How does WhatsApp use hashing or other techniques to anonymize or pseudonymize phone numbers for certain analyses?

[muskanhossain]

WhatsApp utilizes hashing techniques to pseudonymize phone numbers for specific analyses, particularly when dealing with non-users in the context of contact discovery and for detecting misuse of contact syncing. Here's how:

1. Hashing for Contact Discovery (Non-Users):

When a WhatsApp user grants permission to sync their device's address book, WhatsApp accesses the phone numbers of all contacts.
For contacts who are not yet WhatsApp users, WhatsApp employs a cryptographic hash function to process their phone numbers.
Cryptographic Hashing: This process transforms the original india whatsapp number data phone number into a fixed-size string of characters (the hash value) using a one-way function. This means that while it's easy to compute the hash from the phone number, it is computationally infeasible to reverse the process and obtain the original phone number from the hash value alone, especially with a strong hashing algorithm.
Deletion of Original Number: After creating the cryptographic hash of the non-user's phone number, WhatsApp reportedly deletes the original phone number from its servers for these non-user contacts.
Linking to Syncing Users: These hash values are then stored on WhatsApp's servers, linked to the WhatsApp users who synced the corresponding phone numbers before they were hashed.
Efficient Connection Later: When a non-user in a synced address book eventually joins WhatsApp, their phone number is also hashed. WhatsApp can then compare this new hash against the existing store of hashes to efficiently identify and connect the new user with the WhatsApp users who had their number in their address book. This process allows for contact discovery without WhatsApp needing to retain the raw phone numbers of individuals who are not using the service.
2. Hashing for Misuse Detection:

WhatsApp also creates a separate cryptographic hash representation of all phone numbers in a user's device address book (including both WhatsApp users and non-users).
These hashes are used to detect and combat misuse of the contact syncing feature. By analyzing patterns in these hashes, WhatsApp can identify unusual changes in address books that might indicate malicious activity or spamming attempts related to contact syncing.
Why Hashing for Pseudonymization?

Privacy for Non-Users: Hashing ensures that WhatsApp doesn't store the actual phone numbers of individuals who are not using the platform, thus protecting their privacy.
Limited Identifiability: While theoretically, with a large enough dataset and sophisticated techniques, hash collisions or other vulnerabilities could potentially lead to some form of identification, the use of strong cryptographic hashing significantly reduces this risk compared to storing phone numbers in plain text.
Efficient Matching: Hashing allows for efficient matching and comparison of phone numbers for contact discovery without needing to store or transmit the raw, sensitive data.
It's important to note the distinction between pseudonymization and anonymization. Hashing, in this context, primarily serves as a form of pseudonymization. While it makes direct identification difficult without additional information or significant computational effort, it doesn't completely eliminate the possibility of re-identification in all scenarios, especially if the dataset of possible phone numbers is relatively small or if other linked information is available. True anonymization would involve irreversible alteration or aggregation of data to the point where individual identification is no longer possible.

While WhatsApp's specific algorithms and implementations are proprietary, the use of cryptographic hashing as described aligns with common privacy-enhancing techniques for handling sensitive identifiers like phone numbers in large-scale systems.