Authors:

  1. Stephen Kent, BBN Corporation
  2. Randall Atkinson, @Home Network

Introduction

The Encapsulating Security Payload (ESP) header is designed to provide a mix of security services in IPv4 and IPv6. ESP may be applied alone, in combination with the IP Authentication Header (AH) IP Authentication Header, or in a nested fashion, e.g., through the use of tunnel mode (see "Security Architecture for the Internet Protocol" Security Architecture for the Internet Protocol, hereafter referred to as the Security Architecture document). Security services can be provided between a pair of communicating hosts, between a pair of communicating security gateways, or between a security gateway and a host. For more details on how to use ESP and AH in various network environments, see the Security Architecture document Security Architecture for the Internet Protocol.

The ESP header is inserted after the IP header and before the upper layer protocol header (transport mode) or before an encapsulated IP header (tunnel mode). These modes are described in more detail below.

ESP is used to provide confidentiality, data origin authentication, connectionless integrity, an anti-replay service (a form of partial sequence integrity), and limited traffic flow confidentiality. The set of services provided depends on options selected at the time of Security Association establishment and on the placement of the implementation. Confidentiality may be selected independent of all other services. However, use of confidentiality without integrity/authentication (either in ESP or separately in AH) may subject traffic to certain forms of active attacks that could undermine the confidentiality service (see Steven M. Bellovin, Problem Areas for the IP Security Protocols Proceedings of the Sixth Usenix Unix Security Symposium). Data origin authentication and connectionless integrity are joint services (hereafter referred to jointly as "authentication) and are offered as an option in conjunction with (optional) confidentiality. The anti- replay service may be selected only if data origin authentication is selected, and its election is solely at the discretion of the receiver. (Although the default calls for the sender to increment the Sequence Number used for anti-replay, the service is effective only if the receiver checks the Sequence Number.) Traffic flow confidentiality requires selection of tunnel mode, and is most effective if implemented at a security gateway, where traffic aggregation may be able to mask true source-destination patterns. Note that although both confidentiality and authentication are optional, at least one of them MUST be selected.

It is assumed that the reader is familiar with the terms and concepts described in the Security Architecture document. In particular, the reader should be familiar with the definitions of security services offered by ESP and AH, the concept of Security Associations, the ways in which ESP can be used in conjunction with the Authentication Header (AH), and the different key management options available for ESP and AH. (With regard to the last topic, the current key management options required for both AH and ESP are manual keying and automated keying via IKE The Internet Key Exchange (IKE.)

The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in RFC 2119 Key words for use in RFCs to Indicate Requirement Level.

Encapsulating Security Payload Packet Format

The protocol header (IPv4, IPv6, or Extension) immediately preceding the ESP header will contain the value 50 in its Protocol (IPv4) or Next Header (IPv6, Extension) field Assigned Numbers, See also: http://www.iana.org/numbers.html.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ----
|               Security Parameters Index (SPI)                 | ^Auth.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov-
|                      Sequence Number                          | |erage
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ----
|                    Payload Data* (variable)                   | |   ^
~                                                               ~ |   |
|                                                               | |Conf.
+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov-
|               |     Padding (0-255 bytes)                     | |erage*
+-+-+-+-+-+-+-+-+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |   |
|                               |  Pad Length   | Next Header   | v   v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------
|                 Authentication Data (variable)                |
~                                                               ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        * If included in the Payload field, cryptographic
          synchronization data, e.g., an Initialization Vector (IV, see

          Section 2.3), usually is not encrypted per se, although it
          often is referred to as being part of the ciphertext.

The following subsections define the fields in the header format. "Optional" means that the field is omitted if the option is not selected, i.e., it is present in neither the packet as transmitted nor as formatted for computation of an Integrity Check Value (ICV, see Section 2.7). Whether or not an option is selected is defined as part of Security Association (SA) establishment. Thus the format of ESP packets for a given SA is fixed, for the duration of the SA. In contrast, "mandatory" fields are always present in the ESP packet format, for all SAs.

