How are phone numbers hashed or encrypted in messaging apps?
Posted: Wed May 21, 2025 9:22 am
In order to provide a comprehensive and accurate explanation of how phone numbers are hashed or encrypted in messaging apps, I need to gather information on common practices, specifically focusing on how they are used for contact discovery while maintaining privacy.
Here are the queries I will use:
How do messaging apps use phone numbers for contact discovery securely?
What is the role of hashing and encryption for phone numbers in secure messaging apps?
Signal protocol phone number privacy
WhatsApp phone number hashing security
Telegram phone number encryption practices
contact matching privacy messaging apps
phone number hashing security
PII encryption messaging apps
secure contact discovery mechanisms In end-to-end encrypted (E2EE) messaging apps, phone numbers are not directly encrypted as the message content. Instead, they play a crucial role as identifiers and are handled with specific privacy-preserving techniques, often involving hashing, to enable contact discovery while minimizing the exposure of raw PII (Personally Identifiable Information) to the service provider.
Here's how phone numbers are typically managed in the context of E2EE apps:
Primary Identifier and User Registration:
For most popular E2EE messaging apps (like WhatsApp, Signal, Viber, and Google vnpay data Messages with RCS), your phone number is your primary account identifier. You register for the service using your phone number, and it's used to uniquely identify you on the platform.
This choice is driven by convenience: it allows apps to seamlessly integrate with your device's contact list, making it easy to find and connect with friends already on the app without needing to exchange separate usernames.
Contact Discovery Mechanisms (Where Hashing Comes In):
This is where privacy measures for phone numbers are critical. When you sign up or regularly use the app, your device needs to determine which of your phone contacts are also users of the messaging service. There are several approaches:
Hashed Contact Upload (Common but with caveats):
Your device first normalizes all phone numbers in your local address book to a standard format (e.g., E.164: +88017xxxxxxxx).
It then hashes each of these normalized phone numbers using a cryptographic hash function (e.g., SHA256). A hash function produces a fixed-size string of characters that is irreversible (you can't get the original phone number from the hash) and unique (a slight change in the input produces a drastically different hash).
These hashed phone numbers are then sent to the messaging app's servers.
The server also has a database of its registered users' hashed phone numbers. It compares your hashed contacts against its own hashed user list.
If a match is found, the server notifies your app that this contact is a user. The actual phone numbers are never transmitted in the clear to the server, only their hashes.
Limitations of Hashing: While hashing provides a layer of privacy, it's not foolproof. Because phone numbers have relatively low "entropy" (they follow predictable patterns and are within a limited range), they are susceptible to rainbow table attacks or brute-force attacks. An attacker can pre-compute hashes for a vast range of possible phone numbers and then compare them to the hashed numbers leaked from a service. This is why more advanced techniques are preferred by privacy-focused apps.
Private Set Intersection (PSI) / Encrypted Contact Discovery (More Secure):
This is a more advanced cryptographic technique used by apps like Signal to provide stronger privacy for contact discovery.
Instead of simply hashing and uploading, PSI protocols allow two parties (your device and the server) to discover common elements (i.e., mutual contacts) in their datasets without either party revealing their entire set of data to the other.
This involves complex cryptographic operations where your device's hashed contacts are interactively compared with the server's hashed user base in an encrypted manner. The server never learns your full contact list, and you only learn which of your contacts are on the service.
Signal, for instance, has invested significantly in making its contact discovery process as private as possible, even exploring the use of "Trusted Execution Environments" (TEEs) on servers to further isolate this process.
Role in E2EE Key Exchange:
Once contact discovery is complete, and you initiate a chat with a discovered contact, the phone number acts as the identifier to retrieve the correct public cryptographic keys from the messaging app's server.
The actual end-to-end encryption of your messages occurs using a separate cryptographic protocol (e.g., the Signal Protocol used by Signal, WhatsApp, and Google Messages). This protocol generates ephemeral, unique, and symmetric encryption keys for each conversation, and these keys are derived from the exchange of public/private key pairs between the communicating devices.
The phone number merely tells the app who you want to communicate with, so it knows whose keys to fetch to establish the secure, encrypted channel. The phone number itself is not part of the key material used for message encryption.
Storage on Servers:
Even with hashing or PSI for contact discovery, the messaging app's server still needs to store your registered phone number (often in its raw, unhashed E.164 format) because it's your account identifier. This allows the server to route incoming messages, verify your account, and manage your profile.
However, this registered phone number is typically stored in a separate, secure database on the server and is not directly exposed in the same way as contact lists during the discovery process. Many apps also encrypt this PII at rest on their servers.
In essence, phone numbers serve as convenient public "addresses" for users in messaging apps. To protect privacy, they are often hashed or processed using advanced cryptographic techniques like PSI for contact discovery, ensuring that the service provider doesn't gain undue access to your social graph, while the end-to-end encryption of message content relies on separate, dynamic cryptographic keys.
