Encrypted CDN FAQ

As of Telegram 4.2, we support encrypted CDNs for caching media from public channels with over 100.000 members. The CDN caching nodes are located in regions with significant Telegram traffic where we wouldn't want to place Telegram servers for various reasons.

For technical details of the implementation, encryption and verification of data, see the CDN manual.

See this document for a Persian version of this FAQ. بخش فارسی

Why did you decide to use CDNs?

We use our own distributed servers to speed up downloads in regions where freedom of speech is guaranteed — and even there [we don't take this for granted](https://telegram.org/faq#q-do-you-process-data-requests). But when Telegram becomes immensely popular in other areas, we can only rely on CDNs which we treat rather like ISPs from the technical standpoint in that they only get encrypted data they can't decipher.

Thanks to this technology, the download speed for public photos and videos can become significantly higher in regions like Turkey, Indonesia, South America, India, Iran or Iraq without the slightest compromise in security.

Can the CDN decipher any files?

No. Each file that is to be sent to the CDN is encrypted with a unique key using AES-256-CTR encryption. The CDN can't access the data it stores because these keys are only accessible to the main Telegram server and to the authorized client.

Can the CDN substitute the data with their own version?

No. Data downloaded from CDN caching nodes is always verified by the receiving Telegram app by way of hash: attackers won’t be able to replace any files with their own versions.

Can the CDN delete any files?

No. CDN nodes only cache encrypted *copies* of files, originals are stored on the Telegram servers. The user is notified about receiving the file by the Telegram server. If the CDN caching node doesn't give the file to the user, the user will receive the file from the Telegram server directly.

Can CDNs be used for censorship?

No. All original files are stored on the Telegram servers. The CDNs only get encrypted data — and they can't decipher it. They can't substitute any data. And in case of any problems with the CDN, the file will be simply delivered to the users directly from the Telegram servers. Users will always get their data, nobody can stop this.

Can I verify this?

Yes. Anyone can verify our CDN implementation by checking the source code of Telegram apps and inspecting traffic.

Does this affect private data?

No. The CDN caching nodes are not a part of the Telegram cloud. CDN caching nodes are used only for caching popular public media from massive channels. Private data never goes there.

Is this connected with government requests to move private data to their territory?

No. We haven't entered in any agreements with any government regarding the CDNs and the CDNs are not part of any deal. The only purpose of CDNs is to securely improve connectivity in high demand regions where Telegram can't place its servers.

Does this give some countries any influence over Telegram?

No. We have taken special precautions to make sure that no country gains any leverage over Telegram by way of the CDN caching nodes:

• The CDNs do not belong to Telegram – all the risks are on a third-party company that supplies us with CDN nodes around the world.
• We did not invest anything in these CDNs and will only be paying for traffic that is used to pass cached items from our main clusters and to the end users.

As the result, if any country decides to mess with the CDN in their region, they gain nothing except for reducing connectivity for their own citizens – and Telegram loses nothing of value.