| IPSECME | D. Migault, Ed. |
| Internet-Draft | Ericsson |
| Intended status: Standards Track | T. Guggemos, Ed. |
| Expires: December 11, 2016 | LMU Munich |
| Y. Nir | |
| Check Point | |
| June 9, 2016 |
Implicit IV for Counter-based Ciphers in IPsec
draft-mglt-ipsecme-implicit-iv-00.txt
IPsec ESP sends an initialization vector (IV) or nonce in each packet, adding 8 or 16 octets. Some algorithms such as AES-GCM, AES-CCM, AES-CTR and ChaCha20-Poly1305 require a unique nonce but do not require an unpredictable nonce. When using such algorithms the packet counter value can be used to generate a nonce, saving 8 octets per packet. This document describes how to do this.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 11, 2016.
Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
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 [RFC2119].
Counter-based AES modes of operation such as AES-CTR ([RFC3686]), AES-CCM ([RFC4309]), and AES-GCM ([RFC4104]) require the specification of an nonce for each ESP packet. The same applies for ChaCha20-Poly1305 ([RFC7634]. Currently this nonce is sent in each ESP packet ([RFC4303]). 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 compute 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 to negotiate this behavior within the Internet Key Exchange version 2 (IKEv2 - [RFC7296]).
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 ([RFC3602])as AES-CBC requires the IV to be unpredictable. Deriving it directly from the packet counter as described below is insecure.
With the algorithms listed in Section 2, 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. Figure 1 and Figure 2 represent the IV with a regular 4-byte Sequence Number and with an 8-byte Extended Sequence Number respectively.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Zero |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Implicit IV with a 4 byte Sequence Number
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended |
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Implicit IV with an 8 byte Extended Sequence Number
NOTE: THIS SECTION SHOWS SEVERAL WAYS TO DO THE SAME THING. OBVIOUSLY THIS DOCUMENT WILL NOT BE PUBLISHED LIKE THAT. WE EXPECT WG DISCUSSION TO PARE THIS DOWN TO JUST ONE WAY OF NEGOTIATING THE USE OF IMPLICIT IV (IIV).
Negotiation of the use of implicit IV can be done in 3 different ways:
An initiator supporting this feature SHOULD propose implicit IV for all relevant algorithms. To facilitate backward compatibility with non-supporting peers the initiator SHOULD also include those same algorithms without IIV. Depending on the method chosen in Section 5 this may require extra proposals or extra transforms.
An responder supporting this feature SHOULD accept implicit IV for all relevant algorithms. It MUST NOT accept implicit IV for algorithms not specified to be safe for IIV in this or subsequent documents. It SHOULD accept non-IIV proposals for all algorithms if IIV proposals were not included.
The rules of SA payload processing ensure that the responder will never send an SA payload containing the IIV indicator to an initiator that does not support IIV.
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. 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.
TBD: The content of this section depends on our decisions about Section 5.
The following Transform Type is requested: IMPLICIT_IV Transform Type
Values associated to IMPLICIT Transform Type are: