Implicit IV for Counter-based Ciphers in Encapsulating Security Payload (ESP)

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

Counter-based AES modes of operation such as AES-CTR (), AES-CCM (), and AES-GCM () require the specification of an nonce for each ESP packet. The same applies for ChaCha20-Poly1305 (). Currently this nonce is sent in each ESP packet (). This practice is designated in this document as "explicit nonce". In some context, such as IoT, it may be preferable to avoid carrying the extra bytes associated to the IV and instead generate it locally on each peer. The local generation of the nonce is designated in this document as "implicit IV". The size of this nonce depends on the specific algorithm, but all of the algorithms mentioned above take an 8-octet nonce. This document defines how to compute the nonce locally when it is implicit. It also specifies how peers agree with the Internet Key Exchange version 2 (IKEv2 - ) on using an implicit IV versus an explicit IV. This document limits its scope to the algorithms mentioned above. Other algorithms with similar properties may later be defined to use this extension. This document does not consider AES-CBC () as AES-CBC requires the IV to be unpredictable. Deriving it directly from the packet counter as described below is insecure as mentioned in Security Consideration of and has led to real world chosen plain-text attack such as BEAST .

IoT: Internet of Things. IV: Initialization Vector. IIV: Implicit Initialization Vector. Nonce: a fixed-size octet string used only once. This is similar to IV, except that in common usage there is no implication of non-predictability.

With the algorithms listed in , the 8 byte nonce MUST NOT repeat. The binding between a ESP packet and its nonce is provided using the Sequence Number or the Extended Sequence Number. and represent the IV with a regular 4-byte Sequence Number and with an 8-byte Extended Sequence Number respectively.

Sequence Number: the 4 byte Sequence Number carried in the ESP packet. Zero: a 4 byte array with all bits set to zero.

Extended Sequence Number: the 8 byte Extended Sequence Number of the Security Association. The 4 byte low order bytes are carried in the ESP packet. As the IV MUST NOT repeat for one SA when Counter-Mode ciphers are used, Implicit IV as described in this document MUST NOT be used in setups with the chance that the Sequence Number overlaps for one SA. Multicast as described in , and is a prominent example, where many senders share one secret and thus one SA. Section 3.5 of provides a mechanism that MAY be used to prevent IV collisions when the same key is used by multiple users. The mechanism consists in partitioning the IV space between users by assigning the most significant byte to a user. When implicit IV transforms are used, such mechanism cannot be applied as the IV is not sent, but instead it is derived from the Sequence Number. A similar mechanism could be used by associating the most significant byte of the Sequence Number to a sender, while the 3 remaining bytes will be used to carry the counter value. Such mechanism prevents the use of Extended Sequence Number and limits the number of packet to be sent to 2** 24 = 16777216, that is 16 M. Note that associating instead the least significant byte of the Sequence Number to the sender, would enable the system to use Extended Sequence Number and as such extend the limit of packet to be sent to 2 ** ( 24 + 32 ) = 72057594037927936, that is 72 P. Note also that in both cases the Sequence Number are not interpreted as numeric values which impacts the replay window processing defined in and . Unless some mechanism are provided to avoid collision between Sequence Number, ( and so IV ), Implicit IV MUST NOT be used. As such, it is NOT RECOMMENDED to use Implicit IV with Multicast.

An initiator supporting this feature SHOULD propose implicit IV algorithms in the Transform Type 1 (Encryption Algorithm) Substructure of the Proposal Substructure inside the SA Payload. To facilitate backward compatibility with non-supporting peers the initiator SHOULD also include those same algorithms without Implicit IV (IIV) as separate transforms.

The rules of SA Payload processing require that responder picks its algorithms from the proposal sent by the initiator, thus this will ensure that the responder will never send an SA payload containing the IIV transform to an initiator that did not propose it.

Nonce generation for these algorithms has not been explicitly defined. It has been left to the implementation as long as certain security requirements are met. Typically, for AES-GCM, AES-CCM, AES-CTR and ChaCha20-Poly1305, the IV is not allowed being repeated for one particular key. This document provides an explicit and normative way to generate IVs. The mechanism described in this document meets the IV security requirements of all relevant algorithms.

This section assigns new code points to the recommended AEAD suites provided in , thus the new Transform Type 1 - Encryption Algorithm Transform IDs are as defined below: ENCR_AES_CCM_8_IIV: 29 ENCR_AES_GCM_16_IIV: 30 ENCR_CHACHA20_POLY1305_IIV: 31 These algorithms should be added with this document as ESP Reference and "Not Allowed" for IKEv2 Reference.

We would like to thanks people Valery Smyslov for their valuable comments, David Schinazi for its implementation, as well as the ipseceme chairs Tero Kivinen and David Waltermire for moving this work forward.