Network Working Group B. Aboba Request for Comments: 3723 Microsoft Category: Standards Track J. Tseng McDATA Corporation J. Walker Intel V. Rangan Brocade Communications Systems Inc. F. Travostino Nortel Networks April 2004 Securing Block Storage Protocols over IP Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This document discusses how to secure block storage and storage discovery protocols running over IP (Internet Protocol) using IPsec and IKE (Internet Key Exchange). Threat models and security protocols are developed for iSCSI (Internet Protocol Small Computer System Interface), iFCP (Internet Fibre Channel Storage Networking) and FCIP (Fibre Channel over TCP/IP), as well as the iSNS (Internet Storage Name Server) and SLPv2 (Service Location Protocol v2) discovery protocols. Performance issues and resource constraints are analyzed. Table of Contents 1. Introduction ................................................. 3 1.1. iSCSI Overview ......................................... 3 1.2. iFCP Overview .......................................... 4 1.3. FCIP Overview .......................................... 4 1.4. IPsec Overview ......................................... 4 1.5. Terminology ............................................ 6 1.6. Requirements Language .................................. 7
2. Block Storage Protocol Security .............................. 7 2.1. Security Requirements ................................. 7 2.2. Resource Constraints ................................... 10 2.3. Security Protocol ...................................... 12 2.4. iSCSI Authentication ................................... 16 2.5. SLPv2 Security ......................................... 18 2.6. iSNS Security .......................................... 24 3. iSCSI security Inter-Operability Guidelines .................. 28 3.1. iSCSI Security Issues .................................. 28 3.2. iSCSI and IPsec Interaction ............................ 29 3.3. Initiating a New iSCSI Session ......................... 30 3.4. Graceful iSCSI Teardown ................................ 31 3.5. Non-graceful iSCSI Teardown ............................ 31 3.6. Application Layer CRC .................................. 32 4. iFCP and FCIP Security Issues ................................ 34 4.1. iFCP and FCIP Authentication Requirements .............. 34 4.2. iFCP Interaction with IPsec and IKE .................... 34 4.3. FCIP Interaction with IPsec and IKE .................... 35 5. Security Considerations ...................................... 36 5.1. Transport Mode Versus Tunnel Mode ...................... 36 5.2. NAT Traversal .......................................... 39 5.3. IKE Issues ............................................. 40 5.4. Rekeying Issues ........................................ 40 5.5. Transform Issues ....................................... 43 5.6. Fragmentation Issues ................................... 45 5.7. Security Checks ........................................ 46 5.8. Authentication Issues .................................. 47 5.9. Use of AES in Counter Mode ............................. 51 6. IANA Considerations .......................................... 51 6.1. Definition of Terms .................................... 52 6.2. Recommended Registration Policies ...................... 52 7. Normative References ......................................... 52 8. Informative References ....................................... 54 9. Acknowledgments .............................................. 58 Appendix A - Well Known Groups for Use with SRP ................. 59 Appendix B - Software Performance of IPsec Transforms ........... 61 B.1. Authentication Transforms .............................. 61 B.2. Encryption and Authentication Transforms ............... 64 Authors' Addresses ............................................... 69 Full Copyright Statement ......................................... 70
1. Introduction This specification discusses use of the IPsec protocol suite for protecting block storage protocols over IP networks (including iSCSI, iFCP and FCIP), as well as storage discovery protocols (iSNS and SLPv2). 1.1. iSCSI Overview iSCSI, described in [RFC3720], is a connection-oriented command/response protocol that runs over TCP, and is used to access disk, tape and other devices. iSCSI is a client-server protocol in which clients (initiators) open connections to servers (targets) and perform an iSCSI login. This document uses the SCSI terms initiator and target for clarity and to avoid the common assumption that clients have considerably less computational and memory resources than servers; the reverse is often the case for SCSI, as targets are commonly dedicated devices of some form. The iSCSI protocol has a text based negotiation mechanism as part of its initial (login) procedure. The mechanism is extensible in what can be negotiated (new text keys and values can be defined) and also in the number of negotiation rounds (e.g., to accommodate functionality such as challenge-response authentication). After a successful login, the iSCSI initiator may issue SCSI commands for execution by the iSCSI target, which returns a status response for each command, over the same connection. A single connection is used for both command/status messages as well as transfer of data and/or optional command parameters. An iSCSI session may have multiple connections, but a separate login is performed on each. The iSCSI session terminates when its last connection is closed. iSCSI initiators and targets are application layer entities that are independent of TCP ports and IP addresses. Initiators and targets have names whose syntax is defined in [RFC3721]. iSCSI sessions between a given initiator and target are run over one or more TCP connections between those entities. That is, the login process establishes an association between an iSCSI Session and the TCP connection(s) over which iSCSI PDUs will be carried. While the iSCSI login may include mutual authentication of the iSCSI endpoints and negotiation of session parameters, iSCSI does not define its own per-packet authentication, integrity, confidentiality or replay protection mechanisms. Rather, it relies upon the IPsec protocol suite to provide per-packet data confidentiality and
integrity and authentication services, with IKE as the key management protocol. iSCSI uses TCP to provide congestion control, error detection and error recovery. 1.2. iFCP Overview iFCP, defined in [iFCP], is a gateway-to-gateway protocol, which provides transport services to Fibre Channel devices over a TCP/IP network. iFCP allows interconnection and networking of existing Fibre Channel devices at wire speeds over an IP network. iFCP implementations emulate fabric services in order to improve fault tolerance and scalability by fully leveraging IP technology. Each TCP connection is used to support storage traffic between a unique pair of Fibre Channel N_PORTs. iFCP does not have a native, in-band security mechanism. Rather, it relies upon the IPsec protocol suite to provide data confidentiality and authentication services, and IKE as the key management protocol. iFCP uses TCP to provide congestion control, error detection and error recovery. 1.3. FCIP Overview FCIP, defined in [FCIP], is a pure FC encapsulation protocol that transports FC frames. Current specification work intends this for interconnection of Fibre Channel switches over TCP/IP networks, but the protocol is not inherently limited to connecting FC switches. FCIP differs from iFCP in that no interception or emulation of fabric services is involved. One or more TCP connections are bound to an FCIP Link, which is used to realize Inter-Switch Links (ISLs) between pairs of Fibre Channel entities. FCIP Frame Encapsulation is described in [RFC3643]. FCIP does not have a native, in-band security mechanism. Rather, it relies upon the IPsec protocol suite to provide data confidentiality and authentication services, and IKE as the key management protocol. FCIP uses TCP to provide congestion control, error detection and error recovery. 1.4. IPsec Overview IPsec is a protocol suite which is used to secure communication at the network layer between two peers. The IPsec protocol suite is specified within the IP Security Architecture [RFC2401], IKE [RFC2409][RFC2412], IPsec Authentication Header (AH) [RFC2402] and IPsec Encapsulating Security Payload (ESP) [RFC2406] documents. IKE is the key management protocol while AH and ESP are used to protect IP traffic.
