add xmasspirit solve

crypto/XMASSpirit/README.md

@@ -9,4 +9,24 @@ letter?
 
 ## Flag
 
+HTB{4ff1n3_c1ph3r_15_51mpl3_m47h5}
+
+- See letter.pdf
+
+## How to solve
+
+See `decrypt.py`
+
 ## Progress
+
+### Code analysis:
+  - Choose a `randint` `a` from `1` to `mod`, that is not divisible by mod.
+  - Choose a `randint` `b` from `1` to `mod`.
+  - for each byte in `dt`
+    - calculate `(a * byte + b) % mod`
+    - append to output bytes
+  - return bytes when done
+
+### Notes
+
+- `s_next = (a * s_prev + b) mod p` is a linear congruential generator