TextSecure

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TextSecure
TextSecure Blue Icon.png
TextSecure authentication.pngTextSecure conversation list.pngTextSecure conversation.png
Screenshots of TextSecure
Original author(s) Moxie Marlinspike
and Stuart Anderson
(Whisper Systems)
Developer(s) Open Whisper Systems
Initial release May 2010 (2010-05)[1]
Stable release Android 2.17.0 (June 15, 2015; 1 day ago (2015-06-15)) [±]
Development status Active
Written in Java (client and server)
Operating system Android
Size 9.0 MB
Available in 34 languages[2]
Type Encrypted instant messaging and unencrypted SMS/MMS messaging
License GPLv3 (client),[3]
AGPLv3 (server)[4]
Website whispersystems.org

TextSecure is an advanced end-to-end encryption protocol as well as a free and open-source encrypted instant messaging application for Android which uses that protocol. TextSecure enables the secure transmission of instant messages, group messages, attachments and media messages to other TextSecure users. Users can independently verify the identity of their correspondents by comparing key fingerprints out-of-band or by scanning QR codes in person. The Android application can function as a drop-in replacement for Android's native messaging application as it can also fall back to sending unencrypted SMS and MMS messages. The local message database can be encrypted with a passphrase.

TextSecure messages are compatible with Signal messages on iOS. TextSecure and Signal are developed by Open Whisper Systems and are published under the GPLv3 license.

History[edit]

TextSecure's icon from May 2010 to February 2014 and from February 2014 to February 2015.

Whisper Systems and Twitter (2010–2011)[edit]

TextSecure started as an application for sending and receiving encrypted SMS messages.[5] Its beta version was first launched in May 2010 by Whisper Systems,[6] a startup company co-founded by security researcher Moxie Marlinspike and roboticist Stuart Anderson.[7][8] In addition to launching TextSecure, Whisper Systems produced a firewall, tools for encrypting other forms of data, and RedPhone, an application that provides encrypted voice calls.[1][7] All of these were proprietary enterprise mobile security software.

In November 2011, Whisper Systems announced that it had been acquired by Twitter. The financial terms of the deal were not disclosed by either company.[9] Shortly after the acquisition, Whisper Systems' RedPhone service was made unavailable.[10] Some criticized the removal, arguing that the software was "specifically targeted [to help] people under repressive regimes" and that it left people like the Egyptians in "a dangerous position" during the events of the 2011 Egyptian revolution.[11]

Twitter released TextSecure as free and open-source software under the GPLv3 license in December 2011.[7][12][13][14] RedPhone was also released under the same license in July 2012.[15] Marlinspike later left Twitter and founded Open Whisper Systems[16] as a collaborative Open Source project for the continued development of TextSecure and RedPhone.[17]

Open Whisper Systems (2013–present)[edit]

Open Whisper Systems' website was launched in January 2013.[17] Open Whisper Systems started working to bring TextSecure to iOS in March 2013.[18][19]

In February 2014, Open Whisper Systems updated their protocol to version 2, adding group chat and push messaging capabilities.[18][20] Toward the end of July 2014, Open Whisper Systems announced plans to unify its RedPhone and TextSecure applications as Signal.[21] This announcement coincided with the initial release of Signal as a RedPhone counterpart for iOS. The developers said that their next steps would be to provide TextSecure instant messaging capabilities for iOS, unify the RedPhone and TextSecure applications on Android, and launch a web client.[22] Signal was the first iOS app to enable easy, strongly encrypted voice calls for free.[16][23]

In March 2015, Open Whisper Systems released Signal 2.0 with support for TextSecure private messaging on iOS.[24][25] Later that month, Open Whisper Systems ended support for sending and receiving encrypted SMS/MMS messages on Android. As of version 2.7.0, TextSecure only supports sending and receiving encrypted messages via the data channel. Reasons for this included:[5]

  • Complications with the SMS encryption procedure: Users needed to manually initiate a "key exchange", which required a full round trip before any messages could be exchanged. In addition to this, users could not always be sure whether the receiver could receive encrypted SMS/MMS messages or not.
  • Compatibility issues with iOS: Not possible to send or receive encrypted SMS/MMS messages on iOS due to the lack of APIs.
  • The large amounts of metadata that inevitably arise and are uncontrollable when using SMS/MMS for the transportation of messages.
  • Focus on software development: Maintaining SMS/MMS encryption and dealing with edge cases took up valuable resources and inhibited the development of the software.

Reception[edit]

Former NSA contractor Edward Snowden has endorsed Open Whisper Systems' applications on multiple occasions. In his keynote speech at SXSW in March 2014, he praised TextSecure and RedPhone for their ease-of-use.[26][27] During an interview with The New Yorker in October 2014, he recommended using "anything from Moxie Marlinspike and Open Whisper Systems".[28] During a remote appearance at an event hosted by Ryerson University and Canadian Journalists for Free Expression in March 2015, Snowden said that Signal is "very good" and that he knew the security model.[29] Asked about encrypted messaging apps during a Reddit AMA in May 2015, he recommended “Signal for iOS, Redphone/TextSecure for Android”.[30][31]

In October 2014, the Electronic Frontier Foundation (EFF) included TextSecure in their updated surveillance self-defense guide.[32] In November 2014, TextSecure received a top score on the EFF's secure messaging scorecard, along with "ChatSecure + Orbot", Cryptocat, "Signal / RedPhone", Silent Phone, and Silent Text.[33][34] They received points for having communications encrypted in transit, having communications encrypted with keys the providers don't have access to (end-to-end encryption), making it possible for users to independently verify their correspondent's identities, having past communications secure if the keys are stolen (forward secrecy), having their code open to independent review (open source), having their security designs well-documented, and having recent independent security audits.[33]

Features[edit]

The application prevents screenshots of conversations by default.

