Network Working Group R. Salz
Internet-Draft Akamai Technologies
Intended status: Best Current Practice July 08, 2016
Expires: January 9, 2017

No MTI Crypto without Public Review


Cryptography is becoming more important to the IETF and its protocols, and more IETF protocols are using, or looking at, cryptography to increase privacy on the Internet [RFC7258].

This document specifies a proposed best practice for any mechanism (or data format) that uses cryptography; namely, that RFCs cannot specify an algorithm as mandatory-to-implement (MTI) unless that algorithm has had reasonable public review. This document also “sketches out” a rough definition around what such a review would look like.

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This Internet-Draft will expire on January 9, 2017.

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Table of Contents

1. Terminology

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 RFC 2119 [RFC2119].

The term mandatory to implement (MTI) is used in this document to describe a cryptographic algorithm that is listed as a MUST in an RFC.

The term “snake oil” is used as a pejorative for something which appears to do its job acceptably, but actually does not; see It is a goal of the IETF that we never be misled into being, or mistakenly taken as, snake oil salesman.

2. Introduction

Cryptography is becoming more important to the IETF and its protocols, and more IETF protocols are using, or looking at, cryptography to increase privacy on the Internet [RFC7258].

This document specifies a proposed best practice for any protocol (or data format) that uses cryptography. Namely, that such RFCs cannot specify an algorithm as mandatory-to-implement (MTI) unless that algorithm has had reasonable public review. This document also “sketches out” a rough definition around what such a review would look like.

3. Why is Cryptography Hard?

Cryptography is hard because it is not like traditional IETF protocol deployments. In this classic situation, if one party implements a protocol incorrectly, it usually becomes obvious as interoperability suffers or completely fails. But with cryptography, one party can have implementation defects, or known exploitable weaknesses, that expose the entire communication stream to an attacker. Open source and code reviews are not a panacea here, but using only widely-accepted cryptographic mechanisms (e.g., avoiding facilities like will reduce the attack surface.

Cryptography is hard because subtle design characteristics can have disastrous consequences. For example, the US Digital Signature Algorithm requires the random nonce to be protected and never re-used. If those requirements are not met, the private key can be leaked.

Cryptography is hard because adversaries design new attacks and refine existing ones. Attacks get better over time; they never get worse. For example, it is now de riguer to protect against CPU timing attacks, even when the device is only viewable over a network. A recent paper [acoustic] (XXX reference) can identify a private key if your smartphone is just laid next to an innocuous charging device. We understand power differential attacks, timing attacks, and perhaps cache line attacks; we now have to think about RFI emissions from our phone.

Cryptography is hard because the order of operations can matter. It is not intuitively obvious to most developers, which should come first among signing, compression and encryption. This issues was first raised in Spring of 2001 [davis] but was only addressed in TLS by [RFC7366] more than a dozen years later.

Getting the cryptography right is important because the Internet, and therefore the work of the IETF, has become a tempting target for all types of attackers, from individual “script kiddies,” through criminal commercial botnet and phishing ventures, up to national-scale adversaries operating on behalf of their nation-state.

4. Things to avoid

“Sunlight is said to be the best of disinfectants; electric light the most efficient policeman.” – Louis Brandeis, Other People’s Money and How Bankers Use it, first published as a set of articles in Harper’s Weekly in 1914.

Cryptography that is developed in private, such as among an industry consortium is a bad idea. Notable examples of this include:

It is hard to get good public review of patented cryptography, unless there is a strongly compelling need. For example, decades ago RSA was the only practial public-key mechanism available and it was therefore studied pretty extensively.

Part of the concern about patented cryptography is that the patent-holder has every incentive to provide that their system is good, while the rest of the world generally has little interest in proving that their commercial venture is bad. Examples of this include:

Both of these items are “lattice cryptography” and that might also be a reason for lack of review; the field might not have much interest yet.

5. Why limit to MTI?

There is an argument that any new RFC not classified as “historical” should not specify or recommend insufficiently-reviewed cryptography, whether it MTI or not. This document limits itself to MTI for a couple of reasons.

6. How to Do it Right

Cryptographic agility, [RFC7696], is probably a MUST. While it has its detractors, there are no known (to the author) practical considerations to evolving a deployed based to stronger crypto, while still maintaining interoperability with existing entities. This requires being able to make informed choices about when to use old weak crypto, and when to use the “latest and greatest,” and while not much software, and essentially no end-users, are capable of making that choice, it seems sadly the best we can do.

NIST is an important reference for crypto algorithms. Yes, they have made mistakes (DUAL_EC_DRBG), but so has the IETF (opaque-prf) in the same area. But they have run respected international contests and their output receives heavy scrutiny.

The second consideration is to avoid temptation and premature optimization. Do not adopt an algorithm just because it seems “small and fast” or comes from “someone I respect.”

6.1. Public Review

What constitutes sufficient public review? It is hard to say. This section attempts to provide some guidelines.

An open competition, such as those that led to AES (XXX ref) and SHA-3 (XXX ref) seem to be good, even when they come from sources that are under widespread suspicion, like the US Government. These efforts, like the Password Hashing Competition, had wide international participation and analysis by many noted exports.

Papers presented in the various Crypto conferences (XXX need list) are good. Same for various Usenix workshops.

Proof by contest – “Nobody’s Claimed my $200 reward” – are generally useless, for a number of reasons. They tend to be promoted by amateur cryptographers as a way to get attention, and if someone actually looks at them they are always cracked. Numerical analysis is a better approach, albeit much harder work. Contests designed to show the amount of “brute-force” work needed, such as the old RSA factoring challenges, can be useful. But they do not show, for example, if the cryptography under test is fundamentally flawed or not.

Public review is also a natural fit for the IETF, which takes “rough consensus and running code” as an axiom. Theory reduced to practice is much easier, and much less of a limited academic exercise, to review.

7. Acknowledgements

Thanks to Stephen Farrell for instigating this.

8. References

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.

8.2. Informative References

, "
[acoustic] Technion and Tel Aviv University, Weizmann Institute of Science and Tel Aviv University, RSA Key Extraction via Low-Bandwidth Acoustic Cryptanalysis", December 2013.
[davis]Defective Sign & Encrypt in S/MIME, PKCS#7, MOSS, PEM, PGP, and XML.", Usenix Proc. Usenix Tech. Conf., June 2001.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 2014.
[RFC7366] Gutmann, P., "Encrypt-then-MAC for Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", RFC 7366, DOI 10.17487/RFC7366, September 2014.
[RFC7693] Saarinen, M-J. and J-P. Aumasson, "The BLAKE2 Cryptographic Hash and Message Authentication Code (MAC)", RFC 7693, DOI 10.17487/RFC7693, November 2015.
[RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm Agility and Selecting Mandatory-to-Implement Algorithms", BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015.

Author's Address

Rich Salz Akamai Technologies EMail: