This guide will go into detail on how you can get the Orokin Cipher.
Orokin Ciphers are hidden inside the Vaults of Orokin Derelict maps. Not all Orokin Derelict maps have Vaults - Exterminate, Capture, Survival and Assassinate have them but Orokin Derelict Defence doesn't. Vaults are usually hidden quite well, and has to be unlocked by a key that you bring in your item slots.
There are 4 types of keys (called Dragon Key), and the Vault will need in random one of the 4, so to ensure that the Vault can be opened, have your squadmates bring different keys. The keys are built with a minor amount of easily found resources, a Void key (any kind), and a market-bought blueprint. Once the blueprint is bought, there is no need to buy it again.
Each key has a debilitating effect when it is equipped, so it is best that the Arsenal be changed to suit the Key, or the Key is built to suit the current Arsenal.
For the sake of completeness, here's how to get an Orokin Derelict Key. While it is optional (not all members need to have an Orokin Derelict Key, only the squad leader does), this can be helpful if you want to be proactive and recruit people once you have got the gist of how Vault-opening is done. For your first time, you can build one to get people to help you, or tag along.
My experience is to fit your Arsenal around the Dragon Key of your choice, then get yourself an ODKey and ask people to bring the remaining keys that you don't want to have to deal with. People learn differently - maybe you want to learn by looking, maybe you want to by doing. This is for the headstrong types.
The best ODKey to build in my opinion is Orokin Derelict Exterminate for its linear, no-pressure feel once you've killed everything, since there is time to wander about safely then. It is also far simpler to kill off a set quota than chasing about the map for something.
Delve into battle
Method 1 - get in touch with people who are like minded by asking to join their squad (click on image for full size).
Get your rewards
Get 5 runs if you want Mirage, and I'm sure you'll know to get more runs later on!
Click these links for my other Warframe posts:
* Warframe - Failed to Join Session (PC fix): (part 1) (part 2) (part 3)
* Warframe - How to Invite Friends (Update 14)
*Warframe - How to get a Kubrow (step-by-step with pictures)
*Warframe - How to get a Mirage Warframe (step-by-step with pictures)