What is the role of phone numbers in end-to-end encryption?

[jakiyasultana2525]

In the realm of end-to-end encryption (E2EE) for messaging and communication applications, phone numbers play a crucial, but distinct, role. They primarily serve as identifiers for users within the system, rather than being directly involved in the cryptographic key generation or encryption process itself.

Here's a breakdown of their role:

User Registration and Identification:

Most popular E2EE messaging apps (like WhatsApp, Signal, Google Messages with RCS, Viber) use phone numbers as the primary means of user registration and identification. When you sign up, you verify your phone number via an SMS or voice call.
This allows the app to leverage your existing phone contact list. The app can scan your contacts and automatically identify which of your contacts are also using the same messaging service, making it easy to find and connect with friends and family without needing to exchange separate usernames.
The phone number essentially acts as your public "address" within the twitter data secure messaging network, much like an email address acts as an address for email.
Contact Discovery:

By using phone numbers as identifiers, these apps facilitate seamless contact discovery. When you open the app, it checks your phone's contact list against its user database to show you which of your contacts are on the platform. This saves users the effort of manually adding friends.
Some apps, like Signal, are evolving to offer more phone number privacy options, allowing users to hide their number from those who don't already have it, or even to use usernames for initial connections while still requiring a phone number for registration to combat spam.
Establishing Secure Communication Channels (Key Exchange):

While the phone number identifies who you want to communicate with, it's not the key that encrypts your messages. E2EE relies on cryptographic keys, specifically public and private key pairs.
When you initiate a chat with someone, the messaging app (using a secure protocol like the Signal Protocol) facilitates a key exchange between your device and the recipient's device. This exchange happens in the background, typically involving:
Each user's device generating a unique set of cryptographic keys (a public key and a private key).
Public keys are exchanged (often through the messaging service's servers, which are designed not to decrypt messages, only to relay encrypted data and public keys).
Both devices then mathematically derive a shared secret encryption key for that specific conversation, using a process like the Diffie-Hellman key exchange, without ever transmitting their private keys. This shared secret key is unique to that conversation and often changes with every message (forward secrecy).
The phone number's role here is to tell the app whose public keys to fetch from the server to initiate this key exchange. It's the identifier for the recipient of the keys.
Verification of Identity (Security Codes):

To prevent sophisticated "Man-in-the-Middle" (MiTM) attacks, E2EE apps provide security codes (also called safety numbers or fingerprints). These are unique numerical or QR code representations derived from the public keys of the two communicating parties.
Users are encouraged to verify these security codes out-of-band (e.g., by physically comparing QR codes, reading numbers aloud over a phone call, or sending them through a trusted alternative channel). If the codes match, it confirms that you are truly communicating with the intended person's device and that no one is intercepting your communication.
The phone number helps you identify which contact's security code you are verifying. If a contact's phone number changes or they re-register, their security code might change, prompting re-verification.
In summary, phone numbers serve as convenient and widely adopted public identifiers in the context of E2EE messaging. They allow users to register, discover contacts, and initiate secure communication sessions. However, the actual end-to-end encryption of messages relies on robust cryptographic protocols and the exchange of unique, device-generated keys, which are distinct from the phone numbers themselves.