LISP-Security (LISP-SEC)

The Locator/ID Separation Protocol defines a set of functions for routers to exchange information used to map from non-routable Endpoint Identifiers (EIDs) to routable Routing Locators (RLOCs). If these EID-to-RLOC mappings, carried through Map-Reply messages, are transmitted without integrity protection, an adversary can manipulate them and hijack the communication, impersonate the requested EID, or mount Denial of Service or Distributed Denial of Service attacks. Also, if the Map-Reply message is transported unauthenticated, an adversarial LISP entity can overclaim an EID-prefix and maliciously redirect traffic directed to a large number of hosts. A detailed description of "overclaiming" attack is provided in . This memo specifies LISP-SEC, a set of security mechanisms that provides origin authentication, integrity and anti-replay protection to LISP's EID-to-RLOC mapping data conveyed via mapping lookup process. LISP-SEC also enables verification of authorization on EID-prefix claims in Map-Reply messages, ensuring that the sender of a Map-Reply that provides the location for a given EID-prefix is entitled to do so according to the EID prefix registered in the associated Map-Server. Map-Register security, including the right for a LISP entity to register an EID-prefix or to claim presence at an RLOC, is out of the scope of LISP-SEC. Additional security considerations are described in Section 6.

One-Time Key (OTK): An ephemeral randomly generated key that must be used for a single Map-Request/Map-Reply exchange. ITR-OTK: The One-Time Key generated at the ITR. MS-OTK: The One-Time Key generated at the Map-Server. Encapsulated Control Message (ECM): A LISP control message that is prepended with an additional LISP header. ECM is used by ITRs to send LISP control messages to a Map-Resolver, by Map-Resolvers to forward LISP control messages to a Map-Server, and by Map-Resolvers to forward LISP control messages to an ETR. Authentication Data (AD): Metadata that is included either in a LISP ECM header or in a Map-Reply message to support confidentiality, integrity protection, and verification of EID-prefix authorization. OTK-AD: The portion of ECM Authentication Data that contains a One-Time Key. EID-AD: The portion of ECM and Map-Reply Authentication Data used for verification of EID-prefix authorization. PKT-AD: The portion of Map-Reply Authentication Data used to protect the integrity of the Map-Reply message. For definitions of other terms, notably Map-Request, Map-Reply, Ingress Tunnel Router (ITR), Egress Tunnel Router (ETR), Map-Server (MS), and Map-Resolver (MR) please consult the LISP specification .

LISP-SEC addresses the control plane threats, described in , that target EID-to-RLOC mappings, including manipulations of Map-Request and Map-Reply messages, and malicious ETR EID prefix overclaiming. LISP-SEC makes two main assumptions: (1) the LISP mapping system is expected to deliver a Map-Request message to their intended destination ETR as identified by the EID, and (2) no man-in-the-middle (MITM) attack can be mounted within the LISP Mapping System. Furthermore, while LISP-SEC enables detection of EID prefix overclaiming attacks, it assumes that Map-Servers can verify the EID prefix authorization at time of registration. According to the threat model described in LISP-SEC assumes that any kind of attack, including MITM attacks, can be mounted in the access network, outside of the boundaries of the LISP mapping system. An on-path attacker, outside of the LISP mapping system can, for example, hijack Map-Request and Map-Reply messages, spoofing the identity of a LISP node. Another example of on-path attack, called overclaiming attack, can be mounted by a malicious Egress Tunnel Router (ETR), by overclaiming the EID-prefixes for which it is authoritative. In this way the ETR can maliciously redirect traffic directed to a large number of hosts.