Security Parameters Index

The SPI is an arbitrary 32-bit value that, in combination with the destination IP address and security protocol (ESP), uniquely identifies the Security Association for this datagram. The set of SPI values in the range 1 through 255 are reserved by the Internet Assigned Numbers Authority (IANA) for future use; a reserved SPI value will not normally be assigned by IANA unless the use of the assigned SPI value is specified in an RFC. It is ordinarily selected by the destination system upon establishment of an SA (see the Security Architecture document for more details). The SPI field is mandatory.

The SPI value of zero (0) is reserved for local, implementation- specific use and MUST NOT be sent on the wire. For example, a key management implementation MAY use the zero SPI value to mean "No Security Association Exists" during the period when the IPsec implementation has requested that its key management entity establish a new SA, but the SA has not yet been established.

Sequence Number

This unsigned 32-bit field contains a monotonically increasing counter value (sequence number). It is mandatory and is always present even if the receiver does not elect to enable the anti-replay service for a specific SA. Processing of the Sequence Number field is at the discretion of the receiver, i.e., the sender MUST always transmit this field, but the receiver need not act upon it (see the discussion of Sequence Number Verification in the "Inbound Packet Processing" section below).

The sender's counter and the receiver's counter are initialized to 0 when an SA is established. (The first packet sent using a given SA will have a Sequence Number of 1; see Section 3.3.3 for more details on how the Sequence Number is generated.) If anti-replay is enabled

(the default), the transmitted Sequence Number must never be allowed to cycle. Thus, the sender's counter and the receiver's counter MUST be reset (by establishing a new SA and thus a new key) prior to the transmission of the 2^32nd packet on an SA.

Payload Data

Payload Data is a variable-length field containing data described by the Next Header field. The Payload Data field is mandatory and is an integral number of bytes in length. If the algorithm used to encrypt the payload requires cryptographic synchronization data, e.g., an Initialization Vector (IV), then this data MAY be carried explicitly in the Payload field. Any encryption algorithm that requires such explicit, per-packet synchronization data MUST indicate the length, any structure for such data, and the location of this data as part of an RFC specifying how the algorithm is used with ESP. If such synchronization data is implicit, the algorithm for deriving the data MUST be part of the RFC.

Note that with regard to ensuring the alignment of the (real) ciphertext in the presence of an IV:

  • o For some IV-based modes of operation, the receiver treats the IV as the start of the ciphertext, feeding it into the algorithm directly. In these modes, alignment of the start of the (real) ciphertext is not an issue at the receiver.

  • o In some cases, the receiver reads the IV in separately from the ciphertext. In these cases, the algorithm specification MUST address how alignment of the (real) ciphertext is to be achieved.

Padding (for Encryption)

Several factors require or motivate use of the Padding field.

  • o If an encryption algorithm is employed that requires the plaintext to be a multiple of some number of bytes, e.g., the block size of a block cipher, the Padding field is used to fill the plaintext (consisting of the Payload Data, Pad Length and Next Header fields, as well as the Padding) to the size required by the algorithm.

  • o Padding also may be required, irrespective of encryption algorithm requirements, to ensure that the resulting ciphertext terminates on a 4-byte boundary. Specifically, the Pad Length and Next Header fields must be right aligned within a 4-byte word, as illustrated in the ESP packet format figure above, to ensure that the Authentication Data field (if present) is aligned on a 4-byte boundary.

  • o Padding beyond that required for the algorithm or alignment reasons cited above, may be used to conceal the actual length of the payload, in support of (partial) traffic flow confidentiality. However, inclusion of such additional padding has adverse bandwidth implications and thus its use should be undertaken with care.

The sender MAY add 0-255 bytes of padding. Inclusion of the Padding field in an ESP packet is optional, but all implementations MUST support generation and consumption of padding.

  • a. For the purpose of ensuring that the bits to be encrypted are a multiple of the algorithm's blocksize (first bullet above), the padding computation applies to the Payload Data exclusive of the IV, the Pad Length, and Next Header fields.

  • b. For the purposes of ensuring that the Authentication Data is aligned on a 4-byte boundary (second bullet above), the padding computation applies to the Payload Data inclusive of the IV, the Pad Length, and Next Header fields.