An IPsec SA is a one-way security association, uniquely identified by the 3-tuple: Security Parameter Index (SPI), protocol (ESP) and destination IP. The parameters for an IPsec security association are typically established by a key management protocol. These include the encapsulation mode, encapsulation type, session keys and SPI values. IKE is a two phase negotiation protocol based on the modular exchange of messages defined by ISAKMP [RFC2408],and the IP Security Domain of Interpretation (DOI) [RFC2407]. IKE has two phases, and accomplishes the following functions:  Protected cipher suite and options negotiation - using keyed MACs and encryption and anti-replay mechanisms  Master key generation - such as via MODP Diffie-Hellman calculations  Authentication of end-points  IPsec SA management (selector negotiation, options negotiation, create, delete, and rekeying) Items 1 through 3 are accomplished in IKE Phase 1, while item 4 is handled in IKE Phase 2. An IKE Phase 2 negotiation is performed to establish both an inbound and an outbound IPsec SA. The traffic to be protected by an IPsec SA is determined by a selector which has been proposed by the IKE initiator and accepted by the IKE Responder. In IPsec transport mode, the IPsec SA selector can be a "filter" or traffic classifier, defined as the 5-tuple: <Source IP address, Destination IP address, transport protocol (UDP/SCTP/TCP), Source port, Destination port>. The successful establishment of a IKE Phase-2 SA results in the creation of two uni-directional IPsec SAs fully qualified by the tuple <Protocol (ESP/AH), destination address, SPI>. The session keys for each IPsec SA are derived from a master key, typically via a MODP Diffie-Hellman computation. Rekeying of an existing IPsec SA pair is accomplished by creating two new IPsec SAs, making them active, and then optionally deleting the older IPsec SA pair. Typically the new outbound SA is used immediately, and the old inbound SA is left active to receive packets for some locally defined time, perhaps 30 seconds or 1 minute.
1.5. Terminology Fibre Channel Fibre Channel (FC) is a gigabit speed networking technology primarily used to implement Storage Area Networks (SANs), although it also may be used to transport other frames types as well, including IP. FC is standardized under American National Standard for Information Systems of the InterNational Committee for Informational Technology Standards (ANSI-INCITS) in its T11 technical committee. FCIP Fibre Channel over IP (FCIP) is a protocol for interconnecting Fibre Channel islands over IP Networks so as to form a unified SAN in a single Fibre Channel fabric. The principal FCIP interface point to the IP Network is the FCIP Entity. The FCIP Link represents one or more TCP connections that exist between a pair of FCIP Entities. HBA Host Bus Adapter (HBA) is a generic term for a SCSI interface to other device(s); it's roughly analogous to the term Network Interface Card (NIC) for a TCP/IP network interface, except that HBAs generally have on-board SCSI implementations, whereas most NICs do not implement TCP, UDP, or IP. iFCP iFCP is a gateway-to-gateway protocol, which provides Fibre Channel fabric services to Fibre Channel devices over a TCP/IP network. IP block storage protocol Where used within this document, the term "IP block storage protocol" applies to all block storage protocols running over IP, including iSCSI, iFCP and FCIP. iSCSI iSCSI is a client-server protocol in which clients (initiators) open connections to servers (targets).
iSNS The Internet Storage Name Server (iSNS) protocol provides for discovery and management of iSCSI and Fibre Channel (FCP) storage devices. iSNS applications store iSCSI and FC device attributes and monitor their availability and reachability, providing a consolidated information repository for an integrated IP block storage network. iFCP requires iSNS for discovery and management, while iSCSI may use iSNS for discovery, and FCIP does not use iSNS. initiator The iSCSI initiator connects to the target on well-known TCP port 3260. The iSCSI initiator then issues SCSI commands for execution by the iSCSI target. target The iSCSI target listens on a well-known TCP port for incoming connections, and returns a status response for each command issued by the iSCSI initiator, over the same connection. 1.6. Requirements Language In this document, the key words "MAY", "MUST, "MUST NOT", "OPTIONAL", "RECOMMENDED", "SHALL", "SHALL NOT", "SHOULD", and "SHOULD NOT", are to be interpreted as described in [RFC2119]. Note that requirements specified in this document apply only to use of IPsec and IKE with IP block storage protocols. Thus, these requirements do not apply to IPsec implementations in general. Implementation requirements language should therefore be assumed to relate to the availability of features for use with IP block storage security only. Although the security requirements in this document are already incorporated into the iSCSI [RFC3720], iFCP [iFCP] and FCIP [FCIP] standards track documents, they are reproduced here for convenience. In the event of a discrepancy, the individual protocol standards track documents take precedence. 2. Block Storage Protocol Security 2.1. Security Requirements IP Block storage protocols such as iSCSI, iFCP and FCIP are used to transmit SCSI commands over IP networks. Therefore, both the control and data packets of these IP block storage protocols are vulnerable to attack. Examples of attacks include:
 An adversary may attempt to acquire confidential data and identities by snooping data packets.  An adversary may attempt to modify packets containing data and control messages.  An adversary may attempt to inject packets into an IP block storage connection.  An adversary may attempt to hijack TCP connection(s) corresponding to an IP block storage session.  An adversary may launch denial of service attacks against IP block storage devices such as by sending a TCP reset.  An adversary may attempt to disrupt security negotiation process, in order to weaken the authentication, or gain access to user passwords. This includes disruption of application- layer authentication negotiations such as iSCSI Login.  An adversary may attempt to impersonate a legitimate IP block storage entity.  An adversary may launch a variety of attacks (packet modification or injection, denial of service) against the discovery (SLPv2 [RFC2608]) or discovery and management (iSNS [iSNS]) process. iSCSI can use SLPv2 or iSNS. FCIP only uses SLPv2, and iFCP only uses iSNS. Since iFCP and FCIP devices are the last line of defense for a whole Fibre Channel island, the above attacks, if successful, could compromise the security of all the Fibre Channel hosts behind the devices. To address the above threats, IP block storage security protocols must support confidentiality, data origin authentication, integrity, and replay protection on a per-packet basis. Confidentiality services are important since IP block storage traffic may traverse insecure public networks. The IP block storage security protocols must support perfect forward secrecy in the rekeying process. Bi-directional authentication of the communication endpoints MUST be provided. There is no requirement that the identities used in authentication be kept confidential (e.g., from a passive eavesdropper).