The goal of the security mechanisms defined in is to prevent unauthorized insertion of mapping data by providing origin authentication and integrity protection for the Map-Registration, and by using the nonce to detect unsolicited Map-Reply sent by off-path attackers. LISP-SEC builds on top of the security mechanisms defined in to address the threats described in by leveraging the trust relationships existing among the LISP entities participating to the exchange of the Map-Request/Map-Reply messages. Those trust relationships are used to securely distribute a One-Time Key (OTK) that provides origin authentication, integrity and anti-replay protection to mapping data conveyed via the mapping lookup process, and that effectively prevent overclaiming attacks. The processing of security parameters during the Map-Request/Map-Reply exchange is as follows: The ITR-OTK is generated and stored at the ITR, and securely transported to the Map-Server. The Map-Server uses the ITR-OTK to compute an HMAC that protects the integrity of the mapping data known to the Map-Server to prevent overclaiming attacks. The Map-Server also derives a new OTK, the MS-OTK, that is passed to the ETR, by applying a Key Derivation Function (KDF) to the ITR-OTK. The ETR uses the MS-OTK to compute an HMAC that protects the integrity of the Map-Reply sent to the ITR. Finally, the ITR uses the stored ITR-OTK to verify the integrity of the mapping data provided by both the Map-Server and the ETR, and to verify that no overclaiming attacks were mounted along the path between the Map-Server and the ITR. provides the detailed description of the LISP-SEC control messages and their processing, while the rest of this section describes the flow of protocol operations at each entity involved in the Map-Request/Map-Reply exchange: The ITR, upon needing to transmit a Map-Request message, generates and stores an OTK (ITR-OTK). This ITR-OTK is included into the Encapsulated Control Message (ECM) that contains the Map-Request sent to the Map-Resolver. To provide confidentiality to the ITR-OTK over the path between the ITR and its Map-Resolver, the ITR-OTK SHOULD be encrypted using a preconfigured key shared between the ITR and the Map-Resolver, similar to the key shared between the ETR and the Map-Server in order to secure ETR registration . The Map-Resolver decapsulates the ECM message, decrypts the ITR-OTK, if needed, and forwards through the Mapping System the received Map-Request and the ITR-OTK, as part of a new ECM message. As described in , the LISP Mapping System delivers the ECM to the appropriate Map-Server, as identified by the EID destination address of the Map-Request. The Map-Server is configured with the location mappings and policy information for the ETR responsible for the EID destination address. Using this preconfigured information, the Map-Server, after the decapsulation of the ECM message, finds the longest match EID-prefix that covers the requested EID in the received Map-Request. The Map-Server adds this EID-prefix, together with an HMAC computed using the ITR-OTK, to a new Encapsulated Control Message that contains the received Map-Request. The Map-Server derives a new OTK, the MS-OTK, by applying a Key Derivation Function (KDF) to the ITR-OTK. This MS-OTK is included in the Encapsulated Control Message that the Map-Server uses to forward the Map-Request to the ETR. To provide MS-OTK confidentiality over the path between the Map-Server and the ETR, the MS-OTK should be encrypted using the key shared between the ETR and the Map-Server in order to secure ETR registration . If the Map-Server is acting in proxy mode, as specified in , the ETR is not involved in the generation of the Map-Reply. In this case the Map-Server generates the Map-Reply on behalf of the ETR as described below. The ETR, upon receiving the ECM encapsulated Map-Request from the Map-Server, decrypts the MS-OTK, if needed, and originates a standard Map-Reply that contains the EID-to-RLOC mapping information as specified in . The ETR computes an HMAC over this standard Map-Reply, keyed with MS-OTK to protect the integrity of the whole Map-Reply. The ETR also copies the EID-prefix authorization data that the Map-Server included in the ECM encapsulated Map-Request into the Map-Reply message. The ETR then sends this complete Map-Reply message to the requesting ITR. The ITR, upon receiving the Map-Reply, uses the locally stored ITR-OTK to verify the integrity of the EID-prefix authorization data included in the Map-Reply by the Map-Server. The ITR computes the MS-OTK by applying the same KDF used by the Map-Server, and verifies the integrity of the Map-Reply. If the integrity checks fail, the Map-Reply MUST be discarded. Also, if the EID-prefixes claimed by the ETR in the Map-Reply are not equal or more specific than the EID-prefix authorization data inserted by the Map-Server, the ITR MUST discard the Map-Reply.

LISP-SEC metadata associated with a Map-Request is transported within the Encapsulated Control Message that contains the Map-Request. LISP-SEC metadata associated with the Map-Reply is transported within the Map-Reply itself.

LISP-SEC uses the ECM (Encapsulated Control Message) defined in with Type set to 8, and S bit set to 1 to indicate that the LISP header includes Authentication Data (AD). The format of the LISP-SEC ECM Authentication Data is defined in the following figure. OTK-AD stands for One-Time Key Authentication Data and EID-AD stands for EID Authentication Data.