If Padding bytes are needed but the encryption algorithm does not specify the padding contents, then the following default processing MUST be used. The Padding bytes are initialized with a series of (unsigned, 1-byte) integer values. The first padding byte appended to the plaintext is numbered 1, with subsequent padding bytes making up a monotonically increasing sequence: 1, 2, 3, ... When this padding scheme is employed, the receiver SHOULD inspect the Padding field. (This scheme was selected because of its relative simplicity, ease of implementation in hardware, and because it offers limited protection against certain forms of "cut and paste" attacks in the absence of other integrity measures, if the receiver checks the padding values upon decryption.)

Any encryption algorithm that requires Padding other than the default described above, MUST define the Padding contents (e.g., zeros or random data) and any required receiver processing of these Padding bytes in an RFC specifying how the algorithm is used with ESP. In such circumstances, the content of the Padding field will be determined by the encryption algorithm and mode selected and defined in the corresponding algorithm RFC. The relevant algorithm RFC MAY specify that a receiver MUST inspect the Padding field or that a receiver MUST inform senders of how the receiver will handle the Padding field.

Pad Length

The Pad Length field indicates the number of pad bytes immediately preceding it. The range of valid values is 0-255, where a value of zero indicates that no Padding bytes are present. The Pad Length field is mandatory.

Next Header

The Next Header is an 8-bit field that identifies the type of data contained in the Payload Data field, e.g., an extension header in IPv6 or an upper layer protocol identifier. The value of this field is chosen from the set of IP Protocol Numbers defined in the most recent "Assigned Numbers" Assigned Numbers, See also: http://www.iana.org/numbers.html RFC from the Internet Assigned Numbers Authority (IANA). The Next Header field is mandatory.

Authentication Data

The Authentication Data is a variable-length field containing an Integrity Check Value (ICV) computed over the ESP packet minus the Authentication Data. The length of the field is specified by the authentication function selected. The Authentication Data field is optional, and is included only if the authentication service has been selected for the SA in question. The authentication algorithm specification MUST specify the length of the ICV and the comparison rules and processing steps for validation.

Encapsulating Security Protocol Processing

ESP Header Location

Like AH, ESP may be employed in two ways: transport mode or tunnel mode. The former mode is applicable only to host implementations and provides protection for upper layer protocols, but not the IP header. (In this mode, note that for "bump-in-the-stack" or "bump-in-the- wire" implementations, as defined in the Security Architecture document, inbound and outbound IP fragments may require an IPsec implementation to perform extra IP reassembly/fragmentation in order to both conform to this specification and provide transparent IPsec support. Special care is required to perform such operations within these implementations when multiple interfaces are in use.)

In transport mode, ESP is inserted after the IP header and before an upper layer protocol, e.g., TCP, UDP, ICMP, etc. or before any other IPsec headers that have already been inserted. In the context of

IPv4, this translates to placing ESP after the IP header (and any options that it contains), but before the upper layer protocol. (Note that the term "transport" mode should not be misconstrued as restricting its use to TCP and UDP. For example, an ICMP message MAY be sent using either "transport" mode or "tunnel" mode.) The following diagram illustrates ESP transport mode positioning for a typical IPv4 packet, on a "before and after" basis. (The "ESP trailer" encompasses any Padding, plus the Pad Length, and Next Header fields.)

           BEFORE APPLYING ESP
      ----------------------------
IPv4  |orig IP hdr  |     |      |
      |(any options)| TCP | Data |
      ----------------------------

           AFTER APPLYING ESP
      -------------------------------------------------
IPv4  |orig IP hdr  | ESP |     |      |   ESP   | ESP|
      |(any options)| Hdr | TCP | Data | Trailer |Auth|
      -------------------------------------------------
                          |<----- encrypted ---->|
                    |<------ authenticated ----->|

In the IPv6 context, ESP is viewed as an end-to-end payload, and thus should appear after hop-by-hop, routing, and fragmentation extension headers. The destination options extension header(s) could appear either before or after the ESP header depending on the semantics desired. However, since ESP protects only fields after the ESP header, it generally may be desirable to place the destination options header(s) after the ESP header. The following diagram illustrates ESP transport mode positioning for a typical IPv6 packet.

