2.10. Nonces

2.10. Nonces

The IKE_SA_INIT messages each contain a nonce. These nonces are used as inputs to cryptographic functions. The CREATE_CHILD_SA request and the CREATE_CHILD_SA response also contain nonces. These nonces are used to add freshness to the key derivation technique used to obtain keys for Child SA, and to ensure creation of strong pseudorandom bits from the Diffie-Hellman key. Nonces used in IKEv2 MUST be randomly chosen, MUST be at least 128 bits in size, and MUST be at least half the key size of the negotiated pseudorandom function (PRF). However, the initiator chooses the nonce before the outcome of the negotiation is known. Because of that, the nonce has to be long enough for all the PRFs being proposed. If the same random number source is used for both keys and nonces, care must be taken to ensure that the latter use does not compromise the former.