Re: [Cfrg] I-D Action: draft-irtf-cfrg-argon2-02.txt
Dmitry Khovratovich <khovratovich@gmail.com> Mon, 27 March 2017 10:51 UTC
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From: Dmitry Khovratovich <khovratovich@gmail.com>
Date: Mon, 27 Mar 2017 12:51:17 +0200
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Subject: Re: [Cfrg] I-D Action: draft-irtf-cfrg-argon2-02.txt
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Some comments on a new draft:Variants Argon2 fills M bytes of memory in T iterations over it, with M and T being the parameters supplied to Argon2 and determining its performance. Speed on a typical server is linear in the MT product. The Argon2 family has three variants: I, D, and ID, which differ in the way of reusing memory that has been filled. The I variant makes queries with predictable addresses, whereas D determines the addresses on the fly depending on the current state (and thus the password). The ID variant follows I for the first half of the memory used and D for the rest and while overwriting. Side-channels The side-channel attacks, which are of still rising concern in the security community, are applicable to the D variant as the memory addresses and thus information about the password or other secret inputs can be determined from the timing leaks. The I variant is completely invulnerable to this attack, and the ID variant provides only a constant factor improvement for the attacker. Hardware and tradeoffs The M and T parameters determine the cost of bruteforcing passwords on custom hardware, which is proportional to M2T if we follow the traditional time-area product metric. The time-memory tradeoff analysis [2] shows that the bruteforce cost for the I variant can be changed to M2T/Q(M,T) for some quality function Q. For instance, Q(230,1)=5, Q(230,4)=2.5. The D variant is invulnerable to the approach [2], and the savings factor in the ID variant is upper bounded by factor 2 for all parameters. Defender tradeoff and ultimate recommendations In public and private conversations with security architects in the industry we learned that the bottleneck in a system employing the password-hashing function is the function latency rather than memory costs. We then assume that a rational defender would like to maximize the bruteforce costs for the attacker equipped with a list of hashes, salts, and timing information, for fixed computing time on the defender’s machine. In this assumption the defender keeps the MT product constant and maximizes the losses M/Q(M,T). The authors of [2] provides us with attack cost estimates for constant MT = 228,230,232 (measured in iteration-bytes) We ultimately recommend the ID variant with T=1 and maximum M as a default setting for all environments, which is secure against side-channel attacks and prohibit adversarial advantage on dedicated bruteforce hardware. References [1] “Efficiently Computing Data-Independent Memory-Hard Functions” <http://eprint.iacr.org/2016/115.pdf> [2] “Towards Practical Attacks on Argon2i and Balloon Hashing” <http://eprint.iacr.org/2016/759.pdf> On Mon, Mar 27, 2017 at 12:46 PM, <internet-drafts@ietf.org> wrote: > > A New Internet-Draft is available from the on-line Internet-Drafts > directories. > This draft is a work item of the Crypto Forum of the IETF. > > Title : The memory-hard Argon2 password hash and > proof-of-work function > Authors : Alex Biryukov > Daniel Dinu > Dmitry Khovratovich > Simon Josefsson > Filename : draft-irtf-cfrg-argon2-02.txt > Pages : 26 > Date : 2017-03-27 > > Abstract: > This document describes the Argon2 memory-hard function for password > hashing and proof-of-work applications. We provide an implementer > oriented description together with sample code and test vectors. The > purpose is to simplify adoption of Argon2 for Internet protocols. > > > The IETF datatracker status page for this draft is: > https://datatracker.ietf.org/doc/draft-irtf-cfrg-argon2/ > > There are also htmlized versions available at: > https://tools.ietf.org/html/draft-irtf-cfrg-argon2-02 > https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-argon2-02 > > A diff from the previous version is available at: > https://www.ietf.org/rfcdiff?url2=draft-irtf-cfrg-argon2-02 > > > Please note that it may take a couple of minutes from the time of > submission > until the htmlized version and diff are available at tools.ietf.org. > > Internet-Drafts are also available by anonymous FTP at: > ftp://ftp.ietf.org/internet-drafts/ > > _______________________________________________ > Cfrg mailing list > Cfrg@irtf.org > https://www.irtf.org/mailman/listinfo/cfrg > -- Best regards, Dmitry Khovratovich
- [Cfrg] I-D Action: draft-irtf-cfrg-argon2-02.txt internet-drafts
- Re: [Cfrg] I-D Action: draft-irtf-cfrg-argon2-02.… Dmitry Khovratovich
- Re: [Cfrg] I-D Action: draft-irtf-cfrg-argon2-02.… Paterson, Kenny
- Re: [Cfrg] I-D Action: draft-irtf-cfrg-argon2-02.… Blocki, Jeremiah M
- Re: [Cfrg] I-D Action: draft-irtf-cfrg-argon2-02.… Paterson, Kenny
- Re: [Cfrg] I-D Action: draft-irtf-cfrg-argon2-02.… Dmitry Khovratovich