Sponsored-by: On-Waves ehf --- openbsc/doc/osmocom-authn-protocol.txt | 191 +++++++++++++++++++++++++++++++++ 1 file changed, 191 insertions(+) create mode 100644 openbsc/doc/osmocom-authn-protocol.txt
diff --git a/openbsc/doc/osmocom-authn-protocol.txt b/openbsc/doc/osmocom-authn-protocol.txt new file mode 100644 index 0000000..660fdb6 --- /dev/null +++ b/openbsc/doc/osmocom-authn-protocol.txt @@ -0,0 +1,191 @@ + + Osmocom Authentication Protocol (OAP) + +1. General + +This document describes the remote protocol that is used by the SGSN and MAP +proxy to authenticate each other. The protocol and the messages are designed +after the corresponding MAP messages (see GSM 09.02) with the following +differences: + + - The encoding uses TLV structures instead of ASN.1 encodings + - Segmentation is not used + +See the specification of the Gr interface (GSM 03.60). + +1.1. Connection + +The protocol expects that a reliable, ordered, packet boundaries preserving +connection is used (e.g. IPA over TCP). The remote peer is either a service +that understands the protocol natively or a wrapper service that maps the +messages to/from real MAP messages that can be used to directly communicate +with an HLR. + +1.2. Using IPA + +By default, the following identifiers should be used: + - IPA protocol: 0xee (OSMO) + - IPA OSMO protocol extension: 0x06 + +2. Procedures + +Ideal communication sequence: + + SGSN MAP + | | + | Register (Id) | + |----------------------------------->| + | | + | Challenge (RAND+AUTN) | + |<-----------------------------------| + | | + | Challenge Result (SRES) | + |----------------------------------->| + | | + | Register Result | + |<-----------------------------------| + +2.1. Register + +The SGSN sends a REGISTER_REQ message containing an SGSN identifier number. + +2.2. Challenge + +The OAP server (optionally) sends a CHALLENGE_REQ to the SGSN, containing +random bytes and a milenage authentication token generated from these random +bytes, using a shared secret, to authenticate itself to the OAP client (SGSN). +The server may omit this challenge entirely, based on its configuration, and +immediately reply with a Register Result response. If the SGSN cannot be +registered (e.g. id is invalid), the server sends a REGISTER_ERR response. + +2.3. Challenge Result + +When the SGSN has received a Challenge, it may verify the server's +authenticity, and reply with a CHALLENGE_RES message. This shall contain SRES +(and Kc?) authentication tokens generated by milenage from the same random +bytes received from the server and the same shared secet. If the SGSN cannot +verify the server's authenticity, it shall instead send a CHALLENGE_ERR +message. + +2.4. Register Result + +The MAP sends a REGISTER_RES message to indicate that registration has been +successful. If the MAP proxy cannot register the SGSN (e.g. invalid challenge +response), it sends a REGISTER_ERR message. + +3. Message Format + +3.1. General + +Every message is based on the following message format + + IEI Info Element Type Pres. Format Length + Message type 4.2.1 M V 1 + +The receiver shall be able to receive IEs in any order. Unknown IEs shall be +ignored. + +3.2.1. Register Request + +SGSN -> Network peer + + IEI Info Element Type Pres. Format Length + Message type 4.2.1 M V 1 + 30 SGSN Id big endian int (2 oct) M TLV 4 + +3.2.2. Register Error + +Network peer -> SGSN + + IEI Info Element Type Pres. Format Length + Message type 4.2.1 M V 1 + 02 Cause GMM cause, M TLV 3 + 04.08: 10.5.5.14 + +3.2.6. Register Result + +Network peer -> SGSN + + IEI Info Element Type Pres. Format Length + Message type 4.2.1 M V 1 + +3.2.3. Challenge + +Network peer -> SGSN + + IEI Info Element Type Pres. Format Length + Message type 4.2.1 M V 1 + 20 RAND octet string (16) M TLV 18 + 23 AUTN octet string (16) M TLV 18 + +3.2.4. Challenge Error + +SGSN -> Network peer + + IEI Info Element Type Pres. Format Length + Message type 4.2.1 M V 1 + 02 Cause GMM cause, M TLV 3 + 04.08: 10.5.5.14 + +3.2.5. Challenge Result + +SGSN -> Network peer + + IEI Info Element Type Pres. Format Length + Message type 4.2.1 M V 1 + 21 SRES octet string (4) M TLV 6 + 22 Kc octet string (8) M TLV 10 + +4. Information Elements + +4.1. General + +[...] + +4.2.1. Message Type + + +---------------------------------------------------+ + | 8 7 6 5 4 3 2 1 | + | | + | 0 0 0 0 0 1 0 0 - Register Request | + | 0 0 0 0 0 1 0 1 - Register Error | + | 0 0 0 0 0 1 1 0 - Register Result | + | | + | 0 0 0 0 1 0 0 0 - Challenge Request | + | 0 0 0 0 1 0 0 1 - Challenge Error | + | 0 0 0 0 1 0 1 0 - Challenge Result | + | | + +---------------------------------------------------+ + +4.2.2. IE Identifier (informational) + +These are the standard values for the IEI. + + +---------------------------------------------------------+ + | IEI Info Element Type | + | | + | 0x02 Cause GMM cause, 04.08: 10.5.5.14 | + | 0x20 RAND octet string | + | 0x21 SRES octet string | + | 0x22 Kc octet string | + | 0x23 AUTN octet string | + | 0x30 SGSN Id big endian int (2 octets) | + +---------------------------------------------------------+ + +4.2.3. SGSN Id + + 8 7 6 5 4 3 2 1 + +-----------------------------------------------------+ + | | SGSN Id IEI | octet 1 + +-----------------------------------------------------+ + | Length of SGSN Id IE contents (2) | octet 2 + +-----------------------------------------------------+ + | SGSN Id number, most significant byte | octet 3 + +-----------------------------------------------------+ + | SGSN Id number, least significant byte | octet 4 + +-----------------------------------------------------+ + +The SGSN Id number shall be interpreted as an unsigned 16bit integer, where 0 +indicates an invalid / unset Id. + + -- 2.1.4