               BEFORE APPLYING ESP
      ---------------------------------------
IPv6  |             | ext hdrs |     |      |
      | orig IP hdr |if present| TCP | Data |
      ---------------------------------------

               AFTER APPLYING ESP
      ---------------------------------------------------------
IPv6  | orig |hop-by-hop,dest*,|   |dest|   |    | ESP   | ESP|
      |IP hdr|routing,fragment.|ESP|opt*|TCP|Data|Trailer|Auth|
      ---------------------------------------------------------
                                   |<---- encrypted ---->|
                               |<---- authenticated ---->|

          * = if present, could be before ESP, after ESP, or both

ESP and AH headers can be combined in a variety of modes. The IPsec Architecture document describes the combinations of security associations that must be supported.

Tunnel mode ESP may be employed in either hosts or security gateways. When ESP is implemented in a security gateway (to protect subscriber transit traffic), tunnel mode must be used. In tunnel mode, the "inner" IP header carries the ultimate source and destination addresses, while an "outer" IP header may contain distinct IP addresses, e.g., addresses of security gateways. In tunnel mode, ESP protects the entire inner IP packet, including the entire inner IP header. The position of ESP in tunnel mode, relative to the outer IP header, is the same as for ESP in transport mode. The following diagram illustrates ESP tunnel mode positioning for typical IPv4 and IPv6 packets.

      -----------------------------------------------------------
IPv4  | new IP hdr* |     | orig IP hdr*  |   |    | ESP   | ESP|
      |(any options)| ESP | (any options) |TCP|Data|Trailer|Auth|
      -----------------------------------------------------------
                          |<--------- encrypted ---------->|
                    |<----------- authenticated ---------->|

      ------------------------------------------------------------
IPv6  | new* |new ext |   | orig*|orig ext |   |    | ESP   | ESP|
      |IP hdr| hdrs*  |ESP|IP hdr| hdrs *  |TCP|Data|Trailer|Auth|
      ------------------------------------------------------------
                          |<--------- encrypted ----------->|
                      |<---------- authenticated ---------->|

         * = if present, construction of outer IP hdr/extensions
             and modification of inner IP hdr/extensions is
             discussed below.

Algorithms

The mandatory-to-implement algorithms are described in Section 5, "Conformance Requirements". Other algorithms MAY be supported. Note that although both confidentiality and authentication are optional, at least one of these services MUST be selected hence both algorithms MUST NOT be simultaneously NULL.

Encryption Algorithms

The encryption algorithm employed is specified by the SA. ESP is designed for use with symmetric encryption algorithms. Because IP packets may arrive out of order, each packet must carry any data required to allow the receiver to establish cryptographic synchronization for decryption. This data may be carried explicitly in the payload field, e.g., as an IV (as described above), or the data may be derived from the packet header. Since ESP makes provision for padding of the plaintext, encryption algorithms employed with ESP may exhibit either block or stream mode characteristics. Note that since encryption (confidentiality) is optional, this algorithm may be "NULL".

Authentication Algorithms

The authentication algorithm employed for the ICV computation is specified by the SA. For point-to-point communication, suitable authentication algorithms include keyed Message Authentication Codes (MACs) based on symmetric encryption algorithms (e.g., DES) or on one-way hash functions (e.g., MD5 or SHA-1). For multicast communication, one-way hash algorithms combined with asymmetric signature algorithms are appropriate, though performance and space considerations currently preclude use of such algorithms. Note that since authentication is optional, this algorithm may be "NULL".

Outbound Packet Processing

In transport mode, the sender encapsulates the upper layer protocol information in the ESP header/trailer, and retains the specified IP header (and any IP extension headers in the IPv6 context). In tunnel mode, the outer and inner IP header/extensions can be inter-related in a variety of ways. The construction of the outer IP header/extensions during the encapsulation process is described in the Security Architecture document. If there is more than one IPsec header/extension required by security policy, the order of the application of the security headers MUST be defined by security policy.

Security Association Lookup

ESP is applied to an outbound packet only after an IPsec implementation determines that the packet is associated with an SA that calls for ESP processing. The process of determining what, if any, IPsec processing is applied to outbound traffic is described in the Security Architecture document.

Packet Encryption

In this section, we speak in terms of encryption always being applied because of the formatting implications. This is done with the understanding that "no confidentiality" is offered by using the NULL encryption algorithm. Accordingly, the sender:

  • 1. encapsulates (into the ESP Payload field):

    • - for transport mode -- just the original upper layer protocol information.

