diff --git a/pyodide/cryptography-in-pyodide.md b/pyodide/cryptography-in-pyodide.md
new file mode 100644
index 0000000000..af769b22eb
--- /dev/null
+++ b/pyodide/cryptography-in-pyodide.md
@@ -0,0 +1,106 @@
+# Cryptography in Pyodide
+
+Today I was evaluating if the Python [cryptography](https://cryptography.io/) package was a sensible depedency for one of my projects.
+
+It's clearly the most responsible package to use for implementing encryption in Python, but I was nervous about adding it as a dependency to a project that could work without using it at all (with some design changes).
+
+My key concern: I want this project to work running in Pyodide in WebAssembly in the future, and was nervous that `cryptography`, being written partially in Rust, wouldn't work in that environment.
+
+It turns out it works just fine!
+
+I tested it in the Pyodide REPL at https://pyodide.org/en/stable/console.html
+
+![Welcome to the Pyodide terminal emulator 🐍 Python 3.11.3 (main, Sep 25 2023 20:45:01) on WebAssembly/Emscripten Type 'help', 'copyright', 'credits' or 'license' for more information. >>> import micropip >>> await micropip.install('cryptography') >>> from cryptography.fernet import Fernet >>> key = Fernet.generate_key() >>> cipher_suite = Fernet(key) >>> encrypted_text = cipher_suite.encrypt(b'Secret message') >>> encrypted_text b'gAAAAABlY7AZ-OuJAEv1J4KufF8vpreyehPeejdPqluXwD0G_mfFg-Zl1AvEza6F2DXb WMEkIcKc4B0Hb0qJ457pje5FhO5Vyw==' >>> decrypted_text = cipher_suite.decrypt(encrypted_text) >>> decrypted_text b'Secret message' - then on the right the DevTools Network panel shows that this worked by loading a 4.27MB file called cryptography-39.0.2-cp311-cp311-emscripten_3_1_45_wasm32.whl from the Pyodide CDN package.
+](https://github.com/simonw/til/assets/9599/94440423-af87-4c78-b413-35a31dd0a894)
+
+The following import code worked without errors:
+```python
+import micropip
+await micropip.install("cryptography")
+from cryptography.fernet import Fernet
+```
+Then I tested it like this:
+```python
+key = Fernet.generate_key()
+cipher_suite = Fernet(key)
+encrypted_text = cipher_suite.encrypt(b"Secret message")
+print(encrypted_text)
+# b'gAAAAABlY7AZ-OuJAEv1J4KufF8vpreyehPeejdPqluXwD0G_mfFg-Zl1AvEza6F2DXbWMEkIcKc4B0Hb0qJ457pje5FhO5Vyw=='
+decrypted_text = cipher_suite.decrypt(encrypted_text)
+print(decrypted_text)
+# b'Secret message'
+```
+
+From sniffing around in the browser DevTools network panel, it turns out Pyodide provides its own packaged version of the Cryptography package in a file called `cryptography-39.0.2-cp311-cp311-emscripten_3_1_45_wasm32.whl`.
+
+I found the source of this custom package in the Pyodide repository, in the [packages/cryptography](https://github.com/pyodide/pyodide/tree/main/packages/cryptography) folder.
+
+## More packages
+
+That [packages/](https://github.com/pyodide/pyodide/tree/main/packages) folder has a whole bunch of other useful modules that have been custom packaged to work with Pyodide. A few that caught my eye:
+
+- `Jinja2`
+- `Pillow`
+- `Pygments`
+- `biopython`
+- `fastparquet`
+- `ffmpg`
+- `gdal`
+- `geopandas`
+- `geos`
+- `html5lib`
+- `jsonschema`
+- `libheif`
+- `libwebp`
+- `lxml`
+- `msgpack`
+- `shapely`
+- `sqlalchemy`
+- `xgboost`
+- `sqlite3`
+
+Each of these comes with a `meta.yaml` file that defines how it should be compiled, plus a test Python module to verify it and optionally a set of patches to apply before compilation.
+
+Here's what [packages/sqlite3/meta.yaml](https://github.com/pyodide/pyodide/blob/main/packages/sqlite3/meta.yaml) looks like:
+
+```yaml
+package:
+  name: sqlite3
+  version: 1.0.0 # Nonsense
+  tag:
+    - always
+  top-level:
+    - sqlite3
+    - _sqlite3
+source:
+  sha256: $(PYTHON_ARCHIVE_SHA256)
+  url: $(PYTHON_ARCHIVE_URL)
+build:
+  type: cpython_module
+  script: |
+    export FILES=(
+      "Modules/_sqlite/blob.c"
+      "Modules/_sqlite/connection.c"
+      "Modules/_sqlite/cursor.c"
+      "Modules/_sqlite/microprotocols.c"
+      "Modules/_sqlite/module.c"
+      "Modules/_sqlite/prepare_protocol.c"
+      "Modules/_sqlite/row.c"
+      "Modules/_sqlite/statement.c"
+      "Modules/_sqlite/util.c"
+    )
+
+    embuilder build sqlite3 --pic
+
+    for file in "${FILES[@]}"; do
+      emcc $STDLIB_MODULE_CFLAGS -c "${file}" -o "${file/.c/.o}"  \
+        -sUSE_SQLITE3 -DMODULENAME=sqlite
+    done
+
+    OBJECT_FILES=$(find Modules/_sqlite/ -name "*.o")
+    emcc $OBJECT_FILES -o $DISTDIR/_sqlite3.so $SIDE_MODULE_LDFLAGS \
+       -sUSE_SQLITE3 -lsqlite3
+
+    cd Lib && tar --exclude=test -cf - sqlite3 | tar -C $DISTDIR -xf -
+```
+One thing I don't understand is why some pure-Python packages such as Click also [get this treatment](https://github.com/pyodide/pyodide/blob/main/packages/click/meta.yaml) - since those should work if installed directly from PyPI using `micropip.install()`.