1. Introduction

This document considers how to secure Autocrypt-capable mail apps against active network attackers. Autocrypt aims to achieve convenient end-to-end encryption of e-mail. The Level 1 Autocrypt specification offers users opt-in e-mail encryption, but only considers passive adversaries. Active network adversaries, who could, for example, tamper with the Autocrypt header during e-mail message transport, are not considered in the Level 1 specification. Yet, such active attackers might undermine the security of Autocrypt. Therefore, we present and discuss new ways to prevent and detect active network attacks against Autocrypt-capable mail apps.

We aim to help establish a reverse panopticon: a network adversary should not be able to determine whether peers discover malfeasant manipulations, or even whether they exchange information to investigate attacks. If designed and implemented successfully it means that those who (can) care for detecting malfeasance also help to secure the communications of others in the ecosystem.

This document reflects current research of the NEXTLEAP EU project. The NEXTLEAP project aims to secure Autocrypt beyond Level 1. To this end, this document proposes new Autocrypt protocols that focus on securely exchanging and verifying keys. To design these protocols, we considered usability, cryptographic and implementation aspects simultaneously, because they constrain and complement each other. Some of the proposed protocols are already implemented; we link to the repositories in the appropriate places.

1.1. Attack model and terminology

We consider a network adversary that can read, modify, and create network messages. Examples of such an adversary are an ISP, an e-mail provider, an AS, or an eavesdropper on a wireless network. The goal of the adversary is to i) read the content of messages, ii) impersonate peers – communication partners, and iii) to learn who communicates with whom. To achieve these goals, an active adversary might try, for example, to perform a machine-in-the-middle attack on the key exchange protocol between peers. We consider this approach effective against mass surveillance of the encrypted email content while preventing additional meta data leakage.

To enable secure key-exchange and key-verification between peers, we assume that peers have access to a out-of-band communication channel that cannot be observed or manipulated by the adversary. More concretely we expect them to be able to transfer a small amount of data via a QR-code confidentially.

Targeted attacks on end devices or the out-of-band channels can break our assumptions and therefore the security properties of the protocols described. In particular the ability to observe QR-codes in the scan process (for example through CCTV or by getting access to print outs) will allow impersonation attacks. Additional measures can relax the security requirements for the out-of-band channel to also work under a threat of observation.

Passive attackers such as service providers can still learn who communicates with whom at what time and the approximate size of the messages. We recommend using additional meassures such as encrypting the subject to prevent further data leakage. This is beyond the scope of this document though.

Because peers learn the content of the messages, we assume that all peers are honest. They do not collaborate with the adversary and follow the protocols described in this document.

1.2. Problems of current key-verification techniques

An important aspect of secure end-to-end (e2e) encryption is the verification of a peer’s key. In existing e2e-encrypting messengers, users perform key verification by triggering two fingerprint verification workflows: each of the two peers shows and reads the other’s key fingerprint through a trusted channel (often a QR code show+scan).

We observe the following issues with these schemes:

  • The schemes require that both peers start the verification workflow to assert that both of their encryption keys are not manipulated. Such double work has an impact on usability.
  • In the case of a group, every peer needs to verify keys with each group member to be able to assert that messages are coming from and are encrypted to the true keys of members. A peer that joins a group of size \(N\) must perform \(N\) verifications. Forming a group of size \(N\) therefore requires \(N(N-1) / 2\) verifications in total. Thus this approach is impractical even for moderately sized groups.
  • The verification of the fingerprint only checks the current keys. Since protocols do not store any historical information about keys, the verification can not detect if there was a past temporary MITM-exchange of keys (say the network adversary exchanged keys for a few weeks but changed back to the “correct” keys afterwards).
  • Users often fail to distinguish Lost/Reinstalled Device events from Machine-in-the-Middle (MITM) attacks, see for example When Signal hits the Fan.

1.3. Integrating key verification with general workflows

In Securing communications against network adversaries we describe new protocols that aim to resolve these issues, by integrating key verification into existing messaging use cases:

  • the Setup Contact protocol allows a user, say Alice, to establish a verified contact with another user, say Bob. At the end of this protocol, Alice and Bob know each other’s contact information and have verified each other’s keys. To do so, Alice sends bootstrap data using the trusted out-of-band channel to Bob (for example, by showing QR code). The bootstrap data transfers not only the key fingerprint, but also contact information (e.g., email address). After receiving the out-of-band bootstrap data, Alice’s and Bob’s clients communicate via the regular channel to 1) exchange Bob’s key and contact information and 2) to verify each other’s keys. Note that this protocol only uses one out-of-band message requiring involvement of the user. All other messages are transparent.