For a security protocol to be useful, CPU overhead and hardware availability must not preclude implementation at 1 Gbps today. Implementation feasibility at 10 Gbps is highly desirable, but may not be demonstrable at this time. These performance levels apply to aggregate throughput, and include all TCP connections used between IP block storage endpoints. IP block storage communications typically involve multiple TCP connections. Performance issues are discussed further in Appendix B. Enterprise data center networks are considered mission-critical facilities that must be isolated and protected from possible security threats. Such networks are often protected by security gateways, which at a minimum provide a shield against denial of service attacks. The IP block storage security architecture should be able to leverage the protective services of the existing security infrastructure, including firewall protection, NAT and NAPT services, and VPN services available on existing security gateways. When iFCP or FCIP devices are deployed within enterprise networks, IP addresses will be typically be statically assigned as is the case with most routers and switches. Consequently, support for dynamic IP address assignment, as described in [RFC3456], will typically not be required, although it cannot be ruled out. Such facilities will also be relevant to iSCSI hosts whose addresses are dynamically assigned. As a result, the IP block storage security protocols must not introduce additional security vulnerabilities where dynamic address assignment is supported. While IP block storage security is mandatory to implement, it is not mandatory to use. The security services used depend on the configuration and security policies put in place. For example, configuration will influence the authentication algorithm negotiated within iSCSI Login, as well as the security services (confidentiality, data origin authentication, integrity, replay protection) and transforms negotiated when IPsec is used to protect IP block storage protocols such as iSCSI, iFCP and FCIP. FCIP implementations may allow enabling and disabling security mechanisms at the granularity of an FCIP Link. For iFCP, the granularity corresponds to an iFCP Portal. For iSCSI, the granularity of control is typically that of an iSCSI session, although it is possible to exert control down to the granularity of the destination IP address and TCP port. Note that with IPsec, security services are negotiated at the granularity of an IPsec SA, so that IP block storage connections requiring a set of security services different from those negotiated with existing IPsec SAs will need to negotiate a new IPsec SA.
Separate IPsec SAs are also advisable where quality of service considerations dictate different handling of IP block storage connections. Attempting to apply different quality of service to connections handled by the same IPsec SA can result in reordering, and falling outside the replay window. For a discussion of the issues, see [RFC2983]. IP block storage protocols can be expected to carry sensitive data and provide access to systems and data that require protection against security threats. SCSI and Fibre Channel currently contain little in the way of security mechanisms, and rely on physical security, administrative security, and correct configuration of the communication medium and systems/devices attached to it for their security properties. For most IP networks, it is inappropriate to assume physical security, administrative security, and correct configuration of the network and all attached nodes (a physically isolated network in a test lab may be an exception). Therefore, authentication SHOULD be used by IP block storage protocols (e.g., iSCSI SHOULD use one of its in-band authentication mechanisms or the authentication provided by IKE) in order to provide a minimal assurance that connections have initially been opened with the intended counterpart. iSNS, described in [iSNS], is required in all iFCP deployments. iSCSI may use iSNS for discovery, and FCIP does not use iSNS. iSNS applications store iSCSI and FC device attributes and monitor their availability and reachability, providing a consolidated information repository for an integrated IP block storage network. The iSNS specification defines mechanisms to secure communication between an iSNS server and its clients. 2.2. Resource Constraints Resource constraints and performance requirements for iSCSI are discussed in [RFC3347] Section 3.2. iFCP and FCIP devices will typically be embedded systems deployed on racks in air-conditioned data center facilities. Such embedded systems may include hardware chipsets to provide data encryption, authentication, and integrity processing. Therefore, memory and CPU resources are generally not a constraining factor. iSCSI will be implemented on a variety of systems ranging from large servers running general purpose operating systems to embedded host bus adapters (HBAs). In general, a host bus adapter is the most constrained iSCSI implementation environment, although an HBA may draw upon the resources of the system to which it is attached in some cases (e.g., authentication computations required for connection
setup). More resources should be available to iSCSI implementations for embedded and general purpose operating systems. The following guidelines indicate the approximate level of resources that authentication, keying, and rekeying functionality can reasonably expect to draw upon: - Low power processors with small word size are generally not used, as power is usually not a constraining factor, with the possible exception of HBAs, which can draw upon the computational resources of the system into which they are inserted. Computational horsepower should be available to perform a reasonable amount of exponentiation as part of authentication and key derivation for connection setup. The same is true of rekeying, although the ability to avoid exponentiation for rekeying may be desirable (but is not an absolute requirement). - RAM and/or flash resources tend to be constrained in embedded implementations. 8-10 MB of code and data for authentication, keying, and rekeying is clearly excessive, 800-1000 KB is clearly larger than desirable, but tolerable if there is no other alternative and 80-100 KB should be acceptable. These sizes are intended as rough order of magnitude guidance, and should not be taken as hard targets or limits (e.g., smaller code sizes are always better). Software implementations for general purpose operating systems may have more leeway. The primary resource concern for implementation of authentication and keying mechanisms is code size, as iSCSI assumes that the computational horsepower to do exponentiations will be available. There is no dominant iSCSI usage scenario - the scenarios range from a single connection constrained only by media bandwidth to hundreds of initiator connections to a single target or communication endpoint. SCSI sessions and hence the connections they use tend to be relatively long lived; for disk storage, a host typically opens a SCSI connection on boot and closes it on shutdown. Tape session length tends to be measured in hours or fractions thereof (i.e., rapid fire sharing of the same tape device among different initiators is unusual), although tape robot control sessions can be short when the robot is shared among tape drives. On the other hand, tape will not see a large number of initiator connections to a single target or communication endpoint, as each tape drive is dedicated to a single use at a single time, and a dozen tape drives is a large tape device.