    • - for tunnel mode -- the entire original IP datagram.

  • 2. adds any necessary padding.

  • 3. encrypts the result (Payload Data, Padding, Pad Length, and Next Header) using the key, encryption algorithm, algorithm mode indicated by the SA and cryptographic synchronization data (if any).

    • - If explicit cryptographic synchronization data, e.g., an IV, is indicated, it is input to the encryption algorithm per the algorithm specification and placed in the Payload field.

    • - If implicit cryptographic synchronication data, e.g., an IV, is indicated, it is constructed and input to the encryption algorithm as per the algorithm specification.

The exact steps for constructing the outer IP header depend on the mode (transport or tunnel) and are described in the Security Architecture document.

If authentication is selected, encryption is performed first, before the authentication, and the encryption does not encompass the Authentication Data field. This order of processing facilitates rapid detection and rejection of replayed or bogus packets by the receiver, prior to decrypting the packet, hence potentially reducing the impact of denial of service attacks. It also allows for the possibility of parallel processing of packets at the receiver, i.e., decryption can take place in parallel with authentication. Note that since the Authentication Data is not protected by encryption, a keyed authentication algorithm must be employed to compute the ICV.

Sequence Number Generation

The sender's counter is initialized to 0 when an SA is established. The sender increments the Sequence Number for this SA and inserts the new value into the Sequence Number field. Thus the first packet sent using a given SA will have a Sequence Number of 1.

If anti-replay is enabled (the default), the sender checks to ensure that the counter has not cycled before inserting the new value in the Sequence Number field. In other words, the sender MUST NOT send a packet on an SA if doing so would cause the Sequence Number to cycle. An attempt to transmit a packet that would result in Sequence Number overflow is an auditable event. (Note that this approach to Sequence Number management does not require use of modular arithmetic.)

The sender assumes anti-replay is enabled as a default, unless otherwise notified by the receiver (see 3.4.3). Thus, if the counter has cycled, the sender will set up a new SA and key (unless the SA was configured with manual key management).

If anti-replay is disabled, the sender does not need to monitor or reset the counter, e.g., in the case of manual key management (see Section 5). However, the sender still increments the counter and when it reaches the maximum value, the counter rolls over back to zero.

Integrity Check Value Calculation

If authentication is selected for the SA, the sender computes the ICV over the ESP packet minus the Authentication Data. Thus the SPI, Sequence Number, Payload Data, Padding (if present), Pad Length, and Next Header are all encompassed by the ICV computation. Note that the last 4 fields will be in ciphertext form, since encryption is performed prior to authentication.

For some authentication algorithms, the byte string over which the ICV computation is performed must be a multiple of a blocksize specified by the algorithm. If the length of this byte string does not match the blocksize requirements for the algorithm, implicit padding MUST be appended to the end of the ESP packet, (after the Next Header field) prior to ICV computation. The padding octets MUST have a value of zero. The blocksize (and hence the length of the padding) is specified by the algorithm specification. This padding is not transmitted with the packet. Note that MD5 and SHA-1 are viewed as having a 1-byte blocksize because of their internal padding conventions.

Fragmentation

If necessary, fragmentation is performed after ESP processing within an IPsec implementation. Thus, transport mode ESP is applied only to whole IP datagrams (not to IP fragments). An IP packet to which ESP has been applied may itself be fragmented by routers en route, and such fragments must be reassembled prior to ESP processing at a receiver. In tunnel mode, ESP is applied to an IP packet, the payload of which may be a fragmented IP packet. For example, a security gateway or a "bump-in-the-stack" or "bump-in-the-wire" IPsec implementation (as defined in the Security Architecture document) may apply tunnel mode ESP to such fragments.

NOTE: For transport mode -- As mentioned at the beginning of Section 3.1, bump-in-the-stack and bump-in-the-wire implementations may have to first reassemble a packet fragmented by the local IP layer, then apply IPsec, and then fragment the resulting packet.

NOTE: For IPv6 -- For bump-in-the-stack and bump-in-the-wire implementations, it will be necessary to walk through all the extension headers to determine if there is a fragmentation header and hence that the packet needs reassembling prior to IPsec processing.