AD Type: 1 (LISP-SEC Authentication Data) V: Key Version bit. This bit is toggled when the sender switches to a new OTK wrapping key Reserved: Set to 0 on transmission and ignored on receipt. Requested HMAC ID: The HMAC algorithm requested by the ITR. See for details. OTK Length: The length (in bytes) of the OTK Authentication Data (OTK-AD), that contains the OTK Preamble and the OTK. OTK Encryption ID: The identifier of the key wrapping algorithm used to encrypt the One-Time-Key. When a 128-bit OTK is sent unencrypted by the Map-Resolver, the OTK Encryption ID is set to NULL_KEY_WRAP_128. See for more details. One-Time-Key Preamble: set to 0 if the OTK is not encrypted. When the OTK is encrypted, this field may carry additional metadata resulting from the key wrapping operation. When a 128-bit OTK is sent unencrypted by Map-Resolver, the OTK Preamble is set to 0x0000000000000000 (64 bits). See for details. One-Time-Key: the OTK encrypted (or not) as specified by OTK Encryption ID. See for details. EID-AD Length: length (in bytes) of the EID Authentication Data (EID-AD). The ITR MUST set EID-AD Length to 4 bytes, as it only fills the KDF ID field, and all the remaining fields part of the EID-AD are not present. An EID-AD MAY contain multiple EID-records. Each EID-record is 4-byte long plus the length of the AFI-encoded EID-prefix. KDF ID: Identifier of the Key Derivation Function used to derive the MS-OTK. The ITR SHOULD use this field to indicate the recommended KDF algorithm, according to local policy. The Map-Server can overwrite the KDF ID if it does not support the KDF ID recommended by the ITR. See Section 5.4 for more details. Record Count: The number of records in this Map-Request message. A record is comprised of the portion of the packet that is labeled 'Rec' above and occurs the number of times equal to Record Count. Reserved: Set to 0 on transmission and ignored on receipt. EID HMAC ID: Identifier of the HMAC algorithm used to protect the integrity of the EID-AD. This field is filled by Map-Server that computed the EID-prefix HMAC. See Section 5.4 for more details. EID mask-len: Mask length for EID-prefix. EID-AFI: Address family of EID-prefix according to EID-prefix: The Map-Server uses this field to specify the EID-prefix that the destination ETR is authoritative for, and is the longest match for the requested EID. EID HMAC: HMAC of the EID-AD computed and inserted by Map-Server. Before computing the HMAC operation the EID HMAC field MUST be set to 0. The HMAC covers the entire EID-AD.

LISP-SEC uses the Map-Reply defined in , with Type set to 2, and S bit set to 1 to indicate that the Map-Reply message includes Authentication Data (AD). The format of the LISP-SEC Map-Reply Authentication Data is defined in the following figure. PKT-AD is the Packet Authentication Data that covers the Map-Reply payload.

AD Type: 1 (LISP-SEC Authentication Data) EID-AD Length: length (in bytes) of the EID-AD. An EID-AD MAY contain multiple EID-records. Each EID-record is 4-byte long plus the length of the AFI-encoded EID-prefix. KDF ID: Identifier of the Key Derivation Function used to derive MS-OTK. See for more details. Record Count: The number of records in this Map-Reply message. A record is comprised of the portion of the packet that is labeled 'Rec' above and occurs the number of times equal to Record Count. Reserved: Set to 0 on transmission and ignored on receipt. EID HMAC ID: Identifier of the HMAC algorithm used to protect the integrity of the EID-AD. See for more details. EID mask-len: Mask length for EID-prefix. EID-AFI: Address family of EID-prefix according to . EID-prefix: This field contains an EID-prefix that the destination ETR is authoritative for, and is the longest match for the requested EID. EID HMAC: HMAC of the EID-AD, as computed by the Map-Server. Before computing the HMAC operation the EID HMAC field MUST be set to 0. The HMAC covers the entire EID-AD. PKT-AD Length: length (in bytes) of the Packet Authentication Data (PKT-AD). PKT HMAC ID: Identifier of the HMAC algorithm used to protect the integrity of the Map-reply Location Data. PKT HMAC: HMAC of the whole Map-Reply packet, including the LISP-SEC Authentication Data. The scope of the authentication goes from the Map-Reply Type field to the PKT HMAC field included. Before computing the HMAC operation the PKT HMAC field MUST be set to 0. See for more details.

The second bit after the Type field in a Map-Register message is allocated as the S bit. The S bit indicates to the Map-Server that the registering ETR is LISP-SEC enabled. An ETR that supports LISP-SEC MUST set the S bit in its Map-Register messages.

Upon creating a Map-Request, the ITR generates a random ITR-OTK that is stored locally, together with the nonce generated as specified in . The Map-Request MUST be encapsulated in an ECM, with the S-bit set to 1, to indicate the presence of Authentication Data. If the ITR and the Map-Resolver are configured with a shared key, the ITR-OTK confidentiality SHOULD be protected by wrapping the ITR-OTK with the algorithm specified by the OTK Encryption ID field. See for further details on OTK encryption. The Requested HMAC ID field contains the suggested HMAC algorithm to be used by the Map-Server and the ETR to protect the integrity of the ECM Authentication data and of the Map-Reply. The KDF ID field, specifies the suggested key derivation function to be used by the Map-Server to derive the MS-OTK. The EID-AD length is set to 4 bytes, since the Authentication Data does not contain EID-prefix Authentication Data, and the EID-AD contains only the KDF ID field. In response to an encapsulated Map-Request that has the S-bit set, an ITR MUST receive a Map-Reply with the S-bit set, that includes an EID-AD and a PKT-AD. If the Map-Reply does not include both ADs, the ITR MUST discard it. In response to an encapsulated Map-Request with S-bit set to 0, the ITR expects a Map-Reply with S-bit set to 0, and the ITR SHOULD discard the Map-Reply if the S-bit is set. Upon receiving a Map-Reply, the ITR must verify the integrity of both the EID-AD and the PKT-AD, and MUST discard the Map-Reply if one of the integrity checks fails. The integrity of the EID-AD is verified using the locally stored ITR-OTK to re-compute the HMAC of the EID-AD using the algorithm specified in the EID HMAC ID field. If the EID HMAC ID field does not match the Requested HMAC ID the ITR SHOULD discard the Map-Reply and send, at the first opportunity it needs to, a new Map-Request with a different Requested HMAC ID field, according to ITR's local policy. The ITR MUST set the EID HMAC ID field to 0 before computing the HMAC. To verify the integrity of the PKT-AD, first the MS-OTK is derived from the locally stored ITR-OTK using the algorithm specified in the KDF ID field. This is because the PKT-AD is generated by the ETR using the MS-OTK. If the KDF ID in the Map-Reply does not match the KDF ID requested in the Map-Request, the ITR SHOULD discard the Map-Reply and send, at the first opportunity it needs to, a new Map-Request with a different KDF ID, according to ITR's local policy. The derived MS-OTK is then used to re-compute the HMAC of the PKT-AD using the Algorithm specified in the PKT HMAC ID field. If the PKT HMAC ID field does not match the Requested HMAC ID the ITR SHOULD discard the Map-Reply and send, at the first opportunity it needs to, a new Map-Request with a different Requested HMAC ID according to ITR's local policy. Each individual Map-Reply EID-record is considered valid only if: (1) both EID-AD and PKT-AD are valid, and (2) the intersection of the EID-prefix in the Map-Reply EID-record with one of the EID-prefixes contained in the EID-AD is not empty. After identifying the Map-Reply record as valid, the ITR sets the EID-prefix in the Map-Reply record to the value of the intersection set computed before, and adds the Map-Reply EID-record to its EID-to-RLOC cache, as described in . An example of Map-Reply record validation is provided in . The ITR SHOULD send SMR triggered Map-Requests over the mapping system in order to receive a secure Map-Reply. If an ITR accepts piggybacked Map-Replies, it SHOULD also send a Map-Request over the mapping system in order to securely verify the piggybacked Map-Reply.