2.3. Security Protocol 2.3.1. Transforms All IP block storage security compliant implementations MUST support IPsec ESP [RFC2406] to provide security for both control packets and data packets, as well as the replay protection mechanisms of IPsec. When ESP is utilized, per-packet data origin authentication, integrity and replay protection MUST be used. To provide confidentiality with ESP, ESP with 3DES in CBC mode [RFC2451][3DESANSI] MUST be supported, and AES in Counter mode, as described in [RFC3686], SHOULD be supported. To provide data origin authentication and integrity with ESP, HMAC-SHA1 [RFC2404] MUST be supported, and AES in CBC MAC mode with XCBC extensions [RFC3566] SHOULD be supported. DES in CBC mode SHOULD NOT be used due to its inherent weakness. ESP with NULL encryption MUST be supported for authentication. 2.3.2. IPsec Modes Conformant IP block storage protocol implementations MUST support ESP [RFC2406] in tunnel mode and MAY implement IPsec with ESP in transport mode. 2.3.3. IKE Conformant IP block storage security implementations MUST support IKE [RFC2409] for peer authentication, negotiation of security associations, and key management, using the IPsec DOI [RFC2407]. Manual keying MUST NOT be used since it does not provide the necessary rekeying support. Conformant IP block storage security implementations MUST support peer authentication using a pre-shared key, and MAY support certificate-based peer authentication using digital signatures. Peer authentication using the public key encryption methods outlined in IKE's sections 5.2 and 5.3 [RFC2409] SHOULD NOT be used. Conformant IP block storage security implementations MUST support IKE Main Mode and SHOULD support Aggressive Mode. IKE Main Mode with pre-shared key authentication SHOULD NOT be used when either of the peers use a dynamically assigned IP address. While Main Mode with pre-shared key authentication offers good security in many cases, situations where dynamically assigned addresses are used force use of a group pre-shared key, which is vulnerable to man-in-the-middle attack.
When digital signatures are used for authentication, either IKE Main Mode or IKE Aggressive Mode MAY be used. In all cases, access to locally stored secret information (pre-shared key, or private key for digital signing) must be suitably restricted, since compromise of the secret information nullifies the security properties of the IKE/IPsec protocols. When digital signatures are used to achieve authentication, an IKE negotiator SHOULD use IKE Certificate Request Payload(s) to specify the certificate authority (or authorities) that are trusted in accordance with its local policy. IKE negotiators SHOULD check the pertinent Certificate Revocation List (CRL) before accepting a PKI certificate for use in IKE's authentication procedures. The IPsec DOI [RFC2407] provides for several types of identification data. Within IKE Phase 1, for use within the IDii and IDir payloads, conformant IP block storage security implementations MUST support the ID_IPV4_ADDR, ID_IPV6_ADDR (if the protocol stack supports IPv6) and ID_FQDN Identity Payloads. iSCSI security implementations SHOULD support the ID_USER_FQDN Identity Payload; other IP block storage protocols (iFCP, FCIP) SHOULD NOT use the ID_USER_FQDN Identity Payload. Identities other than ID_IPV4_ADDR and ID_IPV6_ADDR (such as ID_FQDN or ID_USER_FQDN) SHOULD be employed in situations where Aggressive mode is utilized along with pre-shared keys and IP addresses are dynamically assigned. The IP Subnet, IP Address Range, ID_DER_ASN1_DN, ID_DER_ASN1_GN formats SHOULD NOT be used for IP block storage protocol security; The ID_KEY_ID Identity Payload MUST NOT be used. As described in [RFC2407], within Phase 1 the ID port and protocol fields MUST be set to zero or to UDP port 500. Also, as noted in [RFC2407]: When an IKE exchange is authenticated using certificates (of any format), any ID's used for input to local policy decisions SHOULD be contained in the certificate used in the authentication of the exchange. The Phase 2 Quick Mode exchanges used by IP block storage protocol implementations MUST explicitly carry the Identity Payload fields (IDci and IDcr). Each Phase 2 IDci and IDcr Payload SHOULD carry a single IP address (ID_IPV4_ADDR, ID_IPV6_ADDR) and SHOULD NOT use the IP Subnet or IP Address Range formats. Other ID payload formats MUST NOT be used. Since IPsec acceleration hardware may only be able to handle a limited number of active IKE Phase 2 SAs, Phase 2 delete messages may be sent for idle SAs, as a means of keeping the number of active Phase 2 SAs to a minimum. The receipt of an IKE Phase 2 delete message MUST NOT be interpreted as a reason for tearing down an IP
block storage connection. Rather, it is preferable to leave the connection up, and if additional traffic is sent on it, to bring up another IKE Phase 2 SA to protect it. This avoids the potential for continually bringing connections up and down. 2.3.4. Security Policy Configuration One of the goals of this specification is to enable a high level of interoperability without requiring extensive configuration. This section provides guidelines on setting of IKE parameters so as to enhance the probability of a successful negotiation. It also describes how information on security policy configuration can be provided so as to further enhance the chances of success. To enhance the prospects for interoperability, some of the actions to consider include:  Transform restriction. Since support for 3DES-CBC and HMAC-SHA1 is required of all implementations, offering these transforms enhances the probability of a successful negotiation. If AES-CTR [RFC3686] with XCBC-MAC [RFC3566] is supported, this transform combination will typically be preferred, with 3DES-CBC/HMAC-SHA1 as a secondary offer.  Group Restriction. If 3DES-CBC/HMAC-SHA1 is offered, and DH groups are offered, then it is recommended that a DH group of at least 1024 bits be offered along with it. If AES-CTR/XCBC-MAC is the preferred offer, and DH groups are offered, then it is recommended that a DH group of at least 2048 bits be offered along with it, as noted in [KeyLen]. If perfect forward secrecy is required in Quick Mode, then it is recommended that the QM PFS DH group be the same as the IKE Phase 1 DH group. This reduces the total number of combinations, enhancing the chances for interoperability.  Key lifetimes. If a key lifetime is offered that is longer than desired, then rather than causing the IKE negotiation to fail, it is recommended that the Responder consider the offered lifetime as a maximum, and accept it. The key can then use a lesser value for the lifetime, and utilize a Lifetime Notify in order to inform the other peer of lifetime expiration.
