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-
- Osmocom Authentication Protocol (OAP)
-
-1. General
-
-The Osmocom Authentication Protocol employs mutual authentication to register a
-client with a server over an IPA connection. Milenage is used as the
-authentication algorithm, where client and server have a shared secret.
-
-For example, an SGSN, as OAP client, may use its SGSN ID to register with a MAP
-proxy, an OAP server.
-
-1.1. Connection
-
-The protocol expects that a reliable, ordered, packet boundaries preserving
-connection is used (e.g. IPA over TCP).
-
-1.2. Using IPA
-
-By default, the following identifiers should be used:
- - IPA protocol: 0xee (OSMO)
- - IPA OSMO protocol extension: 0x06 (OAP)
-
-2. Procedures
-
-Ideal communication sequence:
-
- Client Server
- | |
- | Register (ID) |
- |----------------------------------->|
- | |
- | Challenge (RAND+AUTN) |
- |<-----------------------------------|
- | |
- | Challenge Result (XRES) |
- |----------------------------------->|
- | |
- | Register Result |
- |<-----------------------------------|
-
-Variation "test setup":
-
- Client Server
- | |
- | Register (ID) |
- |----------------------------------->|
- | |
- | Register Result |
- |<-----------------------------------|
-
-Variation "invalid sequence nr":
-
- Client Server
- | |
- | Register (ID) |
- |----------------------------------->|
- | |
- | Challenge (RAND+AUTN) |
- |<-----------------------------------|
- | |
- | Sync Request (AUTS) |
- |----------------------------------->|
- | |
- | Challenge (RAND'+AUTN') |
- |<-----------------------------------|
- | |
- | Challenge Result (XRES) |
- |----------------------------------->|
- | |
- | Register Result |
- |<-----------------------------------|
-
-2.1. Register
-
-The client sends a REGISTER_REQ message containing an identifier number.
-
-2.2. Challenge
-
-The OAP server (optionally) sends back a CHALLENGE_REQ, containing random bytes
-and a milenage authentication token generated from these random bytes, using a
-shared secret, to authenticate itself to the OAP client. The server may omit
-this challenge entirely, based on its configuration, and immediately reply with
-a Register Result response. If the client cannot be registered (e.g. id is
-invalid), the server sends a REGISTER_ERR response.
-
-2.3. Challenge Result
-
-When the client has received a Challenge, it may verify the server's
-authenticity and validity of the sequence number (included in AUTN), and, if
-valid, reply with a CHALLENGE_RES message. This shall contain an XRES
-authentication token generated by milenage from the same random bytes received
-from the server and the same shared secet. If the client decides to cancel the
-registration (e.g. invalid AUTN), it shall not reply to the CHALLENGE_REQ; a
-CHALLENGE_ERR message may be sent, but is not mandatory. For example, the
-client may directly start with a new REGISTER_REQ message.
-
-2.4. Sync Request
-
-When the client has received a Challenge but sees an invalid sequence number
-(embedded in AUTN, according to the milenage algorithm), the client may send a
-SYNC_REQ message containing an AUTS synchronisation token.
-
-2.5. Sync Result
-
-If the server has received a valid Sync Request, it shall answer by directly
-sending another Challenge (see 2.2.). If an invalid Sync Request is received,
-the server shall reply with a REGISTER_ERR message.
-
-2.6. Register Result
-
-The server sends a REGISTER_RES message to indicate that registration has been
-successful. If the server cannot register the client (e.g. invalid challenge
-response), it shall send 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
-
-Client -> Server
-
- IEI Info Element Type Pres. Format Length
- Message type 4.2.1 M V 1
- 30 Client ID big endian int (2 oct) M TLV 4
-
-3.2.2. Register Error
-
-Server -> Client
-
- 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
-
-3.2.3. Register Result
-
-Server -> Client
-
- IEI Info Element Type Pres. Format Length
- Message type 4.2.1 M V 1
-
-3.2.4. Challenge
-
-Server -> Client
-
- 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
-
-3.2.5. Challenge Error
-
-Client -> Server
-
- 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. Challenge Result
-
-Client -> Server
-
- IEI Info Element Type Pres. Format Length
- Message type 4.2.1 M V 1
- 21 XRES octet string (8) M TLV 10
-
-3.2.7. Sync Request
-
-Client -> Server
-
- IEI Info Element Type Pres. Format Length
- Message type 4.2.1 M V 1
- 20 AUTS octet string (14) M TLV 16
-
-3.2.8. Sync Error
-
-Not used.
-
-3.2.9. Sync Result
-
-Not used.
-
-
-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 |
- | |
- | 0 0 0 0 1 1 0 0 - Sync Request |
- | 0 0 0 0 1 1 0 1 - Sync Error (not used) |
- | 0 0 0 0 1 1 1 0 - Sync Result (not used) |
- | |
- +---------------------------------------------------+
-
-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 |
- | 0x23 AUTN octet string |
- | 0x24 XRES octet string |
- | 0x25 AUTS octet string |
- | 0x30 Client ID big endian int (2 octets) |
- +---------------------------------------------------------+
-
-4.2.3. Client ID
-
- 8 7 6 5 4 3 2 1
- +-----------------------------------------------------+
- | | Client ID IEI | octet 1
- +-----------------------------------------------------+
- | Length of Client ID IE contents (2) | octet 2
- +-----------------------------------------------------+
- | Client ID number, most significant byte | octet 3
- +-----------------------------------------------------+
- | Client ID number, least significant byte | octet 4
- +-----------------------------------------------------+
-
-The Client ID number shall be interpreted as an unsigned 16bit integer, where 0
-indicates an invalid / unset ID.
-