TextSecure allows users to send encrypted text messages, audio messages, photos, videos, contact information, and a wide selection of emoticons over a data connection (e.g. Wi-Fi, 3G or 4G) to other TextSecure users with smartphones running Android and to Signal users on iOS. TextSecure also allows users to exchange unencrypted SMS and MMS messages with people who do not have TextSecure or Signal.[20]

Messages sent with TextSecure to other TextSecure users and to Signal users are automatically end-to-end encrypted, which means that they can only be read by the intended recipients. The keys that are used to encrypt the user's messages are stored on the device alone,[33] and they are protected by an additional layer of encryption if the user has a passphrase enabled. In the user interface, encrypted messages are denoted by a lock icon.

Messages that have been sent via SMS/MMS and messages that have been sent via the user's data connection can be distinguished by color. Green text bubbles indicate SMS-based communication and blue text bubbles indicate communication over a data connection.

TextSecure and Signal have a built-in function for verifying that the user is communicating with the right person and that no man-in-the-middle attack has occurred. This verification can be done by comparing key fingerprints out-of-band.[33] Users can also scan each other's personal QR codes.

TextSecure and Signal allow users to chat with more than one person at a time.[35] Group chats are automatically end-to-end encrypted and held over an available data connection if all participants are registered TextSecure or Signal users. Users can create groups with a title and avatar icon, add their friends, join or leave groups, and exchange messages/media, all with the same encryption properties pairwise TextSecure chats provide. The servers do not have access to group metadata such as lists of group members, the group title, or the group avatar icon.[20][36]

Optional features
Feature Description Default
TextSecure messages Uses the data channel for communication with TextSecure and Signal users.[20] On
Passphrase Enables local encryption of the message database and the keys that are used to encrypt the user's messages.[1] Off
Passphrase timeout Forgets the passphrase from memory after a specified time interval. Off
Screen security Blocks screenshots in the recents list and inside the app. On
Old message deletion Automatically deletes older messages once a conversation thread exceeds a specified length. Off
SMS delivery reports Requests a delivery report for each SMS message the user sends. Off
Outgoing SMS/MMS To people who do not have TextSecure or Signal. These messages are not encrypted. Off

Architecture[edit]

Encryption protocol[edit]

The TextSecure encryption protocol is an end-to-end encrypted messaging protocol with deniability guarantees[37] and message-level forward secrecy, similar to the Off-the-Record Messaging (OTR) protocol.[38] It uses Curve25519, AES-256, and HMAC-SHA256 as primitives.[39][38]

The TextSecure protocol is derivative of OTR. The major difference is that the TextSecure protocol uses elliptic curve cryptography (ECC) keys, whereas the OTR protocol uses DSA keys. The TextSecure protocol also compresses some data structure formats[40] and allows the ephemeral key negotiation to work asynchronously.[35][41][38]

There have been two major releases of the TextSecure protocol. Version 1 used the OTR ratchet and custom binary structures. Version 2 uses the no header keys variation of the Axolotl ratchet and protobuf records.[38]

Axolotl[edit]

Axolotl is a cryptographic ratchet that was developed by Moxie Marlinspike and Trevor Perrin.[42] Its stated goals are to:[43]

  • Combine the forward secrecy of symmetric-key updating with the "future secrecy" of an OTR-like Diffie-Hellman ratchet.
    • Forward secrecy means that messages are encrypted with temporary per-message cryptographic keys. The compromise of any long-term keys does not compromise any previous conversations, even if an attacker is in possession of ciphertexts.
    • By "future secrecy", the developers mean that even if a currently used shared key is compromised, future sessions will remain secure.[44] A leak of keys to a passive eavesdropper will be healed by introducing new DH ratchet keys.[42]
  • Improve on OTR's future-secrecy with a "2-step" DH ratchet instead of "3-step".
    • In other words, DH ratchet keys are used in a "send/receive" pattern, instead of "advertise/receive/send".
  • Detect replay / reorder / deletion of messages.
  • Allow decryption of out-of-order messages with minimal reduction in forward secrecy.
  • Not leak metadata in cleartext (such as identities or sequence numbers).

Axolotl ensures that new AES keys are used for every single message, and it provides the application with both forward secrecy and future secrecy properties.[43] The Axolotl specification is in the public domain.[43]

Implementations[edit]

The TextSecure protocol is open and federated,[20][38] making it possible for third parties to independently examine it and to implement it in their systems.