Even when the above advice is taken, it still may be useful to be able to provide additional configuration information in order to enhance the chances of success, and it is useful to be able to manage security configuration regardless of the scale of the deployment. For example, it may be desirable to configure the security policy of an IP block storage device. This can be done manually or automatically via a security policy distribution mechanism. Alternatively, it can be supplied via iSNS or SLPv2. If an IP block storage endpoint can obtain the required security policy by other means (manually, or automatically via a security policy distribution mechanism) then it need not request this information via iSNS or SLPv2. However, if the required security policy configuration is not available via other mechanisms, iSNS or SLPv2 can be used to obtain it. It may also be helpful to obtain information about the preferences of the peer prior to initiating IKE. While it is generally possible to negotiate security parameters within IKE, there are situations in which incompatible parameters can cause the IKE negotiation to fail. The following information can be provided via SLPv2 or iSNS:  IPsec or cleartext support. The minimum piece of peer configuration required is whether an IP block storage endpoint requires IPsec or cleartext. This cannot be determined from the IKE negotiation alone without risking a long timeout, which is highly undesirable for a disk access protocol.  Perfect Forward Secrecy (PFS) support. It is helpful to know whether a peer allows PFS, since an IKE Phase 2 Quick Mode can fail if an initiator proposes PFS to a Responder that does not allow it.  Preference for tunnel mode. While it is legal to propose both transport and tunnel mode within the same offer, not all IKE implementations will support this. As a result, it is useful to know whether a peer prefers tunnel mode or transport mode, so that it is possible to negotiate the preferred mode on the first try.  Main Mode and Aggressive Mode support. Since the IKE negotiation can fail if a mode is proposed to a peer that doesn't allow it, it is helpful to know which modes a peer allows, so that an allowed mode can be negotiated on the first try.
Since iSNS or SLPv2 can be used to distribute IPsec security policy and configuration information for use with IP block storage protocols, these discovery protocols would constitute a 'weak link' were they not secured at least as well as the protocols whose security they configure. Since the major vulnerability is packet modification and replay, when iSNS or SLPv2 are used to distribute security policy or configuration information, at a minimum, per- packet data origin authentication, integrity and replay protection MUST be used to protect the discovery protocol. 2.4. iSCSI Authentication 2.4.1. CHAP Compliant iSCSI implementations MUST implement the CHAP authentication method [RFC1994] (according to [RFC3720], section 11.1.4), which includes support for bi-directional authentication, and the target authentication option. When CHAP is performed over non-encrypted channel, it is vulnerable to an off-line dictionary attack. Implementations MUST support random CHAP secrets of up to 128 bits, including the means to generate such secrets and to accept them from an external generation source. Implementations MUST NOT provide secret generation (or expansion) means other than random generation. If CHAP is used with secret smaller than 96 bits, then IPsec encryption (according to the implementation requirements in [RFC3720] section 8.3.2) MUST be used to protect the connection. Moreover, in this case IKE authentication with group pre-shared keys SHOULD NOT be used. When CHAP is used with a secret smaller then 96 bits, a compliant implementation MUST NOT continue with the iSCSI login unless it can verify that IPsec encryption is being used to protect the connection. Originators MUST NOT reuse the CHAP challenge sent by the Responder for the other direction of a bidirectional authentication. Responders MUST check for this condition and close the iSCSI TCP connection if it occurs. The same CHAP secret SHOULD NOT be configured for authentication of multiple initiators or multiple targets, as this enables any of them to impersonate any other one of them, and compromising one of them enables the attacker to impersonate any of them. It is recommended that iSCSI implementations check for use of identical CHAP secrets by different peers when this check is feasible, and take appropriate measures to warn users and/or administrators when this is detected. A single CHAP secret MAY be used for authentication of an individual
initiator to multiple targets. Likewise, a single CHAP secret MAY be used for authentication of an individual target to multiple initiators. A Responder MUST NOT send its CHAP response if the initiator has not successfully authenticated. For example, the following exchange: I->R CHAP_A=<A1,A2,...> R->I CHAP_A=<A1> CHAP_C=<C> CHAP_I=<I> I->R CHAP_N=<N> CHAP_C=<C> CHAP_I=<I> (Where N, (A1,A2), I, C, and R are correspondingly the Name, Algorithms, Identifier, Challenge, and Response as defined in [RFC1994]) MUST result in the Responder (target) closing the iSCSI TCP connection because the initiator has failed to authenticate (there is no CHAP_R in the third message). Any CHAP secret used for initiator authentication MUST NOT be configured for authentication of any target, and any CHAP secret used for target authentication MUST NOT be configured for authentication of any initiator. If the CHAP response received by one end of an iSCSI connection is the same as the CHAP response that the receiving endpoint would have generated for the same CHAP challenge, the response MUST be treated as an authentication failure and cause the connection to close (this ensures that the same CHAP secret is not used for authentication in both directions). Also, if an iSCSI implementation can function as both initiator and target, different CHAP secrets and identities MUST be configured for these two roles. The following is an example of the attacks prevented by the above requirements: Rogue wants to impersonate Storage to Alice, and knows that a single secret is used for both directions of Storage-Alice authentication. Rogue convinces Alice to open two connections to Rogue, and Rogue identifies itself as Storage on both connections. Rogue issues a CHAP challenge on connection 1, waits for Alice to respond, and then reflects Alice's challenge as the initial challenge to Alice on connection 2. If Alice doesn't check for the reflection across connections, Alice's response on connection 2 enables Rogue to impersonate Storage on connection 1, even though Rogue does not know the Alice-Storage CHAP secret.
Note that RADIUS [RFC2865] does not support bi-directional CHAP authentication. Therefore, while a target acting as a RADIUS client will be able to verify the initiator Response, it will not be able to respond to an initiator challenge unless it has access to an appropriate shared secret by some other means. 2.4.2. SRP iSCSI implementations MAY implement the SRP authentication method [RFC2945] (see [RFC3720], Section 11.1.3). The strength of SRP security is dependent on the characteristics of the group being used (i.e., the prime modulus N and generator g). As described in [RFC2945], N is required to be a Sophie-German prime (of the form N = 2q + 1, where q is also prime) the generator g is a primitive root of GF(n) [SRPNDSS]. SRP well-known groups are included in Appendix A and additional groups may be registered with IANA. iSCSI implementations MUST use one of these well-known groups. All the groups specified in Appendix A up to 1536 bits (i.e., SRP-768, SRP-1024, SRP-1280, SRP-1536) MUST be supported by initiators and targets. To guarantee interoperability, targets MUST always offer "SRP-1536" as one of the proposed groups. 2.5. SLPv2 Security Both iSCSI and FCIP protocols use SLPv2 as a way to discover peer entities and management servers. SLPv2 may also be used to provide information on peer security configuration. When SLPv2 is deployed, the SA advertisements as well as UA requests and/or responses are subject to the following security threats:  An attacker could insert or alter SA advertisements or a response to a UA request in order to masquerade as the real peer or launch a denial of service attack.  An attacker could gain knowledge about an SA or a UA through snooping, and launch an attack against the peer. Given the potential value of iSCSI targets and FCIP entities, leaking of such information not only increases the possibility of an attack over the network; there is also the risk of physical theft.  An attacker could spoof a DAAdvert. This could cause UAs and SAs to use a rogue DAs.
