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3.4 Key Exchange Payload

3.4 Key Exchange Payload​

The Key Exchange payload, denoted KE in this document, is used to exchange Diffie-Hellman public numbers as part of a Diffie-Hellman key exchange. The Key Exchange payload consists of the IKE generic payload header followed by the Diffie-Hellman public value itself.

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Next Payload  |C|  RESERVED   |         Payload Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Diffie-Hellman Group Num    |           RESERVED            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Key Exchange Data                       ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 10: Key Exchange Payload Format

A Key Exchange payload is constructed by copying one's Diffie-Hellman public value into the "Key Exchange Data" portion of the payload. The length of the Diffie-Hellman public value for MODP groups MUST be equal to the length of the prime modulus over which the exponentiation was performed, prepending zero bits to the value if necessary.

The Diffie-Hellman Group Num identifies the Diffie-Hellman group in which the Key Exchange Data was computed (see Section 3.3.2). This Diffie-Hellman Group Num MUST match a Diffie-Hellman group specified in a proposal in the SA payload that is sent in the same message, and SHOULD match the Diffie-Hellman group in the first group in the first proposal, if such exists. If none of the proposals in that SA payload specifies a Diffie-Hellman group, the KE payload MUST NOT be present. If the selected proposal uses a different Diffie-Hellman group (other than NONE), the message MUST be rejected with a Notify payload of type INVALID_KE_PAYLOAD. See also Sections 1.2 and 2.7.

The payload type for the Key Exchange payload is thirty-four (34).

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