Recently Telegram announced using CDN in some territories where they don't want to place main servers.
Although the internals are not explained, according to provided diagram and information, a key is passed from
Telegram server to
Telegram user which CDN is not aware of.
Since the file/file segment is encrypted before being uploaded to CDN, I'm assuming encryption algorithm is of symmetric-key type.
Now, even a new key is used for each file/file segment, that key is the same for everyone (Otherwise each file should has an encrypted version for each user which is not practical).
No matter how the key is sent to
Telegram user (i.e. end-to-end encrypted) at the client side that key could be extracted by every client, including the one in control of the officials. So,
What's the point of encrypting CDN contents and pass such a key directly to the