To address these threats, the following capabilities are required: [a] Service information, as included in SrvRply, AttrRply, SrvReg and SrvDereg messages, needs to be kept confidential. [b] The UA has to be able to distinguish between legitimate and illegitimate service information from SrvRply and AttrRply messages. In the SLPv2 security model SAs are trusted to sign data. [c] The DA has to be able to distinguish between legitimate and illegitimate SrvReg and SrvDereg messages. [d] The UA has to be able to distinguish between legitimate and illegitimate DA Advertisements. This allows the UA to avoid rogue DAs that will return incorrect data or no data at all. In the SLPv2 security model, UAs trust DAs to store, answer queries on and forward data on services, but not necessarily to originate it. [e] SAs may have to trust DAs, especially if 'mesh-enhanced' SLPv2 is used. In this case, SAs register with only one DA and trust that this DA will forward the registration to others. By itself, SLPv2 security, defined in [RFC2608], does not satisfy these security requirements. SLPv2 only provides end-to-end authentication, but does not support confidentiality. In SLPv2 authentication there is no way to authenticate "zero result responses". This enables an attacker to mount a denial of service attack by sending UAs a "zero results" SrvRply or AttrRply as if from a DA with whose source address corresponds to a legitimate DAAdvert. In all cases, there is a potential for denial of service attack against protocol service providers, but such an attack is possible even in the absence of SLPv2 based discovery mechanisms. 2.5.1. SLPv2 Security Protocol SLPv2 message types include: SrvRqst, SrvRply, SrvReg, SrvDereg, SrvAck, AttrRqst, AttrRply, DAAdvert, SrvTypeRqst, SrvTypeRply, SAAdvert. SLPv2 requires that User Agents (UAs) and Service Agents (SAs) support SrvRqst, SrvRply, and DAAdvert. SAs must additionally support SrvReg, SrvAck, and SAAdvert. Where no Directory Agent (DA) exists, the SrvRqst is multicast, but the SrvRply is sent via unicast UDP. DAAdverts are also multicast. However, all other SLPv2 messages are sent via UDP unicast.
In order to provide the required security functionality, iSCSI and FCIP implementations supporting SLPv2 security SHOULD protect SLPv2 messages sent via unicast using IPsec ESP with a non-null transform. SLPv2 authentication blocks (carrying digital signatures), described in [RFC2608] MAY also be used to authenticate unicast and multicast messages. The usage of SLPv2 by iSCSI is described in [iSCSISLP]. iSCSI initiators and targets may enable IKE mechanisms to establish identity. In addition, a subsequent user-level iSCSI session login can protect the initiator-target nexus. This will protect them from any compromise of security in the SLPv2 discovery process. The usage of SLPv2 by FCIP is described in [FCIPSLP]. FCIP Entities assume that once the IKE identity of a peer is established, the FCIP Entity Name carried in FCIP Short Frame is also implicitly accepted as the authenticated peer. Any such association between the IKE identity and the FCIP Entity Name is administratively established. For use in securing SLPv2, when digital signatures are used to achieve authentication in IKE, an IKE negotiator SHOULD use IKE Certificate Request Payload(s) to specify the certificate authority (or authorities) that are trusted in accordance with its local policy. IKE negotiators SHOULD check the pertinent Certificate Revocation List (CRL) before accepting a PKI certificate for use in IKE's authentication procedures. If key management of SLPv2 DAs needs to be coordinated with the SAs and the UAs as well as the protocol service implementations, one may use certificate based key management, with a shared root Certificate Authority (CA). One of the reasons for utilizing IPsec for SLPv2 security is that is more likely that certificates will be deployed for IPsec than for SLPv2. This both simplifies SLPv2 security and makes it more likely that it will be implemented interoperably and more importantly, that it will be used. As a result, it is desirable that little additional effort be required to enable IPsec protection of SLPv2. However, just because a certificate is trusted for use with IPsec does not necessarily imply that the host is authorized to perform SLPv2 operations. When using IPsec to secure SLPv2, it may be desirable to distinguish between certificates appropriate for use by UAs, SAs, and DAs. For example, while a UA might be allowed to use any certificate conforming to IKE certificate policy, the certificate used by an SA might indicate that it is a legitimate source of service advertisements. Similarly, a DA certificate might indicate that it is a valid DA. This can be accomplished by using special CAs to issue certificates valid for use by SAs and DAs; alternatively, SA and DA authorizations can be employed.
Assume that the policy for issuing and distributing SLPv2 authorized certificates to SAs and DAs limits them only to legitimate SAs and DAs. In this case, IPsec is used to provide SLPv2 security as follows: [a] SLPv2 messages sent via unicast are IPsec protected, using ESP with a non-null transform. [b] SrvRply and AttrRply messages from either a DA or SA are unicast to UAs. Assuming that the SA used a certificate authorized for SLPv2 service advertisement in establishing the IKE Phase 1 SA, or that the DA used a certificate authorized for DA usage, the UA can accept the information sent, even if it has no SLPv2 authentication block. Note that where SrvRqst messages are multicast, they are not protected. An attacker may attempt to exploit this by spoofing a multicast SrvRqst from the UA, generating a SrvRply from an SA of the attacker's choosing. Although the SrvRply is secured, it does not correspond to a legitimate SrvRqst sent by the UA. To avoid this attack, where SrvRqst messages are multicast, the UA MUST check that SrvRply messages represent a legitimate reply to the SrvRqst that was sent. [c] SrvReg and SrvDereg messages from a SA are unicast to DAs. Assuming that the SA used a certificate authorized for SLPv2 service advertisement in establishing the IKE Phase 1 SA, the DA can accept the de/registration even if it has no SLPv2 authentication block. Typically, the SA will check the DA authorization prior to sending the service advertisement. [d] Multicast DAAdverts can be considered advisory. The UA will attempt to contact DAs via unicast. Assuming that the DA used a certificate authorized for SLPv2 DAAdverts in establishing the IKE Phase 1 SA, the UA can accept the DAAdvert even if it has no SLPv2 authentication block. [e] SAs can accept DAAdverts as described in [d]. 2.5.2. Confidentiality of Service Information Since SLPv2 messages can contain information that can potentially reveal the vendor of the device or its other associated characteristics, revealing service information constitutes a security risk. As an example, the FCIP Entity Name may reveal a WWN from which an attacker can learn potentially useful information about the Entity's characteristics.
