doc: start to write about decryption and reencryption in fundamentals

docs/fundamentals/gateway/decryption.md

@@ -1 +1,30 @@
 # Decryption
+
+Everything in fhEVM is encrypted, at some point one could need to decrypt somes values. Let's give as illustration a blind auction application.
+After reaching the end of the auction, one need to discover (only) the winner, here is where a asynchronous decrypt could appear. 
+
+
+> :warning: **Decryption is public**: It means everyone will be able to see the value. If this is a personal information see [Reencryption](./reencryption.md)
+
+## How it's working
+
+The Gateway acts as an oracle service: it will listen to decryption request events and return the decrypted value through a callback function.
+The responsabilities of the Gateway are:
+- Listening decryption request from fhEVM that contains a handle `h` to the associated ciphertext `C`
+- Computing a storage proof `P` to attest C is decryptable
+- Retrieve C from fhEVM using `h` as key
+- Send a decyption request to TKMS which in turn is running an internal blockchain aka `KMS BC`
+- Wait and listen for `decyptionResponse` (containing the plaitext and a few signatures from KMS to attest the integrity of the palintext) event from `KMS BC`
+- Return `decyptionResponse` through the callback function
+
+## High level overview of the decryption flow 
+
+We allow explicit decryption requests for any encrypted type. The values are decrypted with the network private key.
+
+![](asyncDecrypt.png)
+
+
+
+
+
+

docs/fundamentals/gateway/reencryption.md

@@ -1 +1,9 @@
 # Reencryption
+
+
+Reencryption is performed on the client side by calling the gateway service using the [fhevmjs](https://github.com/zama-ai/fhevmjs/) library. To do this, you need to provide a view function that returns the ciphertext to be reencrypted.
+
+1. The dApp retrieves the ciphertext from the view function (e.g., balanceOf).
+2. The dApp generates a keypair for the user and requests the user to sign the public key.
+3. The dApp calls the gateway, providing the ciphertext, public key, user address, contract address, and the user's signature.
+4. The dApp decrypts the received value with the private key.