Last Call Review of draft-ietf-core-coap-14
I have reviewed this document as part of the security directorate's
ongoing effort to review all IETF documents being processed by the IESG.
These comments were written primarily for the benefit of the security
area directors. Document editors and WG chairs should treat these
comments just like any other last call comments.
This document defines the Constrained Application Protocol (CoAP), which
is a specialized web transfer protocol (basically a binary protocol that
provides a subset of HTTP functionality) for use with constrained nodes
and constrained (e.g., low-power, lossy) networks. The nodes often have
8-bit microcontrollers with small amounts of ROM and RAM, while
constrained networks such as 6LoWPAN often have high packet error rates
and a typical throughput of 10s of kbit/s. The protocol is designed for
machine-to-machine applications such as smart energy and building
The Security Consideration points to Section 15 of RFC 2616 (among other
things). I think this is quite appropriate, as RFC 2616 covers lots of
relevant issues, including disclosure of sensitive information.
The Security Consideration section correctly points out
that Protocol Parsing complexity can lead to vulnerabilities,
in particular parsing/processing of URIs.
Section 11.3 talks about risk of amplification attacks (causing bigger
packets to be sent to a victim based on smaller packets sent by an
attacker) and possible mitigations. Section 11.4 talks about IP Address
Spoofing Attacks (message spoofing, making endpoints "deaf", cache
poisoning, etc.). Section 11.2 talks about Proxying and Caching attacks
(Denial-of-service, threat to confidentiality and integrity of
request/response data). Many mitigation techniques depend on use of DTLS
(modes other than NoSec), but this is fine, as one of DTLS modes is
mandatory to implement for all compliant CoAP nodes.
I appreciated text describing possible cross protocol attacks, in
particular when DNS packets are sent to a CoAP endpoint.
Section 9 (Securing CoAP) talks about several modes of securing CoAP
with DTLS. It talks about how certificate verification should be done
and provisioning of raw public keys. These sections seem to be well
written and sufficiently detailed to implement.
Overall I think that the document has very good and detailed security
184.108.40.206. Raw Public Key Certificates
In this mode the device has an asymmetric key pair but without an
X.509 certificate (called a raw public key). A device MAY be
configured with multiple raw public keys. The type and length of the
raw public key depends on the cipher suite used. Implementations in
RawPublicKey mode MUST support the mandatory to implement cipher
suite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 as specified in
It looks like this reference is Normative, while you have it as Informative.
[RFC5246], [RFC4492]. The mechanism
for using raw public keys with TLS is specified in
The RawPublicKey mode was designed to be easily provisioned in M2M
deployments. It is assumed that each device has an appropriate
asymmetric public key pair installed. An identifier is calculated
from the public key as described in Section 2 of
[I-D.farrell-decade-ni]. All implementations that support checking
RawPublicKey identities MUST support at least the sha-256-120 mode
(SHA-256 truncated to 120 bits). Implementations SHOULD support also
I think you need a Normative reference to SHA-256 here.
longer length identifiers and MAY support shorter lengths. Note that
the shorter lengths provide less security against attacks and their
use is NOT RECOMMENDED.
P.S. I have some Apps specific issues with the document, but I send
these separately in my AppsDir review.