The SLPv2 security model assumes that service information is public, and therefore does not provide for confidentiality. However, storage devices represent mission critical infrastructure of substantial value, and so iSCSI and FCIP security implementations supporting SLPv2 security SHOULD encrypt as well as authenticate and integrity- protect unicast SLPv2 messages. Assuming that all unicast SLPv2 messages are protected by IPsec, and that confidentiality is provided, then the risk of disclosure can be limited to SLPv2 messages sent via multicast, namely the SrvRqst and DAAdvert. The information leaked in a multicast SrvRqst depends on the level of detail in the query. If leakage is a concern, then a DA can be provided. If this is not feasible, then a general query can be sent via multicast, and then further detail can be obtained from the replying entities via additional unicast queries, protected by IPsec. Information leakage via a multicast DAAdvert is less of a concern than the authenticity of the message, since knowing that a DA is present on the network only enables an attacker to know that SLPv2 is in use, and possibly that a directory service is also present. This information is not considered very valuable. 2.5.3. SLPv2 Security Implications Through the definition of security attributes, it is possible to use SLPv2 to distribute information about security settings for IP block storage entities. SLPv2 distribution of security policy is not necessary if the security settings can be determined by other means, such as manual configuration or IPsec security policy distribution. If an entity has already obtained its security configuration via other mechanisms, then it MUST NOT request security policy via SLPv2. Where SLPv2 is used to provide security policy information for use with IP block storage protocols, SLPv2 MUST be protected by IPsec as described in this document. Where SLPv2 is not used to distribute security policy information, implementations MAY implement SLPv2 security as described in this document. Where SLPv2 is used, but security is not implemented, IP block storage protocol implementations MUST support a negative cache for authentication failures. This allows implementations to avoid continually contacting discovered endpoints that fail authentication within IPsec or at the application layer (in the case of iSCSI Login). The negative cache need not be maintained within the IPsec implementation, but rather within the IP block storage protocol implementation.
Since this document proposes that hop-by-hop security be used as the primary mechanism to protect SLPv2, UAs have to trust DAs to accurately relay data from SAs. This is a change to the SLPv2 security model described in [RFC2608]. However, SLPv2 authentication as defined in [RFC2608] does not provide a way to authenticate "zero result responses", leaving SLPv2 vulnerable to a denial of service attack. Such an attack can be carried out on a UA by sending it a "zero results" SrvRply or AttrRply, sent from a source address corresponding to a DA issuing a legitimate DAAdvert. In addition, SLPv2 security as defined in [RFC2608] does not support confidentiality. When IPsec with ESP and a non-null transform is used to protect SLPv2, not only can unicast requests and replies be authenticated, but confidentiality can also be provided. This includes unicast requests to DAs and SAs as well as replies. It is also possible to actively discover SAs using multicast SA discovery, and then to send unicast requests to the discovered SAs. As a result, for use with IP block storage protocols, it is believed that use of IPsec for security is more appropriate than the SLPv2 security model defined in [RFC2608]. Using IPsec to secure SLPv2 has performance implications. Security associations established between: - UAs and SAs may be reused (the client on the UA host will use the service on the SA host). - SAs and DAs may be reused (the SAs will reregister services) - UAs and DAs will probably not be reused (many idle security associations are likely to result, and build up on the DA). When IPsec is used to protect SLPv2, it is not necessarily appropriate for all hosts with whom an IPsec security association can be established to be trusted to originate SLPv2 service advertisements. This is particularly the case in environments where it is easy to obtain certificates valid for use with IPsec (for example, where anyone with access to the network can obtain a machine certificate valid for use with IPsec). If not all hosts are authorized to originate service advertisements, then it is necessary to distinguish between authorized and unauthorized hosts. This can be accomplished by the following mechanisms:  Configuring SAs with the identities or certificate characteristics of valid DAs, and configuring DAs with the identities of SAs allowed to advertise IP block storage
services. The DAs are then trusted to enforce policies on service registration. This approach involves manual configuration, but avoids certificate customization for SLPv2.  Restricting the issuance of certificates valid for use in SLPv2 service advertisement. While all certificates allowed for use with IPsec will chain to a trusted root, certificates for hosts authorized to originate service advertisements could be signed by an SLPv2-authorized CA, or could contain explicit SLPv2 authorizations within the certificate. After the IPsec security association is set up between the SLPv2 entities, the SLPv2 implementations can then retrieve the certificates used in the negotiation in order to determine whether the entities are authorized for the operations that are being performed. This approach requires less configuration, but requires some certificate customization for use with SLPv2. 2.6. iSNS Security The iSCSI protocol may use iSNS for discovery and management services, while the iFCP protocol is required to use iSNS for such services. In addition, iSNS can be used to store and distribute security policy and authorization information to iSCSI and iFCP devices. When the iSNS protocol is deployed, the interaction between iSNS server and iSNS clients are subject to the following additional security threats:  An attacker can alter iSNS protocol messages, directing iSCSI and iFCP devices to establish connections with rogue devices, or weakening IPsec protection for iSCSI or iFCP traffic.  An attacker can masquerade as the real iSNS server by sending false iSNS heartbeat messages. This could deceive iSCSI and iFCP devices into using rogue iSNS servers.  An attacker can gain knowledge about iSCSI and iFCP devices by snooping iSNS protocol messages. Such information could aid an attacker in mounting a direct attack on iSCSI and iFCP devices, such as a denial-of-service attack or outright physical theft. To address these threats, the following capabilities are needed: [a] Unicast iSNS protocol messages may need to be authenticated. In addition, to protect against threat  above, confidentiality support is desirable, and REQUIRED when certain functions of iSNS are used.
