Finalize the "concepts" part

content/documentation/design/_index.md

@@ -7,13 +7,27 @@ template = "documentation.html"
 
 ## Goals
 
+
+**Highly resilient** - Multiple instances of Aerogramme can been run in parallel without coordination.
+Multi-region support, survive datacenter failures.
+
+**Easy to operate** - Transparently replicate mailbox and solve conflicts. Integrate with your LDAP server. Privacy friendly
+
+**Per-user encryption of mailboxes.**
+Can be run as a local proxy to hide your mailbox content from the server.
+
+
 ## Main concepts
 
-[Per-user encryption](@/documentation/design/per-user-encryption.md) - TODO
+[Per-user encryption](@/documentation/design/per-user-encryption.md) - Aerogramme can't persist data in plain text,
+instead its whole data model is built upon the idea that a mailbox is a series of encrypted blob. These blobs do not reveal
+the mailbox name, the metadata of stored emails or even the flags that have been put on them.
 
-[Unbreakable mailboxes](@/documentation/design/unbreakable-mailboxes.md) - TODO
+**Continuous Mailbox Merging** - As multiple instances of Aerogramme can be run simultaneously, and that's possible
+that 2 instances interact with the same mailbox (over Garage), each process monitors external writes for the mailbox
+they track and automatically do the merging [in a correct way](@/documentation/internals/imap_uid.md).
 
 **Modular design** - Login and Mailbox storage is abstracted behind an interface: multiple
 implementations are thus possible.
 
-**Micro-service** - Stateless. 12 factor app.
+**Microservice** - Aerogramme is stateless and tries to adhere as much as possible to the [12 factor app](https://12factor.net/) principles so it's easy to run in a cluster.

content/documentation/design/per-user-encryption.md

@@ -8,6 +8,11 @@ Aerogramme can't store plaintext data, instead all users data must be encrypted
 Of course, cryptography is always a tradeoff with other properties (usability, compatibility, features, etc.),
 so the way the key is derived and where the encryption/decryption can take place can be configured.
 
+## Compared to PGP
+
+PGP only encrypts the body of the email, it keeps in cleartext the metadata of your email (fields like From:, To:, or Subject: are readable by an attacker), 
+it can't protect your flags, your mailbox names, etc. Conversely, all this data is encrypted in Aerogramme.
+
 ## Security flavors
 
 These different configurations are identified as flavors:

content/documentation/design/unbreakable-mailboxes.md

@@ -1,6 +0,0 @@
-+++
-title = "Unbreakable mailboxes"
-weight = 20
-+++
-
-Test