diff --git a/src/ISAAC-cipher.md b/src/ISAAC-cipher.md
new file mode 100644
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--- /dev/null
+++ b/src/ISAAC-cipher.md
@@ -0,0 +1,22 @@
+# ISAAC cipher
+
+ISAAC is a cryptographically secure pseudorandom number generator, and a stream cipher designed by Robert J. Jenkins Jr (see [ISAAC](http://burtleburtle.net/bob/rand/isaacafa.html)).
+
+You can find the standard Java implementation used [here](https://github.com/PureCS/rs317-client/blob/master/src/com/runescape/client/io/ISAACCipher.java).
+
+## ISAAC instance creation
+When a client logs into the server, two instances of ISAAC are created,
+and the generated seeds are transmitted between them.  
+One instance is for encryption, and one for decryption.
+
+The seed is comprised of 128-bits, half is generated by the client, and
+half by the server.  
+This is to ensure that no individual entity has entire control of
+the key generation process.
+
+## Use in the client
+ISAAC is used to encrypt and decrypt packet opcodes before transmission, to make packet manipulation harder.
+
+When a packet is sent, its opcode becomes the original opcode added to the generated ISAAC opcode from the encryption instance.
+
+The packet opcode is decrypted analogously on the server.
diff --git a/src/SUMMARY.md b/src/SUMMARY.md
index 8e1f2e7..b74cc3d 100644
--- a/src/SUMMARY.md
+++ b/src/SUMMARY.md
@@ -8,7 +8,8 @@
   - [DMCA policy](./DMCA-policy.md)
   - [Privacy policy](./Privacy-policy.md)
 - [Data](./Data.md)
-  - [Data-Types](./Data-Types.md)
+  - [Data-types](./Data-Types.md)
+    - [ISAAC cipher](./ISAAC-cipher.md)
     - [Word](./Word.md)
     - [DWord](./DWord.md)
     - [QWord](./QWord.md)