tokens (such as KI for 2G, or K and OP/OPC for UMTS) are
+present in the HLR, OsmoMSC will only attach a subscriber after successful
+authentication. Note that the 3G authentication keys are also used on 2G when
+the MS indicates UMTS compatibility, in which case the full UMTS style mutual
+authentication may indeed take place on 2G (GERAN).
+On 2G, if no authentication keys are present in the HLR for a given subscriber,
+OsmoMSC will attach the subscriber _without_ authentication. Subscribers that
+lack authentication keys can always be rejected with this setting:
+==== Authentication on 3G
+3G (UTRAN) always requires authentication (a.k.a. Integrity Protection) by
+specification, and hence authentication keys must be present in the HLR for a
+subscriber to be able to attach on 3G.
+OsmoMSC always indicates UIA1 and UIA2 as permitted Integrity Protection
+algorithms on 3G.
To enable ciphering on the radio link, authentication must take place first:
-the Kc resulting from authentication is the key used for ciphering. Hence, all
-subscribers must have authentication tokens available in the HLR for ciphering.
+the Kc resulting from authentication is the key used for ciphering. Hence, to
+be able to use ciphering, a subscriber must have authentication tokens
+available in the HLR.
+==== Ciphering on 2G
The MS, BTS and MSC must agree on a ciphering algorithm to use.
- The MS sends its supported ciphering algorithms via Classmark IEs during
- Typically the BSC needs to know which A5 ciphers are supported by connected
+ BTSes, see the `network / encryption a5` configuration item for OsmoBSC
- Finally, OsmoMSC may impose that specific A5 ciphers shall not be considered.
It is the responsibility of the BSC to then pick an A5 cipher that satisfies
@@ -143,12 +167,28 @@
- encryption a5 3
+ encryption a5 1 3
- Never use A5/2: it is an "export grade cipher" and has been deprecated for
its low ciphering strength.
-NOTE: At the time of writing, OsmoMSC supports setting only a single A5 cipher,
-while it should be able to allow a set of ciphers. This is subject to ongoing
+- To allow either no encryption or any of A5/1 or A5/3 based on the presence of
+ authentication keys and abilities of the MS, SIM and BSC configuration, it is
+ recommended to enable all ciphers in OsmoMSC. The highest available A5 cipher
+ will be used; the order in which the A5 options are configured does not
+ affect the choice.
+ encryption a5 0 1 3
+==== Ciphering on 3G
+While authentication is always required on 3G, ciphering is optional.
+So far OsmoMSC lacks explicit configuration for ciphering on 3G. As an interim
+solution, ciphering is always enabled on 3G.
+OsmoMSC indicates UEA1 and UEA2 as permitted encryption algorithms on 3G.
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Gerrit-Owner: neels <nhofmeyr at sysmocom.de>
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