== Osmocom Authentication Protocol (OAP) === 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. === Connection The protocol expects that a reliable, ordered, packet boundaries preserving connection is used (e.g. IPA over TCP). === Using IPA By default, the following identifiers should be used: - IPA protocol: 0xee (OSMO) - IPA OSMO protocol extension: 0x06 (OAP) === Procedures .Ideal communication sequence [mscgen] ---- msc { cli [label="Client"], srv [label="Server"]; cli => srv [label="Register (ID)"]; cli <= srv [label="Challenge (RAND+AUTN)"]; cli => srv [label="Challenge Result (XRES)"]; cli <= srv [label="Register Result"]; } ---- .Variation "test setup" [mscgen] ---- msc { cli [label="Client"], srv [label="Server"]; cli => srv [label="Register (ID)"]; cli <= srv [label="Register Result"]; } ---- .Variation "invalid sequence nr": [mscgen] ---- msc { cli [label="Client"], srv [label="Server"]; cli => srv [label="Register (ID)"]; cli <= srv [label="Challenge (RAND+AUTN)"]; cli => srv [label="Sync Request (AUTS)"]; cli <= srv [label="Challenge (RAND+AUTN')"]; cli => srv [label="Challenge Result (XRES)"]; cli <= srv [label="Register Result"]; } ---- ==== Register The client sends a REGISTER_REQ message containing an identifier number. [[oap-challenge]] ==== 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. ==== 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 secret. 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. ==== 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. ==== Sync Result If the server has received a valid Sync Request, it shall answer by directly sending another Challenge (see <>). If an invalid Sync Request is received, the server shall reply with a REGISTER_ERR message. ==== 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. === Message Format Every message is based on the following message format [options="header",cols="5%,20%,45%,10%,10%,10%"] |=== |IEI|IE|Type|Presence|Format|Length | |Message Type|<>|M|V|1 |=== The receiver shall be able to receive IEs in any order. Unknown IEs shall be ignored. ==== Register Request Direction: Client -> Server [options="header",cols="5%,20%,45%,10%,10%,10%"] |=== |IEI|IE|Type|Presence|Format|Length | |Message Type|<>|M|V|1 |30|Client ID|<>|M|TLV|4 |=== ==== Register Error Direction: Server -> Client [options="header",cols="5%,20%,45%,10%,10%,10%"] |=== |IEI|IE|Type|Presence|Format|Length | |Message Type|<>|M|V|1 |02|Cause|GMM Cause, TS 24.008: 10.5.5.14|M|TLV|3 |=== ==== Register Result Direction: Server -> Client [options="header",cols="5%,20%,45%,10%,10%,10%"] |=== |IEI|IE|Type|Presence|Format|Length | |Message Type|<>|M|V|1 |=== ==== Challenge Direction: Server -> Client [options="header",cols="5%,20%,45%,10%,10%,10%"] |=== |IEI|IE|Type|Presence|Format|Length | |Message Type|<>|M|V|1 |20|RAND|octet string (16)|TLV|18 |23|AUTN|octet string (16)|TLV|18 |=== ==== Challenge Error Direction: Client -> Server [options="header",cols="5%,20%,45%,10%,10%,10%"] |=== |IEI|IE|Type|Presence|Format|Length | |Message Type|<>|M|V|1 |02|Cause|GMM Cause, TS 24.008: 10.5.5.14|M|TLV|3 |=== ==== Challenge Result Direction: Client -> Server [options="header",cols="5%,20%,45%,10%,10%,10%"] |=== |IEI|IE|Type|Presence|Format|Length | |Message Type|<>|M|V|1 |21|XRES|octet string (8)|TLV|10 |=== ==== Sync Request Direction: Client -> Server [options="header",cols="5%,20%,45%,10%,10%,10%"] |=== |IEI|IE|Type|Presence|Format|Length | |Message Type|<>|M|V|1 |20|AUTS|octet string (14)|TLV|16 |=== ==== Sync Error Not used. ==== Sync Result Not used. === Information Elements [[common-oap-ie-msgtype]] ==== Message Type [options="header",cols="15%,85%"] |=== |0x04|Register Request |0x05|Register Error |0x06|Register Result |0x08|Challenge Request |0x09|Challenge Error |0x0a|Challenge Result |0x0c|Sync Request |0x0d|Sync Error (not used) |0x0e|Sync Result (not used) |=== [[common-oap-iei]] ==== IE Identifier (informational) These are the standard values for the IEI. [options="header",cols="15%,15%,70%"] |=== |IEI|Info Element|Type |0x02|Cause|GMM Cause, 04.08: 10.5.5.14 |0x20|RAND|Octet String |0x23|AUTN|Octet Strong |0x24|XRES|Octet String |0x25|AUTS|Octet String |0x30|Client ID|big endian integer, 16 bit |=== [[common-oap-ie-clientid]] ==== Client ID [packetdiag] ---- { colwidth = 32 node_height = 24 0-7: Client ID IEI 8-15: Length (2) 16-31: Client ID (big endian) } ---- The Client ID number shall be interpreted as an unsigned 16bit integer, where 0 indicates an invalid / unset ID.