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Integrity Check Value Calculation
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The ICV, a truncated version of a message authentication code (MAC), is calculated by a MAC algorithm. IPSec requires that all implementations support at least HMAC-MD5 and HMAC-SHA1 (the HMAC symmetric authentication scheme supported by MD5 or SHA-1 hashes; see 6. To guarantee minimal interoperability, an IPSec implementation must support at least these schemes.
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Network and Transport Security Protocols
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The ICV is computed using the following fields:
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The IP header fields that either do not change in transit or whose values are predictable upon arrival at the endpoint for the AH security association. Other fields are set to zero for the purpose of calculation. The entire contents of the AH header except for the Authentication Data field. The Authentication Data field is set to zero for the purpose of calculation. All upper-level protocol data, which is assumed to be immutable in transit.
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The HMAC value is calculated completely, although it is truncated to 96 bytes (the default size for the Authentication Data field).
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Transport and Tunnel Modes
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AH services can be employed in two ways: in transport mode or in tunnel mode. The actual placement of the AH depends on which mode is used and on whether the AH is being applied to an IPv4 or an IPv6 packet. Figure 7-3 illustrates IPv4 and IPv6 packets before authentication services are applied. In transport mode, the AH applies only to host implementations and provides protection for upper-layer protocols in addition to selected IP header fields. In this mode, AH is inserted after the IP header but before
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Figure 7-3 IPv4 and IPv6 before AH is applied
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any upper-layer protocol (such as, TCP, UDP) and before any other IPSec headers that have already been inserted. In IPv4, this calls for placing AH after the original IP header but before the upper-layer protocol. In IPv6, AH is viewed as an end-to-end payload; this means that intermediate routers should not process it. For this reason, the AH should appear after the original IP header, hop-by-hop, routing, and fragmentation extension headers. This mode is provided via the transport security association (SA). Figure 7-4 illustrates the AH transport mode positioning in typical IPv4 and IPv6 packets.
Figure 7-4 IPv4 and IPv6 header placement in transport mode
In tunnel mode, the AH can be employed in either host or security gateways. When AH is implemented in a security gateway (to protect transit traffic), tunnel mode must be used. In this mode, the AH is inserted between the original IP header and the new outer IP header. Whereas the inner IP header carries the ultimate source and destination addresses, the new outer IP header may contain distinct IP addresses (such as, addresses of firewalls or other security gateways). In tunnel mode, AH protects the entire inner IP packet, including the entire inner IP header. In tunnel mode, the position of AH relative to the outer IP header is the same as for AH in transport mode. This mode is provided via the tunnel SA. Figure 7-5 illustrates AH tunnel mode positioning for typical IPv4 and IPv6 packets.
NOTE:
ESP and AH headers can be combined in a variety of modes. The IPSec architecture document (RFC2401) describes the combinations of security associations that must be supported.
Network and Transport Security Protocols
Figure 7-5 IPv4 and IPv6 header placement in tunnel mode
The Encapsulating Security Payload Protocol
The encapsulating security payload (ESP) protocol provides confidentiality services for IP data while in transit across untrusted networks. Optionally, ESP also provides authentication services. The format of ESP varies according to the type and mode of the encryption being used. In all cases the key associated with the encryption is selected using the SPI. Figure 7-6 illustrates the components of an ESP header.
Figure 7-6 Components of an ESP header
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The ESP header contains the following fields:
Security Parameters Index This field, as in the AH packet, is used to help uniquely identify a security association to be used. Sequence Number This field, again as in the AH packet, contains a counter that increases each time a packet is sent to the same address using the same SPI. It lets the recipient keep track of the packet order. Payload Data This variable-length field contains the actual encrypted data contents being carried by the IP packet. Padding This field provides space for adding bytes, as required by certain types of encryption algorithms (see 2). Data padding confuses sniffers, who try to access information about encrypted data in transit, in this case by trying to estimate how much data is being transmitted. Pad Length This field identifies how much of the encrypted payload is padding. Next Header This field identifies the type of data carried in the Payload Data field. Authentication Data This variable-length field contains a value that represents the ICV computed over the ESP packet minus the Authentication Data field. This field is optional and is included only if the authentication service is selected within the SA.
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