The payload of a Map-Reply may contain multiple EID-records. The whole Map-Reply is signed by the ETR, with the PKT HMAC, to provide integrity protection and origin authentication to the EID-prefix records claimed by the ETR. The Authentication Data field of a Map-Reply may contain multiple EID-records in the EID-AD. The EID-AD is signed by the Map-Server, with the EID HMAC, to provide integrity protection and origin authentication to the EID-prefix records inserted by the Map-Server. Upon receiving a Map-Reply with the S-bit set, the ITR first checks the validity of both the EID HMAC and of the PKT-AD HMAC. If either one of the HMACs is not valid, a log message is issued and the Map-Reply is not processed any further. If both HMACs are valid, the ITR proceeds with validating each individual EID-record claimed by the ETR by computing the intersection of each one of the EID-prefix contained in the payload of the Map-Reply with each one of the EID-prefixes contained in the EID-AD. An EID-record is valid only if at least one of the intersections is not the empty set. For instance, the Map-Reply payload contains 3 mapping record EID-prefixes: 1.1.1.0/24 1.1.2.0/24 1.2.0.0/16 The EID-AD contains two EID-prefixes: 1.1.2.0/24 1.2.3.0/24 The EID-record with EID-prefix 1.1.1.0/24 is not processed since it is not included in any of the EID-ADs signed by the Map-Server. A log message is issued. The EID-record with EID-prefix 1.1.2.0/24 is stored in the map-cache because it matches the second EID-prefix contained in the EID-AD. The EID-record with EID-prefix 1.2.0.0/16 is not processed since it is not included in any of the EID-ADs signed by the Map-Server. A log message is issued. In this last example the ETR is trying to over claim the EID-prefix 1.2.0.0/16, but the Map-Server authorized only 1.2.3.0/24, hence the EID-record is discarded.

The processing performed by a PITR is equivalent to the processing of an ITR. However, if the PITR is directly connected to the ALT, the PITR performs the functions of both the ITR and the Map-Resolver forwarding the Map-Request encapsulated in an ECM header that includes the Authentication Data fields as described in .

MS-OTK confidentiality is required in the path between the Map-Server and the ETR, the MS-OTK SHOULD be encrypted using the preconfigured key shared between the Map-Server and the ETR for the purpose of securing ETR registration . Similarly, if ITR-OTK confidentiality is required in the path between the ITR and the Map-Resolver, the ITR-OTK SHOULD be encrypted with a key shared between the ITR and the Map-Resolver. The OTK is encrypted using the algorithm specified in the OTK Encryption ID field. When the AES Key Wrap algorithm is used to encrypt a 128-bit OTK, according to [RFC3339], the AES Key Wrap Initialization Value MUST be set to 0xA6A6A6A6A6A6A6A6 (64 bits). The output of the AES Key Wrap operation is 192-bit long. The most significant 64-bit are copied in the One-Time Key Preamble field, while the 128 less significant bits are copied in the One-Time Key field of the LISP-SEC Authentication Data. When decrypting an encrypted OTK the receiver MUST verify that the Initialization Value resulting from the AES Key Wrap decryption operation is equal to 0xA6A6A6A6A6A6A6A6. If this verification fails the receiver MUST discard the entire message. When a 128-bit OTK is sent unencrypted the OTK Encryption ID is set to NULL_KEY_WRAP_128, and the OTK Preamble is set to 0x0000000000000000 (64 bits).

Upon receiving an encapsulated Map-Request with the S-bit set, the Map-Resolver decapsulates the ECM message. The ITR-OTK, if encrypted, is decrypted as specified in . The Map-Resolver, as specified in , originates a new ECM header with the S-bit set, that contains the unencrypted ITR-OTK, as specified in , and the other data derived from the ECM Authentication Data of the received encapsulated Map-Request. The Map-Resolver then forwards the received Map-Request, encapsulated in the new ECM header that includes the newly computed Authentication Data fields.