Here are the queries I will use:
How do messaging apps use phone numbers for contact discovery securely?
What is the role of hashing and encryption for phone numbers in secure messaging apps?
Signal protocol phone number privacy
WhatsApp phone number hashing security
Telegram phone number encryption practices
contact matching privacy messaging apps
phone number hashing security
PII encryption messaging apps
secure contact discovery mechanisms In end-to-end encrypted (E2EE) messaging apps, phone numbers are not directly encrypted as the message content. Instead, they play a crucial role as identifiers and are handled with specific privacy-preserving techniques, often involving hashing, to enable contact discovery while minimizing the exposure of raw PII (Personally Identifiable Information) to the service provider.
Here's how phone numbers are typically managed in the context of E2EE apps:
Primary Identifier and User Registration:
For most popular E2EE messaging apps (like WhatsApp, Signal, Viber, and Google vnpay data Messages with RCS), your phone number is your primary account identifier. You register for the service using your phone number, and it's used to uniquely identify you on the platform.
This choice is driven by convenience: it allows apps to seamlessly integrate with your device's contact list, making it easy to find and connect with friends already on the app without needing to exchange separate usernames.
Contact Discovery Mechanisms (Where Hashing Comes In):
This is where privacy measures for phone numbers are critical. When you sign up or regularly use the app, your device needs to determine which of your phone contacts are also users of the messaging service. There are several approaches:
Hashed Contact Upload (Common but with caveats):
Your device first normalizes all phone numbers in your local address book to a standard format (e.g., E.164: +88017xxxxxxxx).
It then hashes each of these normalized phone numbers using a cryptographic hash function (e.g., SHA256). A hash function produces a fixed-size string of characters that is irreversible (you can't get the original phone number from the hash) and unique (a slight change in the input produces a drastically different hash).
These hashed phone numbers are then sent to the messaging app's servers.
The server also has a database of its registered users' hashed phone numbers. It compares your hashed contacts against its own hashed user list.
If a match is found, the server notifies your app that this contact is a user. The actual phone numbers are never transmitted in the clear to the server, only their hashes.
Limitations of Hashing: While hashing provides a layer of privacy, it's not foolproof. Because phone numbers have relatively low "entropy" (they follow predictable patterns and are within a limited range), they are susceptible to rainbow table attacks or brute-force attacks. An attacker can pre-compute hashes for a vast range of possible phone numbers and then compare them to the hashed numbers leaked from a service. This is why more advanced techniques are preferred by privacy-focused apps.
Private Set Intersection (PSI) / Encrypted Contact Discovery (More Secure):
This is a more advanced cryptographic technique used by apps like Signal to provide stronger privacy for contact discovery.
Instead of simply hashing and uploading, PSI protocols allow two parties (your device and the server) to discover common elements (i.e., mutual contacts) in their datasets without either party revealing their entire set of data to the other.
This involves complex cryptographic operations where your device's hashed contacts are interactively compared with the server's hashed user base in an encrypted manner. The server never learns your full contact list, and you only learn which of your contacts are on the service.
Signal, for instance, has invested significantly in making its contact discovery process as private as possible, even exploring the use of "Trusted Execution Environments" (TEEs) on servers to further isolate this process.
Role in E2EE Key Exchange:
Once contact discovery is complete, and you initiate a chat with a discovered contact, the phone number acts as the identifier to retrieve the correct public cryptographic keys from the messaging app's server.
The actual end-to-end encryption of your messages occurs using a separate cryptographic protocol (e.g., the Signal Protocol used by Signal, WhatsApp, and Google Messages). This protocol generates ephemeral, unique, and symmetric encryption keys for each conversation, and these keys are derived from the exchange of public/private key pairs between the communicating devices.
The phone number merely tells the app who you want to communicate with, so it knows whose keys to fetch to establish the secure, encrypted channel. The phone number itself is not part of the key material used for message encryption.
Storage on Servers:
Even with hashing or PSI for contact discovery, the messaging app's server still needs to store your registered phone number (often in its raw, unhashed E.164 format) because it's your account identifier. This allows the server to route incoming messages, verify your account, and manage your profile.
However, this registered phone number is typically stored in a separate, secure database on the server and is not directly exposed in the same way as contact lists during the discovery process. Many apps also encrypt this PII at rest on their servers.
In essence, phone numbers serve as convenient public "addresses" for users in messaging apps. To protect privacy, they are often hashed or processed using advanced cryptographic techniques like PSI for contact discovery, ensuring that the service provider doesn't gain undue access to your social graph, while the end-to-end encryption of message content relies on separate, dynamic cryptographic keys.