What are the privacy differences between end-to-end encryption in ProtonMail web versus Bridge+local mail clients?
Executive summary
Proton’s web and mobile clients perform end-to-end (zero‑access) encryption for message bodies between Proton accounts; Bridge claims to provide the same end‑to‑end encryption while integrating with local IMAP/SMTP clients, but it introduces local storage and client‑side exposure risks — Bridge stores decrypted keys in memory and may leave messages on disk depending on the desktop mail app [1] [2] [3]. Proton’s documentation and reviews note subject lines are not E2EE on Proton (web) and Bridge follows Proton’s models for interoperability, so subject metadata remains exposed to servers or legal process [4] [1].
1. What Proton says about end‑to‑end parity: same crypto, different execution
Proton’s support pages state messages exchanged between Proton Mail accounts via Bridge are “end‑to‑end encrypted just like they are in our web and mobile apps,” and Bridge adds zero‑access encryption while letting desktop clients use IMAP/SMTP [2] [5]. The company positions Bridge as providing equivalent cryptographic protections to the web client for Proton‑to‑Proton messages, while translating Proton’s encryption into formats desktop clients can use [6] [5].
2. Local client changes the threat model: decrypted data, client discretion
Proton’s own documentation warns that Bridge “stores the emails on your device in an encrypted format. However, the email client you are using can store the emails locally unencrypted.” That means although Bridge holds keys and performs decryption, a traditional mail app (Outlook, Thunderbird, Apple Mail) may cache or index plaintext on disk, exposing messages to local attackers or forensic seizure even if server‑side access remains zero‑knowledge [1] [6].
3. Key handling and memory tradeoffs: Bridge’s security design
Proton describes Bridge as downloading encrypted private PGP keys, unlocking them in memory, and never storing those keys on disk; keys live in device memory while the user is logged in and are wiped on shutdown [3]. That model reduces persistent key leakage risk, but it still requires trusting the local machine’s runtime environment — malware or improper client configuration can capture decrypted messages while Bridge holds keys in RAM [3] [6].
4. Metadata and subject lines: an unencrypted leakage common to both modes
Independent reviews and Proton material call out that subject lines are not end‑to‑end encrypted on Proton Mail; reviewers note Proton can turn over subject lines under legal process. Bridge’s interoperability preserves Proton’s behavior and therefore does not change that metadata exposure — subject lines remain a weak point whether you use web or Bridge [4] [1].
5. Sending to non‑Proton recipients: different UX and protections
Proton’s web and mobile clients support password‑protected messages for non‑Proton recipients and will use PGP when configured; Bridge will use configured PGP settings for external recipients, but practical usability differs: web clients offer built‑in password flows while Bridge relies on the desktop client’s PGP setup and storage practices [1] [7]. Reviews emphasize the web UX redirects non‑Proton recipients through Proton’s web unlock flow, a behavior Bridge may not replicate automatically [7] [1].
6. Operational considerations: convenience vs. attack surface
Proton frames Bridge as a convenience that “adds end‑to‑end encryption to popular email apps” and supports TLS+certificate pinning back to Proton servers [6] [3]. Reviews and guides caution that integrating into a general‑purpose mail client increases attack surface (local indexing, client plugins, system backups) compared with the sandboxed web/mobile apps where Proton controls client code and ephemeral decryption contexts [7] [6].
7. Bottom line and user guidance
If your primary worry is server‑side access, Proton’s web/mobile and Bridge aim to provide equivalent E2EE between Proton accounts [2] [1]. If you are concerned about local compromise, forensic seizure, or unencrypted client caches, Bridge materially changes privacy: it decrypts on your machine and lets third‑party clients potentially persist plaintext [1] [3]. Available sources do not mention third‑party plugin risks or platform‑specific backup behaviors in detail; for those gaps consult Proton’s current docs and client vendor guidance (not found in current reporting).
Limitations: this analysis relies on Proton’s published statements and secondary reviews in the provided results; technical audits or independent tests beyond these sources are not reported here [3] [4].