Inbound Packet Processing

Reassembly

If required, reassembly is performed prior to ESP processing. If a packet offered to ESP for processing appears to be an IP fragment, i.e., the OFFSET field is non-zero or the MORE FRAGMENTS flag is set, the receiver MUST discard the packet; this is an auditable event. The audit log entry for this event SHOULD include the SPI value, date/time received, Source Address, Destination Address, Sequence Number, and (in IPv6) the Flow ID.

NOTE: For packet reassembly, the current IPv4 spec does NOT require either the zero'ing of the OFFSET field or the clearing of the MORE FRAGMENTS flag. In order for a reassembled packet to be processed by IPsec (as opposed to discarded as an apparent fragment), the IP code must do these two things after it reassembles a packet.

Security Association Lookup

Upon receipt of a (reassembled) packet containing an ESP Header, the receiver determines the appropriate (unidirectional) SA, based on the destination IP address, security protocol (ESP), and the SPI. (This process is described in more detail in the Security Architecture document.) The SA indicates whether the Sequence Number field will

be checked, whether the Authentication Data field should be present, and it will specify the algorithms and keys to be employed for decryption and ICV computations (if applicable).

If no valid Security Association exists for this session (for example, the receiver has no key), the receiver MUST discard the packet; this is an auditable event. The audit log entry for this event SHOULD include the SPI value, date/time received, Source Address, Destination Address, Sequence Number, and (in IPv6) the cleartext Flow ID.

Sequence Number Verification

All ESP implementations MUST support the anti-replay service, though its use may be enabled or disabled by the receiver on a per-SA basis. This service MUST NOT be enabled unless the authentication service also is enabled for the SA, since otherwise the Sequence Number field has not been integrity protected. (Note that there are no provisions for managing transmitted Sequence Number values among multiple senders directing traffic to a single SA (irrespective of whether the destination address is unicast, broadcast, or multicast). Thus the anti-replay service SHOULD NOT be used in a multi-sender environment that employs a single SA.)

If the receiver does not enable anti-replay for an SA, no inbound checks are performed on the Sequence Number. However, from the perspective of the sender, the default is to assume that anti-replay is enabled at the receiver. To avoid having the sender do unnecessary sequence number monitoring and SA setup (see section 3.3.3), if an SA establishment protocol such as IKE is employed, the receiver SHOULD notify the sender, during SA establishment, if the receiver will not provide anti-replay protection.

If the receiver has enabled the anti-replay service for this SA, the receive packet counter for the SA MUST be initialized to zero when the SA is established. For each received packet, the receiver MUST verify that the packet contains a Sequence Number that does not duplicate the Sequence Number of any other packets received during the life of this SA. This SHOULD be the first ESP check applied to a packet after it has been matched to an SA, to speed rejection of duplicate packets.

Duplicates are rejected through the use of a sliding receive window. (How the window is implemented is a local matter, but the following text describes the functionality that the implementation must exhibit.) A MINIMUM window size of 32 MUST be supported; but a window size of 64 is preferred and SHOULD be employed as the default.

Another window size (larger than the MINIMUM) MAY be chosen by the receiver. (The receiver does NOT notify the sender of the window size.)

The "right" edge of the window represents the highest, validated Sequence Number value received on this SA. Packets that contain Sequence Numbers lower than the "left" edge of the window are rejected. Packets falling within the window are checked against a list of received packets within the window. An efficient means for performing this check, based on the use of a bit mask, is described in the Security Architecture document.

If the received packet falls within the window and is new, or if the packet is to the right of the window, then the receiver proceeds to ICV verification. If the ICV validation fails, the receiver MUST discard the received IP datagram as invalid; this is an auditable event. The audit log entry for this event SHOULD include the SPI value, date/time received, Source Address, Destination Address, the Sequence Number, and (in IPv6) the Flow ID. The receive window is updated only if the ICV verification succeeds.

DISCUSSION:

Note that if the packet is either inside the window and new, or is outside the window on the "right" side, the receiver MUST authenticate the packet before updating the Sequence Number window data.

Integrity Check Value Verification

If authentication has been selected, the receiver computes the ICV over the ESP packet minus the Authentication Data using the specified authentication algorithm and verifies that it is the same as the ICV included in the Authentication Data field of the packet. Details of the computation are provided below.

