I am looking for either an online service or an open-source program installed locally which would support end-to-end encryption and allow me to store calendar entries which only I would have access too. If there exists an online service I would only want them to store encrypted data and not have any access to my private encryption keys. If the service has to be installed locally, either on my computer or on a server (private calendar server) that is fine. All I am interested in is finding the most secure method of having a calendar. Ideally it would be similar in use to Google Calendar or iCal.
To the best of my knowledge, no good end-to-end encrypted calendar is available at the moment (Dec 2016). The only option I'm familiar with is with a Swedish private email company called https://www.countermail.com. However, their calendar, whilst end-to-end encrypted (stored on their side encrypted, transmitted encrypted and decrypted only locally on your machine), has some downsides: it is very basic in functionality and cannot be synced to different devices. It also comes as part of a larger email package - not just the calendar.
Two private email companies, Protonmail and Tutanota, have annoucned their intention to provide encrypted calendars soon (Tutanota specify the first half of 2017). This should provide more decent options.
You should also know that Izzy's previous answer is wrong: using ownCloud (or NextCloud) and syncing your calendar via https is NOT the same as end-to-end encrypted. In fact, it is ONLY encrypted in transit, and not at either end. Https is used for transmitting data, not for storing it. End-to-end encryption means that the data is stored encrypted at both ends as well as in transit. Izzy is not using end-to-end encryption.
Disclaimer: I'm the developer of the product mentioned in this answer. I did not post earlier as I misread the question to be absolutely limited to open source.
If you're searching for a solution for Android and Thunderbird, GeneralSync might be an option: it performs an automatic and decentral synchronization of contacts and calendars within devices in the same local network. By carrying your smartphone or a similar device with you, this also permits to bridge multiple networks without any connection through the internet¹. As any communication occurs directly between your devices and is encrypted, and thus satisfies your end-to-end requirement.
GeneralSync is currently available for free after registering for the public beta and can be installed on all major desktop operating systems (requires Java) and Android devices. I intend to add support for more applications and data types in the future, but as that requires money GeneralSync is not available under an open source license.
¹ If you know how to setup port forwarding and DNS-resolveable hostnames you can of course also sync via internet
it's been a couple of months since i discovered Mailfence, which is a complete email suit with a diversity of tools such as; calendar,documents and groups.As i was searching more about them i found out that everything you do it's an end to end encryption, basically privacy and security is very important. There are also other providers, who offer almost the same things but this was referred to me by a friend and i am actually very delighted with their service. Recently i discovered they also have a blog where they post Tutorials and articles not only for their customers.
Article about Calendar For those who are interested for a end to end encrypted calendar they also do have a very updated article.
I hope this was usefull. If you have a better solution please do share with us.
Disclaimer: I'm the developer of one of the product (Calendar Lock PEA) mentioned in this answer.
As far as I know, there are still not many options at the moment.
The CalDAV protocol does not allow encryption, therefore all applications using this protocol in its pure form (Fruxx, Nextcloud, ownCloud, Mailfence...) are excluded.
As far as I know, today (2019) only EteSync exists with end-to-end encryption and if you have an email address at Tutanota, there seems to be an encrypted calendar there as well. Both are open source.
If you are looking for an offline calendar or synchronize the data yourself, you can use also the Calendar Lock PEA.