Re: [openpgp] Mitigation of Attacks on Email End-to-End Encryption
Marcus Brinkmann <marcus.brinkmann@rub.de> Tue, 03 November 2020 17:02 UTC
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To: Stephen Farrell <stephen.farrell@cs.tcd.ie>, openpgp@ietf.org
Cc: Jörg Schwenk <joerg.schwenk@rub.de>, Jens Müller <jens.a.mueller@rub.de>, Sebastian Schinzel <schinzel@fh-muenster.de>, Damian Poddebniak <poddebniak@fh-muenster.de>, Juraj Somorovsky <juraj.somorovsky@rub.de>
References: <80af2d9c-f646-4167-209d-5fb35c880682@rub.de> <81da533b-094f-b16c-a71b-927441e8c18b@cs.tcd.ie>
From: Marcus Brinkmann <marcus.brinkmann@rub.de>
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Date: Tue, 03 Nov 2020 18:02:37 +0100
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Subject: Re: [openpgp] Mitigation of Attacks on Email End-to-End Encryption
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Hi, sorry, I did not test the DOI. The URL is: https://dl.acm.org/doi/10.1145/3372297.3417878 The PDF is at: https://dl.acm.org/doi/pdf/10.1145/3372297.3417878 Thanks, Marcus On 11/3/20 5:35 PM, Stephen Farrell wrote: > > Hiya, > > Interesting work and I look forward to seeing your > suggestions as we go. > > One thing: the DOI URL below gets me a "not found" > error but indicates that may change in future. Is > that correct? If so, be great if you could send a > ping when the paper is available (or another URL > to a preprint or whatever). > > Thanks, > S. > > > On 03/11/2020 16:24, Marcus Brinkmann wrote: >> Hello, >> >> we have just published a paper on our research how to mitigate attacks >> on email end-to-end encryption. The full paper is available here with >> open access: >> >> Jörg Schwenk, Marcus Brinkmann, Damian Poddebniak, Jens Müller, Juraj >> Somorovsky, and Sebastian Schinzel. 2020. Mitigation of Attacks on Email >> End-to-End Encryption. In Proceedings of the 2020 ACM SIGSAC Conference >> on Computer and Communications Security (CCS '20). Association for >> Computing Machinery, New York, NY, USA, 1647–1664. >> DOI:https://doi.org/10.1145/3372297.3417878 >> >> We have analyzed three attack vectors: EFAIL malleability gadgets (MG), >> EFAIL direct exfiltration (DE), and REPLY attacks. >> >> MG attacks exploit unauthenticated block cipher modes such as CBC and >> CFB, and are mitigated by using an authenticated encryption mode such as >> AEAD, or by a strict implementation of OpenPGP's modification detection >> code. S/MIME 4.0 and OpenPGP RFC4880bis have added AEAD encryption >> modes, and assuming a strict implementation, can also protect against MG >> attacks that way. (Currently, RFC4880bis does allow unsafe >> implementations of AEAD, and even encourages them due to unrestricted >> chunk sizes. This has been previously discussed, and I will re-raise >> this issue when the WG has been reinstantiated). >> >> EFAIL DE attacks rely on modifications of the MIME structure to embed >> authentic ciphertexts in a context that allows exfiltration of the >> plaintext after decryption, for example through image source URLs in a >> HTML MIME element before the ciphertext. These attacks have also been >> published in the EFAIL paper, and so far were only mitigated at the >> recipient side by ad-hoc measures in email clients. Our experience was >> that developers were struggling to mitigate these attacks. For example, >> we found several bypasses after attempts at mitigation. >> >> REPLY attacks are known for 20+ years: They rely on modifications of the >> email header (SMTP) context, that allow the attacker to receive replies >> to authentic ciphertexts, were the victim quotes the plaintext back to >> the attacker. To our knowledge, these attacks have not been mitigated so >> far. We have looked at reply attacks in our paper on covert content >> attacks [COVERT]. >> >> We have looked systematically at these issues, and propose to protect >> the MIME and SMTP context of an email by adding a summary of this >> decryption contexts (DC) as associated data (AD) in the AEAD encryption. >> This way, any significant modification to these contexts that indicate >> an attack would lead to a decryption error, rather than emitting the >> plaintext to the application, where it would be subject to a large >> attack surface to launch DE or REPLY attacks. >> >> To support this mechanism, OpenPGP RFC4880bis would need to be amended >> to allow applications to add arbitrary data to the AD, either directly >> (length+value) or by adding a hash representation (constant length). I >> plan to introduce a proposal for these changes when the WG is >> reinstantiated. >> >> We have evaluated which SMTP headers are relevant for REPLY actions in >> email clients, and which MIME contexts can be considered safe. Based on >> this (and inspired by DKIM), we make a specific proposal for calculating >> the decryption context which is sender-enforced and extensible. >> >> We have implemented this solution with GnuPG and Thunderbird/Enigmail. >> It was easy to implement, offered excellent compatibility (low false >> positive rate when detecting attacks) and mitigated all REPLY and DE >> attacks conclusively without introducing new cryptographic primitives. >> >> I hope these findings are interesting to the OpenPGP community. For >> example, some of the problems described in >> >> https://www.ietf.org/archive/id/draft-dkg-lamps-e2e-mail-guidance-00.txt >> >> can be mitigated using our techniques. Also, the REPLY action behavior >> of email clients in our evaluation should be useful for the memory hole >> project, for example. >> >> Thanks, >> Marcus >> >> >> Bibliography: >> >> [COVERT] Jens Müller, Marcus Brinkmann, Damian Poddebniak, Sebastian >> Schinzel, and Jörg Schwenk. 2019. Re: What’s Up Johnny? – Covert Content >> Attacks on Email End-to-End Encryption. >> https://arxiv.org/ftp/arxiv/papers/1904/1904.07550.pdf. >> >> [EFAIL] Damian Poddebniak, Christian Dresen, Jens Müller, Fabian Ising, >> Sebastian Schinzel, Simon Friedberger, Juraj Somorovsky, and Jörg >> Schwenk. 2018. Efail: Breaking S/MIME and OpenPGP Email Encryption using >> Exfiltration Channels. In 27th USENIX Security Symposium, USENIX >> Security 2018, Baltimore, MD, USA, August 15-17, 2018., William Enck and >> Adrienne Porter Felt (Eds.). USENIX Association, 549–566. >> https://www.usenix.org/conference/usenixsecurity18/presentation/poddebniak >> >> -- Dipl.-Math. Marcus Brinkmann Lehrstuhl für Netz- und Datensicherheit Ruhr Universität Bochum Universitätsstr. 150, Geb. ID 2/461 D-44780 Bochum Telefon: +49 (0) 234 / 32-25030 http://www.nds.rub.de/chair/people/mbrinkmann
- [openpgp] Mitigation of Attacks on Email End-to-E… Marcus Brinkmann
- Re: [openpgp] Mitigation of Attacks on Email End-… Stephen Farrell
- Re: [openpgp] Mitigation of Attacks on Email End-… Marcus Brinkmann
- Re: [openpgp] Mitigation of Attacks on Email End-… Benjamin Kaduk
- Re: [openpgp] Mitigation of Attacks on Email End-… Marcus Brinkmann
- Re: [openpgp] Mitigation of Attacks on Email End-… Benjamin Kaduk
- Re: [openpgp] Mitigation of Attacks on Email End-… Daniel Kahn Gillmor