If the computed and received ICV's match, then the datagram is valid, and it is accepted. If the test fails, then the receiver MUST discard the received IP datagram as invalid; this is an auditable event. The log data SHOULD include the SPI value, date/time received, Source Address, Destination Address, the Sequence Number, and (in IPv6) the cleartext Flow ID.

DISCUSSION:

Begin by removing and saving the ICV value (Authentication Data field). Next check the overall length of the ESP packet minus the Authentication Data. If implicit padding is required, based on

the blocksize of the authentication algorithm, append zero-filled bytes to the end of the ESP packet directly after the Next Header field. Perform the ICV computation and compare the result with the saved value, using the comparison rules defined by the algorithm specification. (For example, if a digital signature and one-way hash are used for the ICV computation, the matching process is more complex.)

Packet Decryption

As in section 3.3.2, "Packet Encryption", we speak here in terms of encryption always being applied because of the formatting implications. This is done with the understanding that "no confidentiality" is offered by using the NULL encryption algorithm. Accordingly, the receiver:

  • 1. decrypts the ESP Payload Data, Padding, Pad Length, and Next Header using the key, encryption algorithm, algorithm mode, and cryptographic synchronization data (if any), indicated by the SA.

    • - If explicit cryptographic synchronization data, e.g., an IV, is indicated, it is taken from the Payload field and input to the decryption algorithm as per the algorithm specification.

    • - If implicit cryptographic synchronization data, e.g., an IV, is indicated, a local version of the IV is constructed and input to the decryption algorithm as per the algorithm specification.

  • 2. processes any padding as specified in the encryption algorithm specification. If the default padding scheme (see Section 2.4) has been employed, the receiver SHOULD inspect the Padding field before removing the padding prior to passing the decrypted data to the next layer.

  • 3. reconstructs the original IP datagram from:

    • - for transport mode -- original IP header plus the original upper layer protocol information in the ESP Payload field

    • - for tunnel mode -- tunnel IP header + the entire IP datagram in the ESP Payload field.

The exact steps for reconstructing the original datagram depend on the mode (transport or tunnel) and are described in the Security Architecture document. At a minimum, in an IPv6 context, the receiver SHOULD ensure that the decrypted data is 8-byte aligned, to facilitate processing by the protocol identified in the Next Header field.

If authentication has been selected, verification and decryption MAY be performed serially or in parallel. If performed serially, then ICV verification SHOULD be performed first. If performed in parallel, verification MUST be completed before the decrypted packet is passed on for further processing. This order of processing facilitates rapid detection and rejection of replayed or bogus packets by the receiver, prior to decrypting the packet, hence potentially reducing the impact of denial of service attacks. Note:

If the receiver performs decryption in parallel with authentication, care must be taken to avoid possible race conditions with regard to packet access and reconstruction of the decrypted packet.

Note that there are several ways in which the decryption can "fail":

  • a. The selected SA may not be correct -- The SA may be mis-selected due to tampering with the SPI, destination address, or IPsec protocol type fields. Such errors, if they map the packet to another extant SA, will be indistinguishable from a corrupted packet, (case c). Tampering with the SPI can be detected by use of authentication. However, an SA mismatch might still occur due to tampering with the IP Destination Address or the IPsec protocol type field.

  • b. The pad length or pad values could be erroneous -- Bad pad lengths or pad values can be detected irrespective of the use of authentication.

  • c. The encrypted ESP packet could be corrupted -- This can be detected if authentication is selected for the SA.,

In case (a) or (c), the erroneous result of the decryption operation (an invalid IP datagram or transport-layer frame) will not necessarily be detected by IPsec, and is the responsibility of later protocol processing.

Auditing

Not all systems that implement ESP will implement auditing. However, if ESP is incorporated into a system that supports auditing, then the ESP implementation MUST also support auditing and MUST allow a system administrator to enable or disable auditing for ESP. For the most part, the granularity of auditing is a local matter. However, several auditable events are identified in this specification and for each of these events a minimum set of information that SHOULD be included in an audit log is defined. Additional information also MAY be included in the audit log for each of these events, and additional

events, not explicitly called out in this specification, also MAY result in audit log entries. There is no requirement for the receiver to transmit any message to the purported sender in response to the detection of an auditable event, because of the potential to induce denial of service via such action.