In December 2013, it was announced that the TextSecure protocol had successfully been integrated into the Android-based open-source operating system CyanogenMod.[45][46][47] As of CyanogenMod 11.0, the client logic is contained in a system app called WhisperPush. According to Open Whisper Systems, "the Cyanogen team runs their own TextSecure server for WhisperPush clients, which federates with [Open Whisper Systems'] TextSecure server, so that both clients can exchange messages with each-other seamlessly".[47] The WhisperPush source code is available under the GPLv3 license.[48]

On November 18, 2014, Open Whisper Systems announced a partnership with WhatsApp to provide end-to-end encryption by incorporating the protocol used in TextSecure into each WhatsApp client platform.[49] Open Whisper Systems asserted that they have already incorporated the protocol into the latest WhatsApp client for Android and that support for other clients, group/media messages, and key verification would be coming soon.[50] WhatsApp confirmed the partnership to reporters, but there was no announcement or documentation about the encryption feature on the official website, and further requests for comment were declined.[51]

Servers[edit]

Client-server communication is protected by TLS.[39] Communication is handled by a REST API and push messaging (both GCM and APN).[4] Support for WebSocket has been added.[52]

In order to determine which contacts are also TextSecure users, cryptographic hashes of the user's contact numbers are periodically transmitted to the server.[53] The server then checks to see if those match any of the SHA256 hashes of registered users and tells the client if any matches are found. The contact discovery mechanism is designed so that the server does not need to store the hashes that are transmitted to it,[53][4] and Moxie Marlinspike has asserted that Open Whisper Systems' server does not.[54]

The group messaging mechanism is designed so that the servers do not have access to any group metadata such as the membership list, group title, or group icon.[36] Instead, the creation, updating, joining, and leaving of groups is done by the clients, which deliver pairwise messages to the participants in exactly the same way that group conversation messages are delivered.[3][20][36]

Open Whisper Systems' server infrastructure is funded through grants and donations they receive. The server-side architecture is federated. The developers hope this will help spread the cost over time. The developers of CyanogenMod already host the servers that handle the traffic for their users. As Open Whisper Systems will launch more clients, their hope is that other stakeholders will take on hosting as well.[55]

Licensing[edit]

The complete source code of TextSecure is available on GitHub under a free software license. This enables interested parties to examine the code and help the developers verify that everything is behaving as expected. It also allows advanced users to compile their own copy of the application and compare it with the version that is distributed by Open Whisper Systems.[3] The same is true of the software that handles message routing for the TextSecure data channel.[4]

Distribution[edit]

Open Whisper Systems distributes TextSecure through Google Play. Following an incident in August 2012, they have declined requests to distribute the application through 3rd party sources, such as F-Droid.[52][56][57] Open Whisper Systems has acknowledged that this is an important issue for some of TextSecure's users, and have assured that they are working on it. They have, however, chosen to focus on serving the millions of users who have Google Cloud Messaging (GCM) capabilities first. They have invited the community to help them add WebSocket support to TextSecure for Android.[52]

Until March 2015, TextSecure used GCM as the transport for message delivery over the data channel. In March 2015, Open Whisper Systems moved to a model of handling message delivery themselves. As of March 2015, TextSecure relies on GCM only for a wakeup event.[5]

Audits[edit]

In October 2013, iSEC Partners published a blog post in which they said that they had audited several of the projects supported by the Open Technology Fund over the past year, including TextSecure.[58]

In October 2014, researchers from Ruhr University Bochum published a protocol analysis of TextSecure.[39] Among other findings, they presented an unknown key-share attack on the protocol, but in general, they found that the encrypted chat client is secure.[59]

Developers[edit]

Main article: Open Whisper Systems

TextSecure is developed by Open Whisper Systems, a nonprofit software group[60] that develops collaborative Open Source projects with a mission to "make private communication simple".[61] The group consists of a large community of volunteer Open Source contributors, as well as a small team of dedicated grant-funded developers.[61] Open Whisper Systems is funded by a combination of donations and grants, and all of its products are published as free and open-source software under the terms of the GNU General Public License (GPL) version 3.

The project has received financial support from, among others, the Freedom of the Press Foundation,[62] the Knight Foundation,[63] the Shuttleworth Foundation,[64] and the Open Technology Fund,[65] a U.S. government program that has also funded other privacy projects like the anonymity software Tor and the encrypted instant messaging website Cryptocat.

See also[edit]

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References[edit]

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  55. ^ Moxie Marlinspike (11 March 2014). "How is openwhispersystems paying for the its server costs?". Open Whisper Systems. Retrieved 16 March 2014. 
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  60. ^ Franceschi-Bicchierai, Lorenzo (18 November 2014). "WhatsApp messages now have Snowden-approved encryption on Android". Mashable. Retrieved 23 January 2015. 
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  62. ^ "Open Whisper Systems". Freedom of the Press Foundation. Retrieved 18 January 2015. 
  63. ^ "TextSecure". Knight Foundation. Retrieved 5 January 2015. 
  64. ^ "Moxie Marlinspike". Shuttleworth Foundation. Retrieved 14 January 2015. 
  65. ^ "Projects". Open Technology Fund. Retrieved 14 January 2015. 

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