A Project of the Electronic Frontier Foundation 
Signal – Private Messenger is a free and open source software application for iPhone that employs end-to-end encryption
, allowing users to send end-to-end encrypted group, text, picture, and video messages and have encrypted phone conversations between Signal users on iPhone and Android. Although Signal uses telephone numbers as contacts, calls and messages actually use your data connection; therefore both parties to the conversation must have Internet access on their mobile devices. Due to this, Signal users don’t incur SMS and MMS fees.
Installing Signal – Private Messenger on your iPhone
Step 1: Download and Install Signal – Private Messenger
On your iOS device, enter the App Store and search for “Signal.” Select the app “Signal – Private Messenger” by Open Whisper Systems.
Click to download the app and accept the iTunes Store Terms & Conditions by selecting “Accept.” The app will download and install automatically. Click “Open” to launch the app.
Step 2: Register and Verify your Phone Number
You will see a screen that looks like this:
Enter your mobile phone number and click “Verify This Device.” In order to verify your phone number, you will be sent an SMS text with a six-digit code; enter the code in the space provided. If you are unable to receive SMS texts, you have the option of receiving a phone call to verify your phone number. Click “Submit Verification Code.”
Using Signal
In order to use Signal, the person that you are calling must have Signal installed on their iPhone or Android device. If you try to call or send a message to someone using the Signal app and they do not have any of the aforementioned apps installed, the app will ask if you would like to invite them via SMS, but it will not allow you to complete your call or send a message to them from inside the app.
Signal provides you with a list of other Signal users in your contacts. To do this, data representing the phone numbers in your contact list is uploaded to the Signal servers, although this data is deleted almost immediately.
To get started, click the "+" button.
You’ll see a list of your contacts that have already installed Signal. You have the choice to either call them or send them a message.
How to Initiate an Encrypted Call
To initiate an encrypted call to a contact, click on the phone icon next to the contact's name.
Once a call is established, both parties to the call will be shown a random pair of words. This word pair will allow you to verify your identity and keys with the other user—also known as key verification.
The most trustworthy way to verify the identity of a caller is to use out-of-band verification to verify the word pair. You can also read the words aloud if you recognize the caller’s voice, although very sophisticated attackers might be able to defeat this if they needed to. The word pair must be identical on both users' phones for you to be sure your message is not being intercepted.
How to Send an Encrypted Message
In order to send an end-to-end encrypted text, picture, or video message, navigate to your contact list, click on the contact’s name, and send your message.
You can send an encrypted group message by navigating to your list of contacts, clicking on the group chat icon in the upper right-hand corner, and creating a new group.
Using this app can help to keep your communications safer since everything sent via Signal - Private Messenger is always end-to-end encrypted.
Note that Open Whisper Systems, the makers of Signal, use other companies' infrastructure to send its users alerts when they receive a new message. It uses Google on Android, and Apple on iPhone. That means information about who is receiving messages and when they were received may leak to these companies.