Key takeaways
- Phone numbers are hashed with SHA-256 on your device before matching.
- The C Chat server does not store the hashes used for matching.
- You can skip phone contacts entirely and connect by @username.
- Hashing protects your address book; it does not hide your own number from people who already have it.
- Direct messages you start with a matched contact are private, outside team audits.
Many messaging apps ask for access to your contacts on day one. For a personal app that may feel normal; for a work tool, it raises a fair question: what happens to the numbers of your family, your doctor and your other clients? C Chat’s answer is to do the matching with hashed numbers and not keep them afterwards.
Short answer: hashed on the device, not stored on the server
Contact matching privacy in C Chat rests on two facts:
- Numbers are hashed with SHA-256 on your device. The plain phone numbers in your address book are converted into hashes before they are used for matching.
- The server does not store the hashes. Once the matching is done, there is no saved list of your contacts on C Chat’s side.
The result is that you can find colleagues already on C Chat without turning your personal address book into company data. The feature page on private phone contact matching shows how it looks in the app.
How contact matching privacy works in C Chat
A hash is a one-way calculation. If you want the technical background, the glossary entry on SHA-256 hashing explains it in plain language. In short:
- The same input always gives the same output, which is what makes matching possible.
- The output does not show the original number in readable form.
- Changing one digit produces a completely different hash.
When a hash corresponds to a registered C Chat user, the app shows that person as someone you can message. From there you start a direct message with no request step.
The alternative: usernames
Contact matching is one of three ways to reach people. You can also message team members directly or use an @username. Usernames are the better choice when you work with clients or contractors and prefer not to exchange phone numbers at all.
Who can see what
- You see which of your contacts are on C Chat.
- Your contacts can find you only if they have the phone number you signed up with.
- Team owners and admins do not see your contacts. Their audit rights cover the team’s group conversations only.
- C Chat’s servers do not store the hashes used for matching.
For the complete picture of access to messages, see who can read my messages.
What it means for owners, admins and members
For owners and admins, contact matching makes adoption easier: staff find each other without a manual directory. You do not collect anyone’s address book, which keeps one category of personal data out of your systems entirely.
For members, especially people using a personal phone for work, matching means you do not have to hand over the numbers of everyone you know to join the team. That is one of the practical differences with the risks of using personal WhatsApp for work.
For HR, it is worth explaining in onboarding that contact matching is optional in practice: an employee can rely on team membership and @usernames instead.
Honest limits and caveats
- Hashing is not magic. Phone numbers follow known formats, so a determined attacker with a hash could try to guess the number by brute force. Not storing the hashes is what closes that door on C Chat’s side.
- Your own number is findable by people who have it. If that is a problem, sign up with email and share your @username.
- Metadata still exists. Who messages whom and when is information in itself; read about metadata in messaging to understand the concept.
- Leaving is your choice. If you stop using C Chat, follow the steps on what happens when you delete your account.
Getting started
Sign up at getcchat.com/c with your phone number or email, pick an @username, and decide whether you want to find colleagues through your contacts or only through your team. Either way, your direct messages stay private, as explained in why direct messages stay private.
Frequently asked questions
Does C Chat upload my contacts?
C Chat does not keep your address book. Numbers are converted into SHA-256 hashes on your device for matching, and the server does not store those hashes.
What is a SHA-256 hash in simple terms?
It is a fixed-length fingerprint calculated from a piece of data. The same phone number always produces the same fingerprint, which allows matching, but the fingerprint itself does not show the number in readable form.
Can someone find me on C Chat if they have my phone number?
If you signed up with your phone number, a person who has that number in their contacts can find you through contact matching. If you prefer not to share your number, sign up with email and share your @username instead.
Is hashing the same as anonymizing my contacts?
Not exactly. Phone numbers come from a limited range, so hashes of numbers can in principle be guessed by trying many numbers. That is why it matters that C Chat does not store the hashes after matching.
Do team admins see my phone contacts?
No. Contact matching only helps you find people to message. Admins can audit group conversations of their team, not your contacts and not your direct messages.