  • the Verified Group protocol enables a user to invite another user to join a verified group. The “joining” peer establishes verified contact with the inviter, and the inviter then announces the joiner as a new member. At the end of this protocol, the “joining” peer has learned the keys of all members of the group. This protocol builds on top of the previous protocol. But, this time, the bootstrap data functions as an invite code to the group.

    Any member may invite new members. By introducing members in this incremental way, a group of size \(N\) requires only \(N-1\) verifications overall to ensure that a network adversary can not compromise end-to-end encryption between group members. If one group member loses her key (e.g. through device loss), she must re-join the group via invitation of the remaining members of the verified group.

  • the History verification protocol verifies the cryptograhic integrity of past messages and keys. It can precisely point to messages where cryptographic key information has been modified by the network.

Moreover, in Securing communications against network adversaries we also discuss a privacy issue with the Autocrypt Key gossiping mechanism. The continuous gossipping of keys may enable an observer to infer who recently communicated with each other. We present an “onion-key-lookup” protocol which allows peers to verify keys without other peers learning who is querying a key from whom. Users may make onion key lookups to learn and verify key updates from group members: if a peer notices inconsistent key information for a peer it can send an onion-key query to resolve the inconsistency.

Onion key lookups also act as cover traffic which make it harder for the network to know which user is actually communicating with whom.

1.4. Supplementary key consistency through ClaimChains

We discuss a variant of ClaimChain, a distributed key consistency scheme, in which all cryptographic checks are performed on the end-point side. ClaimChains are self-authenticated hash chains whose blocks contain statements about key material of the ClaimChain owner and the key material of her contacts. The “head” of the ClaimChain, the latest block, represents a commitment to the current state, and the full history of past states.

ClaimChain data structures track all claims about public keys and enable other peers to automatically verify the integrity of claims. ClaimChains include cryptographic mechanisms to ensure the privacy of the claim it stores and the privacy of the user’s social graph. Only authorized users can access the key material and the cross-references being distributed. In other words, neither providers nor unauthorized users can learn anything about the key material in the ClaimChain and the social graph of users by just observing the data structure.

Private claims could be used by malicious users (or a network adversary who impersonates users) to equivocate, i.e., present a different view of they keys they have seen to their peers. For example, Alice could try to equivocate by showing different versions of a cross-reference of Bob’s key to Carol and Donald. Such equivocations would hinder the ability to resolve correct public keys. Therefore, ClaimChain prevents users (or a network adversaries) from equivocating to other users about their cross-references.

The implementation of ClaimChains considered in this document relies on a self-authenticating storage which, given a hash, replies with a matching data block. We suggest that providers provide a “dumb” block storage for their e-mail customers, re-using existing authentication techniques for guarding writes to the block storage. The head hashes that allow to verify a full chain are distributed along with Autocrypt Gossip headers. Given a head, peers can verify that a chain has not been tampered with and represents the latest belief of another peer. Peers can use the information in the chain to perform consistency checks.

ClaimChain permits users to check the evolution of others’ keys over time. If inspection of the Claimchains reveals inconsistencies in the keys of a peer – for example, because an adversary tampered with the keys – the AutoCrypt client can advice the user to run the History-verification protocol with this inconsistent peer. This protocol will then reveal conclusive evidence of malfeasance.

1.5. Detecting inconsistencies through Gossip and DKIM

The protocols for key verification and key inconsistency aid to detect malfeasance. However, even if they were not added, mail apps can use existing Autocrypt Level 1 Key Gossip and DKIM signatures to detect key inconsistencies.

Key inconsistencies or broken signatures found using these methods can not be interpreted unequivocally as proof of malfeasance. Yet, mail apps can track such events and provide recommendations to users about “Who is the most interesting peer to verify keys with?” so as to detect real attacks.

We note that if the adversary isolates a user by consistently injecting MITM-keys on her communications, the adversary can avoid the “inconsistency detection” via Autocrypt’s basic mechanisms. However, any out-of-band key-history verification of that user will result in conclusive evidence of malfeasance.