WhatsApp

WhatsApp Messenger is a proprietary, cross-platform instant messaging subscription service for smartphones with Internet access. In addition to text messaging, users can send each other images, video, and audio media messages. The client software is available for Google, BlackBerry OS, Apple iOS,Android selected Nokia Series 40, Symbian, selected Nokia Asha platform and Microsoft Windows Phone. WhatsApp Inc. was founded in 2009 by American Brian Acton and Ukrainian Jan Koum (also the CEO), both former employees of Yahoo!, and is based in Mountain View, Santa Clara County, California.

Competing with a number of Asian-based messaging services (like LINE, KakaoTalk, WeChat and Zalo), WhatsApp handled ten billion messages per day in August 2012, growing from two billion in April 2012, and one billion the previous October. On June 13, 2013, WhatsApp announced on Twitter that they had reached their new daily record by processing 27 billion messages. According to the Financial Times, WhatsApp "has done to SMS on mobile phones what Skype did to international calling on landlines."

As of November 10, 2013, WhatsApp has over 190 million monthly active users, 400 million photos are shared each day, and the messaging system handles more than 10 billion messages each day. In a December 2013 blog post, WhatsApp claimed that 400 million active users use the service each month.

Technical
WhatsApp uses a customized version of the open standard Extensible Messaging and Presence Protocol (XMPP). Upon installation, it creates a user account using one's phone number as username (Jabber ID: ). WhatsApp software automatically compares all the phone numbers from the device's address book with its central database of WhatsApp users to automatically add contacts to the user's WhatsApp contact list. Previously the Android and S40 versions used an MD5-hashed, reversed-version of the phone's IMEI as password, while the iOS version used the phone's Wi-Fi MAC address instead of IMEI. A 2012 update now generates a random password on the server side.

WhatsApp is supported on most Android, BlackBerry, iPhone, Nokia, and Windows smartphones. All Android phones running the Android 2.1 and above, all Blackberry devices running OS 4.7 and later, and all iPhones running iOS 4.3 and later. However, some Dual SIM devices may not be compatible with WhatsApp, though there are some workarounds for this.

Multimedia messages are sent by uploading the image, audio or video to be sent to a HTTP server and then sending a link to the content along with its Base64 encoded thumbnail (if applicable).

Security
In May 2011, a security hole was reported which left WhatsApp user accounts open for session hijacking and packet analysis. WhatsApp communications were not encrypted, and data was sent and received in plaintext, meaning messages were easily to be read if packet traces are available. In September 2011, WhatsApp released a new version of the Messenger application for iPhones, closing critical security holes that allowed forged messages to be sent and messages from any WhatsApp user to be read.

On January 6, 2012, an unknown hacker published a website (WhatsAppStatus.net) that made it possible to change the status of an arbitrary WhatsApp user, as long as the phone number was known. To make it work, it only required a restart of the app. According to the hacker, it is only one of the many security problems in WhatsApp. On January 9, WhatsApp reported that it had resolved the problem, although the only measure actually taken was to block the website's IP address. As a reaction, a Windows tool was made available for download providing the same functionality. This problem has since been resolved in the form of an IP check on currently logged in session.

On January 13, 2012, WhatsApp was pulled from the iOS App Store, and the reason was not disclosed. The app was added back to the App Store four days later.

In May 2012, security researchers noticed that new updates of WhatsApp no longer sent messages as plaintext,  the cryptographic method implemented was subsequently described as "broken". As of August 15, 2012, the WhatsApp support staff claim messages are encrypted in the "latest version" of the WhatsApp software for iOS and Android (not including BlackBerry, Windows Phone, and Symbian), without specifying the implemented cryptographic method.

German Tech site The H demonstrated how to use WhatsAPI to hijack any WhatsApp account on September 14, 2012. Shortly after a legal threat to WhatsAPI's developers was alleged, characterized by The H as "an apparent reaction" to security reports, and WhatsAPI's source code was taken down for some days. The WhatsAPI team has since returned to active development.

Privacy
A major privacy and security problem has been the subject of a joint Canadian-Dutch government investigation. The primary concern was that WhatsApp required users to upload their entire mobile phone's address book to WhatsApp servers so that WhatsApp could discover who, among the users' existing contacts, is available via WhatsApp. While this is a fast and convenient way to quickly find and connect the user with contacts who are also using WhatsApp, it means that their address book was then mirrored on the WhatsApp servers, including contact information for contacts who are not using WhatsApp. This information was stored in hashed, though not salted form and without "additional" identifying information such as a name, although the stored identifying information is sufficient to identify every contact.

WhatsApp uses a timestamp feature, which gives the ability to view the last time a user was logged-on (or "Last seen"), unless expressly blocked by said user. This feature is considered to be a privacy problem. While iPhone users can choose to disable this feature, Android users cannot.

WhatsApp allows you to view the profile picture and current "Status" of every user, as long as that user is enlisted in Contacts.

On March 31, 2013, the telecommunications authority in Saudi Arabia, the Communications and Information Technology Commission (CITC), issued a statement regarding possible measures against WhatsApp, among other applications, unless the service providers took serious steps in order to comply with monitoring and privacy regulations.

One of the drawback of WhatsApp is that the user does not need to send a friend request to send messages to another user. However, users can block numbers on Whatsapp.

Open WhatsApp Project
The Open WhatsApp Project is an open-source re-implementation of the WhatsApp client software for mobile phones done by an independent group. Initially targeted at the Nokia N9 (which was officially not supported by WhatsApp), it was later ported to other platforms, including Blackberry 10. It uses the WhatsApp service behind-the-scenes, and is thus not a competitor to WhatsApp, being just a different front-end, and is also subject to the same privacy and security concerns as WhatsApp.