Telegram API: How do I get messages from a public channel, which I am not participant of?

I am able to successfully retrieve messages from a channel via channels.getMessages request, once I know their message IDs. I find channel id by contacts.search, by the way.

At the moment, mesage IDs are consequtive integers, so getting max_id would solve the issue.

I am sure it possible, since official clients do this (view a channel without joining it). I will try to find out how official desktop app does this by reading its sources, but any help will be much appreciated.

I need this because am writing a simple public telegram channel -> rss/web interface.

Please do not confuse Telegram Client API with Telegram Bot API. Bot API allows to receive 'push' messages on new messages, but no 'reading historical logs'.

Solution

Here are the steps you have to do in order to get messages from a channel you're not joined:

Resolve the username into ID and access_hash with contacts.resolveUsername
Invoke messages.getHistory to get your desired messages

Here is a short description of messages.getHistory parameters:

peer:        The channel from whom to retrieve the message history
limit:       Number of messages to be retrieved
offset_date: Offset date (messages *previous* to this date will be retrieved)
offset_id:   Offset message ID (only messages *previous* to the given ID will be retrieved)
max_id:      All the messages with a higher (newer) ID or equal to this will be excluded
min_id:      All the messages with a lower (older) ID or equal to this will be excluded
add_offset:  Additional message offset (all of the specified offsets + this offset = older messages)