Conformance Requirements

Implementations that claim conformance or compliance with this specification MUST implement the ESP syntax and processing described here and MUST comply with all requirements of the Security Architecture document. If the key used to compute an ICV is manually distributed, correct provision of the anti-replay service would require correct maintenance of the counter state at the sender, until the key is replaced, and there likely would be no automated recovery provision if counter overflow were imminent. Thus a compliant implementation SHOULD NOT provide this service in conjunction with SAs that are manually keyed. A compliant ESP implementation MUST support the following mandatory-to-implement algorithms:

Since ESP encryption and authentication are optional, support for the 2 "NULL" algorithms is required to maintain consistency with the way these services are negotiated. NOTE that while authentication and encryption can each be "NULL", they MUST NOT both be "NULL".

Security Considerations

Security is central to the design of this protocol, and thus security considerations permeate the specification. Additional security- relevant aspects of using the IPsec protocol are discussed in the Security Architecture document.

Differences from RFC 1827

This document differs from RFC 1827 IP Encapsulating Security Payload (ESP in several significant ways. The major difference is that, this document attempts to specify a complete framework and context for ESP, whereas RFC 1827 provided a "shell" that was completed through the definition of transforms. The combinatorial growth of transforms motivated the reformulation of the ESP specification as a more complete document, with options for security services that may be offered in the context of ESP. Thus, fields previously defined in transform documents are

now part of this base ESP specification. For example, the fields necessary to support authentication (and anti-replay) are now defined here, even though the provision of this service is an option. The fields used to support padding for encryption, and for next protocol identification, are now defined here as well. Packet processing consistent with the definition of these fields also is included in the document.

Acknowledgements

Many of the concepts embodied in this specification were derived from or influenced by the US Government's SP3 security protocol, ISO/IEC's NLSP, or from the proposed swIPe security protocol. [SDNS89, ISO92, IB93].

For over 3 years, this document has evolved through multiple versions and iterations. During this time, many people have contributed significant ideas and energy to the process and the documents themselves. The authors would like to thank Karen Seo for providing extensive help in the review, editing, background research, and coordination for this version of the specification. The authors would also like to thank the members of the IPsec and IPng working groups, with special mention of the efforts of (in alphabetic order): Steve Bellovin, Steve Deering, Phil Karn, Perry Metzger, David Mihelcic, Hilarie Orman, Norman Shulman, William Simpson and Nina Yuan.

Disclaimer

The views and specification here are those of the authors and are not necessarily those of their employers. The authors and their employers specifically disclaim responsibility for any problems arising from correct or incorrect implementation or use of this specification.

References

  1. IP Encapsulating Security Payload (ESP ( ), August 1995
  2. Steven M. Bellovin, Problem Areas for the IP Security Protocols Proceedings of the Sixth Usenix Unix Security Symposium , July 1996
  3. Key words for use in RFCs to Indicate Requirement Level ( , ), March 1997
  4. The Internet Key Exchange (IKE ( ), November 1998
  5. John Ioannidis & Matt Blaze, Architecture and Implementation of Network-layer Security Under Unix Proceedings of the USENIX Security Symposium, Santa Clara, CA , October 1993
  6. ISO/IEC JTC1/SC6, Network Layer Security Protocol, ISO-IEC DIS 11577, International Standards Organisation, Geneva, Switzerland, 29 , November 1992
  7. Security Architecture for the Internet Protocol ( ), November 1998
  8. IP Authentication Header ( ), November 1998
  9. The ESP DES-CBC Cipher Algorithm With Explicit IV ( ), November 1998
  10. The Use of HMAC-MD5-96 within ESP and AH ( ), November 1998
  11. The Use of HMAC-SHA-1-96 within ESP and AH ( ), November 1998
  12. Assigned Numbers, See also: http://www.iana.org/numbers.html ( , ), October 1994
  13. SDNS Secure Data Network System, Security Protocol 3, SP3, Document SDN.301, Revision 1.5, 15, as published in NIST Publication NIST-IR-90-4250, February 1990 , May 1989