diff --git a/pyproject.toml b/pyproject.toml
index bf0236d..fe48ab2 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -10,10 +10,15 @@ requires-python = ">=3.11"
 classifiers = [
     # TODO
 ]
+dynamic = ["version"]
 dependencies = [
-    "tomlkit"
+    "tomlkit",
+]
+
+[project.optional-dependencies]
+test = [
+    "pytest",
 ]
-dynamic = ["version"]
 
 [project.urls]
 # TODO
diff --git a/src/bsv/command/init.py b/src/bsv/command/init.py
index 1d14d45..765f1cc 100644
--- a/src/bsv/command/init.py
+++ b/src/bsv/command/init.py
@@ -87,11 +87,17 @@ def init(
     bsv_table.add(tomlkit.comment("Mapping between bsv tree and the actual filesystem."))
     bsv_table.add("path_map", tomlkit.array())
 
+    cas_table = tomlkit.table()
+    cas_table.add("type", "simple")
+    cas_table.add("hash", "sha256")
+
     doc = tomlkit.document()
     doc.add(tomlkit.comment("bsv repository configuration"))
     doc.add(tomlkit.comment(f"Created by {getlogin()} on {DateTime.now().isoformat()}."))
     doc.add(tomlkit.nl())
     doc.add("bsv", bsv_table)
+    doc.add(tomlkit.nl())
+    doc.add("cas", cas_table)
 