Upon receiving an ECM encapsulated Map-Request with the S-bit set, the Map-Server process the Map-Request according to the value of the S-bit contained in the Map-Register sent by the ETR during registration. If the S-bit contained in the Map-Register was clear the Map-Server decapsulates the ECM and generates a new ECM encapsulated Map-Request that does not contain an ECM Authentication Data, as specified in . The Map-Server does not perform any further LISP-SEC processing. If the S-bit contained in the Map-Register was set the Map-Server decapsulates the ECM and generates a new ECM Authentication Data. The Authentication Data includes the OTK-AD and the EID-AD, that contains EID-prefix authorization information, that are ultimately sent to the requesting ITR. The Map-Server updates the OTK-AD by deriving a new OTK (MS-OTK) from the ITR-OTK received with the Map-Request. MS-OTK is derived applying the key derivation function specified in the KDF ID field. If the algorithm specified in the KDF ID field is not supported, the Map-Server uses a different algorithm to derive the key and updates the KDF ID field accordingly. The Map-Server and the ETR MUST be configured with a shared key for mapping registration according to . If MS-OTK confidentiality is required, then the MS-OTK SHOULD be encrypted, by wrapping the MS-OTK with the algorithm specified by the OTK Encryption ID field as specified in . The Map-Server includes in the EID-AD the longest match registered EID-prefix for the destination EID, and an HMAC of this EID-prefix. The HMAC is keyed with the ITR-OTK contained in the received ECM Authentication Data, and the HMAC algorithm is chosen according to the Requested HMAC ID field. If The Map-Server does not support this algorithm, the Map-Server uses a different algorithm and specifies it in the EID HMAC ID field. The scope of the HMAC operation covers the entire EID-AD, from the EID-AD Length field to the EID HMAC field, which must be set to 0 before the computation. The Map-Server then forwards the updated ECM encapsulated Map-Request, that contains the OTK-AD, the EID-AD, and the received Map-Request to an authoritative ETR as specified in .

If the Map-Server is in proxy mode, it generates a Map-Reply, as specified in , with the S-bit set to 1. The Map-Reply includes the Authentication Data that contains the EID-AD, computed as specified in , as well as the PKT-AD computed as specified in .

Upon receiving an ECM encapsulated Map-Request with the S-bit set, the ETR decapsulates the ECM message. The OTK field, if encrypted, is decrypted as specified in to obtain the unencrypted MS-OTK. The ETR then generates a Map-Reply as specified in and includes the Authentication Data that contains the EID-AD, as received in the encapsulated Map-Request, as well as the PKT-AD. The EID-AD is copied from the Authentication Data of the received encapsulated Map-Request. The PKT-AD contains the HMAC of the whole Map-Reply packet, keyed with the MS-OTK and computed using the HMAC algorithm specified in the Requested HMAC ID field of the received encapsulated Map-Request. If the ETR does not support the Requested HMAC ID, it uses a different algorithm and updates the PKT HMAC ID field accordingly. The scope of the HMAC operation covers the entire PKT-AD, from the Map-Reply Type field to the PKT HMAC field, which must be set to 0 before the computation. Finally the ETR sends the Map-Reply to the requesting ITR as specified in .

The LISP-SEC threat model described in , assumes that the LISP Mapping System is working properly and eventually delivers Map-Request messages to a Map-Server that is authoritative for the requested EID. Map-Register security, including the right for a LISP entity to register an EID-prefix or to claim presence at an RLOC, is out of the scope of LISP-SEC.

The ITR-OTK MUST be generated by a properly seeded pseudo-random (or strong random) source. See for advice on generating security-sensitive random data

If the Map-Server and the ETR are colocated, LISP-SEC does not provide protection from overclaiming attacks mounted by the ETR. However, in this particular case, since the ETR is within the trust boundaries of the Map-Server, ETR's overclaiming attacks are not included in the threat model.

The following HMAC ID values are defined by this memo for use as Requested HMAC ID, EID HMAC ID, and PKT HMAC ID in the LISP-SEC Authentication Data:

AUTH-HMAC-SHA-1-96 MUST be supported, AUTH-HMAC-SHA-256-128 should be supported.

The following OTK Encryption ID values are defined by this memo for use as OTK key wrap algorithms ID in the LISP-SEC Authentication Data:

NULL-KEY-WRAP-128, and AES-KEY-WRAP-128 MUST be supported. NULL-KEY-WRAP-128 is used to carry an unencrypted 128-bit OTK, with a 64-bit preamble set to 0x0000000000000000 (64 bits).

The following KDF ID values are defined by this memo for use as KDF ID in the LISP-SEC Authentication Data:

HKDF-SHA1-128 MUST be supported

The authors would like to acknowledge Pere Monclus, Dave Meyer, Dino Farinacci, Brian Weis, David McGrew, Darrel Lewis and Landon Curt Noll for their valuable suggestions provided during the preparation of this document.