[b] Multicast iSNS protocol messages such as the iSNS heartbeat message need to be authenticated. These messages need not be confidential since they do not leak critical information. There is no requirement that the identities of iSNS entities be kept confidential. Specifically, the identity and location of the iSNS server need not be kept confidential. In order to protect against an attacker masquerading as an iSNS server, client devices MUST support authentication of broadcast or multicast messages such as the iSNS heartbeat. The iSNS authentication block (which is identical in format to the SLP authentication block) MAY be used for this purpose. Note that the authentication block is used only for iSNS broadcast or multicast messages, and SHOULD NOT be used in unicast iSNS messages. Since iSNS is used to distribute authorizations determining which client devices can communicate, IPsec authentication and data integrity MUST be supported. In addition, if iSNS is used to distribute security policy for iFCP and iSCSI devices, then authentication, data integrity, and confidentiality MUST be supported and used. Where iSNS is used without security, IP block storage protocol implementations MUST support a negative cache for authentication failures. This allows implementations to avoid continually contacting discovered endpoints that fail authentication within IPsec or at the application layer (in the case of iSCSI Login). The negative cache need not be maintained within the IPsec implementation, but rather within the IP block storage protocol implementation. 2.6.1. Use of iSNS to Discover Security Configuration of Peer Devices In practice, within a single installation, iSCSI and/or iFCP devices may have different security settings. For example, some devices may be configured to initiate secure communication, while other devices may be configured to respond to a request for secure communication, but not to require security. Still other devices, while security capable, may neither initiate nor respond securely. In practice, these variations in configuration can result in devices being unable to communicate with each other. For example, a device that is configured to always initiate secure communication will experience difficulties in communicating with a device that neither initiates nor responds securely.
The iSNS protocol is used to transfer naming, discovery, and management information between iSCSI devices, iFCP gateways, management stations, and the iSNS server. This includes the ability to enable discovery of security settings used for communication via the iSCSI and/or iFCP protocols. The iSNS server stores security settings for each iSCSI and iFCP device interface. These security settings, which can be retrieved by authorized hosts, include use or non-use of IPsec, IKE, Main Mode, Aggressive Mode, PFS, Pre-shared Key, and certificates. For example, IKE may not be enabled for a particular device interface. If a peer device can learn of this in advance by consulting the iSNS server, it will not need to waste time and resources attempting to initiate an IKE Phase 1 SA with that device interface. If iSNS is used to distribute security policy, then the minimum information that should be learned from the iSNS server is the use or non-use of IKE and IPsec by each iFCP or iSCSI peer device interface. This information is encoded in the Security Bitmap field of each Portal of the peer device, and is applicable on a per-interface basis for the peer device. iSNS queries to acquire security configuration data about peer devices MUST be protected by IPsec/ESP authentication. 2.6.2. Use of iSNS to Distribute iSCSI and iFCP Security Policies Once communication between iSNS clients and the iSNS server are secured through use of IPsec, iSNS clients have the capability to discover the security settings required for communication via the iSCSI and/or iFCP protocols. Use of iSNS for distribution of security policies offers the potential to reduce the burden of manual device configuration, and decrease the probability of communications failures due to incompatible security policies. If iSNS is used to distribute security policies, then IPsec authentication, data integrity, and confidentiality MUST be used to protect all iSNS protocol messages. The complete IKE/IPsec configuration of each iFCP and/or iSCSI device can be stored in the iSNS server, including policies that are used for IKE Phase 1 and Phase 2 negotiations between client devices. The IKE payload format includes a series of one or more proposals that the iSCSI or iFCP device will use when negotiating the appropriate IPsec policy to use to protect iSCSI or iFCP traffic.
Note that iSNS distribution of security policy is not necessary if the security settings can be determined by other means, such as manual configuration or IPsec security policy distribution. If an entity has already obtained its security configuration via other mechanisms, then it MUST NOT request security policy via iSNS. For further details on how to store and retrieve IKE policy proposals in the iSNS server, see [iSNS]. 2.6.3. iSNS Interaction with IKE and IPsec When IPsec security is enabled, each iSNS client that is registered in the iSNS database maintains at least one Phase 1 and one Phase 2 security association with the iSNS server. All iSNS protocol messages between iSNS clients and the iSNS server are to be protected by a phase-2 security association. 2.6.4. iSNS Server Implementation Requirements All iSNS implementations MUST support the replay protection mechanisms of IPsec. ESP in tunnel mode MUST be implemented, and IPsec with ESP in transport mode MAY be implemented. To provide data origin authentication and integrity with ESP, HMAC- SHA1 MUST be supported, and AES in CBC MAC mode with XCBC extensions [RFC3566] SHOULD be supported. When confidentiality is implemented, 3DES in CBC mode MUST be supported, and AES in Counter mode, as described in [RFC3686], SHOULD be supported. DES in CBC mode SHOULD NOT be used due to its inherent weakness. If confidentiality is not required but data origin authentication and integrity is enabled, ESP with NULL Encryption MUST be used. Conformant iSNS implementations MUST support IKE for authentication, negotiation of security associations, and key management, using the IPsec DOI, described in [RFC2407]. IP block storage protocols can be expected to send data in high volumes, thereby requiring rekey. Since manual keying does not provide rekeying support, its use is prohibited with IP block storage protocols. Although iSNS does not send a high volume of data, and therefore rekey is not a major concern, manual keying SHOULD NOT be used. This is for consistency, since dynamic keying support is already required in IP storage security implementations. Conformant iSNS security implementations MUST support authentication using a pre- shared key, and MAY support certificate-based peer authentication using digital signatures. Peer authentication using the public key encryption methods outlined in [RFC2409] sections 5.2 and 5.3 SHOULD NOT be used.
Conformant iSNS implementations MUST support IKE Main Mode and SHOULD support Aggressive Mode. IKE Main Mode with pre-shared key authentication SHOULD NOT be used when either of the peers use dynamically assigned IP addresses. While Main Mode with pre-shared key authentication offers good security in many cases, situations where dynamically assigned addresses are used force use of a group pre-shared key, which is vulnerable to man-in-the-middle attack. When digital signatures are used for authentication, either IKE Main Mode or IKE Aggressive Mode MAY be used. In all cases, access to locally stored secret information (pre-shared key or private key for digital signing) MUST be suitably restricted, since compromise of the secret information nullifies the security properties of the IKE/IPsec protocols. When digital signatures are used to achieve authentication, an IKE negotiator SHOULD use IKE Certificate Request Payload(s) to specify the certificate authority (or authorities) that are trusted in accordance with its local policy. IKE negotiators SHOULD check the pertinent Certificate Revocation List (CRL) before accepting a PKI certificate for use in IKE's authentication procedures.