     config_path = destination / "bsv_config.toml"
     try:
diff --git a/src/bsv/simple_cas/__init__.py b/src/bsv/simple_cas/__init__.py
index 3e9d59f..a3e483a 100644
--- a/src/bsv/simple_cas/__init__.py
+++ b/src/bsv/simple_cas/__init__.py
@@ -14,3 +14,6 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 from __future__ import annotations
+
+
+from bsv.simple_cas.cas import SimpleCas as Cas
diff --git a/src/bsv/simple_cas/cas.py b/src/bsv/simple_cas/cas.py
new file mode 100644
index 0000000..504fe43
--- /dev/null
+++ b/src/bsv/simple_cas/cas.py
@@ -0,0 +1,134 @@
+# bsv - Backup, Synchronization, Versioning
+# Copyright (C) 2023  Simon Boyé
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+from __future__ import annotations
+
+from dataclasses import dataclass
+import hashlib
+from pathlib import Path
+from typing import Any, BinaryIO, Callable, Optional
+
+from bsv.simple_cas.util import Hash
+
+
+class SimpleCas:
+    _root_dir: Path
+    _hash_factory: Callable[[], Hash]
+    _digest_size: int
+
+    _index: dict[bytes, IndexItem]
+
+    _is_inside_context: bool = False
+
+    def __init__(self, root_dir: Path, hash_factory: Callable[[], Hash]):
+        self._root_dir = root_dir
+        self._hash_factory = hash_factory
+        self._digest_size = self._hash_factory().digest_size
+
+        self._index = {}
+        if (self._root_dir / "cas.idx").exists():
+            with (self._root_dir / "cas.idx").open("rb") as stream:
+                while True:
+                    digest = stream.read(self._digest_size)
+                    if not digest:
+                        break
+                    object_type = stream.read(4)
+                    offset = int.from_bytes(stream.read(4))
+                    size = int.from_bytes(stream.read(4))
+                    self._index[digest] = IndexItem(object_type, offset, size)
+
+    @classmethod
+    def from_obj(cls, root_dir: Path, obj: dict[str, Any]) -> SimpleCas:
+        return SimpleCas(
+            root_dir = root_dir,
+            hash_factory = hashlib.new(obj["hash"]), # type: ignore
+        )
+
+    def __enter__(self):
+        assert not self._is_inside_context
+        self._is_inside_context = True
+
+    def __exit__(self, exc_type, exc_value, traceback):
+        assert self._is_inside_context
+        self._is_inside_context = False
+
+    def __len__(self) -> int:
+        return len(self._index)
+
+    def __contains__(self, digest: bytes) -> bool:
+        assert len(digest) == self._digest_size
+        return digest in self._index
+
+    def read(self, digest: bytes) -> Optional[Object]:
+        item = self._index.get(digest)
+        if item is None:
+            return None
+
+        with (self._root_dir / "cas.dat").open("rb") as stream:
+            stream.seek(item.offset)
+            assert stream.read(self._digest_size) == digest
+            object_type = stream.read(4)
+            assert object_type == item.object_type
+            size = int.from_bytes(stream.read(4))
+            assert size == item.size
+            data = stream.read(size)
+
+        return Object(object_type, data)
+
+    def write(self, object_type: bytes, data: bytes) -> bytes:
+        assert len(object_type) == 4
+        assert len(data) < 2**32
+
+        hash = self._hash_factory()
+        hash.update(object_type)
+        hash.update(b"\0")
+        hash.update(len(data).to_bytes(4))
+        hash.update(b"\0")
+        hash.update(data)
+        digest = hash.digest()
+
+        if digest not in self:
+            with self._open_writer(digest, object_type, len(data)) as out:
+                out.write(digest)
+                out.write(object_type)
+                out.write(len(data).to_bytes(4))
+                out.write(data)
+
+        return digest
+
+    def _open_writer(self, digest: bytes, object_type: bytes, size: int) -> BinaryIO:
+        dat_file = (self._root_dir / "cas.dat").open("ab")
+        offset = dat_file.tell()
+        self._index[digest] = IndexItem(object_type, offset, size)
+
+        with (self._root_dir / "cas.idx").open("ab") as idx_file:
+            idx_file.write(digest)
+            idx_file.write(object_type)
+            idx_file.write(offset.to_bytes(4))
+            idx_file.write(size.to_bytes(4))
+
+        return dat_file
+
+
+@dataclass
+class Object:
+    object_type: bytes
+    data: bytes
+
+@dataclass
+class IndexItem:
+    object_type: bytes
+    offset: int
+    size: int
diff --git a/src/bsv/simple_cas/util.py b/src/bsv/simple_cas/util.py
new file mode 100644
index 0000000..fd31b33
--- /dev/null
+++ b/src/bsv/simple_cas/util.py
@@ -0,0 +1,31 @@
+# bsv - Backup, Synchronization, Versioning
+# Copyright (C) 2023  Simon Boyé
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+
+
+class Hash(ABC):
+    name: str
+    digest_size: int
+
+    @abstractmethod
+    def update(self, *data: bytes | bytearray | memoryview):
+        ...
+
+    @abstractmethod
+    def digest(self) -> bytes:
+        ...
diff --git a/tests/test_simple_cas.py b/tests/test_simple_cas.py
new file mode 100644
index 0000000..c219fe7
--- /dev/null
+++ b/tests/test_simple_cas.py
@@ -0,0 +1,76 @@
+# bsv - Backup, Synchronization, Versioning
+# Copyright (C) 2023  Simon Boyé
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+from __future__ import annotations
+from hashlib import sha256
+from pathlib import Path
+
+from tempfile import TemporaryDirectory
+import pytest
+
+from bsv.simple_cas.cas import SimpleCas
+
+
+@pytest.fixture
+def tmp_dir():
+    with TemporaryDirectory(prefix="simple_cas_") as tmp_dir:
+        yield Path(tmp_dir)
+
+
+@pytest.fixture
+def cas(tmp_dir):
+    cas = SimpleCas(
+        tmp_dir,
+        sha256, # type: ignore
+    )
+    with cas:
+        yield cas
+
+
+def test_simple_cas(tmp_dir: Path):
+    cas = SimpleCas(
+        tmp_dir,
+        sha256, # type: ignore
+    )
+    with cas:
+        assert len(cas) == 0
+
+        data = b"Hello World!"
+        digest = cas.write(b"blob", data)
+
+        assert len(cas) == 1
+        assert digest in cas
+
+        obj = cas.read(digest)
+        assert obj is not None
+        assert obj.object_type == b"blob"
+        assert obj.data == data
+
+    cas = SimpleCas(
+        tmp_dir,
+        sha256, # type: ignore
+    )
+    with cas:
+        assert len(cas) == 1
+        assert digest in cas
+
+        obj = cas.read(digest)
+        assert obj is not None
+        assert obj.object_type == b"blob"
+        assert obj.data == data
+
+        digest2 = cas.write(b"blob", data)
+        assert digest2 == digest
+        assert len(cas) == 1