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authorNeels Hofmeyr <nhofmeyr@sysmocom.de>2015-10-01 15:23:52 +0200
committerHolger Hans Peter Freyther <holger@moiji-mobile.com>2015-10-07 18:59:55 +0200
commit0e62e595a697eb6075349b586ed264bdb9462cd0 (patch)
tree6068ddd6b8f268d1536dc5f9fcad600b2e19c977
parent06cfe00c711a037981b53946dbaa5b042acb2fbb (diff)
Add OAP design document.
Sponsored-by: On-Waves ehf
-rw-r--r--openbsc/doc/osmocom-authn-protocol.txt250
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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
+
+3.2.6. Register Result
+
+Server -> Client
+
+ IEI Info Element Type Pres. Format Length
+ Message type 4.2.1 M V 1
+
+3.2.3. 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
+
+3.2.4. 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
+
+3.2.5. 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
+
+3.2.3. Sync Request
+
+Client -> Server
+
+ IEI Info Element Type Pres. Format Length
+ Message type 4.2.1 M V 1
+ 20 AUTS octet string (16) M TLV 18
+
+3.2.4. Sync 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
+
+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.
+