diff --git a/src/cas/README.md b/src/cas/README.md
new file mode 100644
index 000000000..a2342fac8
--- /dev/null
+++ b/src/cas/README.md
@@ -0,0 +1,6 @@
+`casfs` is a Content Addressable Storage (CAS) abstraction over a file system.
+It has no folders nor files. Instead, it has *blobs* which are identified by their content.
+
+Essentially, it provides two main operations: `put` and `get`. The `put` operation
+takes a blob and stores it in the underlying file system and returns the blob's hash digest.
+The `get` operation takes a hash and returns the blob, which matches the hash digest, if it exists.
diff --git a/src/cas/types.ts b/src/cas/types.ts
new file mode 100644
index 000000000..4d9a303e4
--- /dev/null
+++ b/src/cas/types.ts
@@ -0,0 +1,12 @@
+import type {CrudResourceInfo} from "../crud/types";
+
+export interface CasApi {
+  put(blob: Uint8Array): Promise<string>;
+  get(hash: string, options?: CasGetOptions): Promise<Uint8Array>;
+  del(hash: string, silent?: boolean): Promise<void>;
+  info(hash: string): Promise<CrudResourceInfo>;
+}
+
+export interface CasGetOptions {
+  skipVerification?: boolean;
+}
diff --git a/src/crud-to-cas/CrudCas.ts b/src/crud-to-cas/CrudCas.ts
new file mode 100644
index 000000000..cd4e1be05
--- /dev/null
+++ b/src/crud-to-cas/CrudCas.ts
@@ -0,0 +1,54 @@
+import {hashToLocation} from "./util";
+import type {CasApi} from "../cas/types";
+import type {CrudApi, CrudResourceInfo} from "../crud/types";
+
+export interface CrudCasOptions {
+  hash: (blob: Uint8Array) => Promise<string>;
+}
+
+const normalizeErrors = async <T>(code: () => Promise<T>): Promise<T> => {
+  try {
+    return await code();
+  } catch (error) {
+    if (error && typeof error === 'object') {
+      switch (error.name) {
+        case 'ResourceNotFound':
+        case 'CollectionNotFound':
+          throw new DOMException(error.message, 'BlobNotFound');
+      }
+    }
+    throw error;
+  }
+};
+
+export class CrudCas implements CasApi {
+  constructor(protected readonly crud: CrudApi, protected readonly options: CrudCasOptions) {}
+
+  public readonly put = async (blob: Uint8Array): Promise<string> => {
+    const digest = await this.options.hash(blob);
+    const [collection, resource] = hashToLocation(digest);
+    await this.crud.put(collection, resource, blob);
+    return digest;
+  };
+
+  public readonly get = async (hash: string): Promise<Uint8Array> => {
+    const [collection, resource] = hashToLocation(hash);
+    return await normalizeErrors(async () => {
+      return await this.crud.get(collection, resource);
+    });
+  };
+
+  public readonly del = async (hash: string, silent?: boolean): Promise<void> => {
+    const [collection, resource] = hashToLocation(hash);
+    await normalizeErrors(async () => {
+      return await this.crud.del(collection, resource, silent);
+    });
+  };
+
+  public readonly info = async (hash: string): Promise<CrudResourceInfo> => {
+    const [collection, resource] = hashToLocation(hash);
+    return await normalizeErrors(async () => {
+      return await this.crud.info(collection, resource);
+    });
+  };
+}
diff --git a/src/crud-to-cas/__tests__/CrudCas.test.ts b/src/crud-to-cas/__tests__/CrudCas.test.ts
new file mode 100644
index 000000000..6b116ab6b
--- /dev/null
+++ b/src/crud-to-cas/__tests__/CrudCas.test.ts
@@ -0,0 +1,105 @@
+import { of } from 'thingies';
+import { createHash } from 'crypto';
+import { memfs } from '../..';
+import { onlyOnNode20 } from '../../__tests__/util';
+import { NodeFileSystemDirectoryHandle } from '../../node-to-fsa';
+import { FsaCrud } from '../../fsa-to-crud/FsaCrud';
+import { CrudCas } from '../CrudCas';
+
+const hash = async (blob: Uint8Array): Promise<string> => {
+  const shasum = createHash('sha1')
+  shasum.update(blob)
+  return shasum.digest('hex') 
+};
+
+const setup = () => {
+  const fs = memfs();
+  const fsa = new NodeFileSystemDirectoryHandle(fs, '/', { mode: 'readwrite' });
+  const crud = new FsaCrud(fsa);
+  const cas = new CrudCas(crud, {hash});
+  return { fs, fsa, crud, cas, snapshot: () => (<any>fs).__vol.toJSON() };
+};
+
+const b = (str: string) => {
+  const buf = Buffer.from(str);
+  return new Uint8Array(buf.buffer, buf.byteOffset, buf.byteLength);
+};
+
+onlyOnNode20('CrudCas', () => {
+  describe('.put()', () => {
+    test('can store a blob', async () => {
+      const blob = b('hello world');
+      const { cas, snapshot } = setup();
+      const hash = await cas.put(blob);
+      expect(hash).toBe('2aae6c35c94fcfb415dbe95f408b9ce91ee846ed');
+      expect(snapshot()).toMatchSnapshot();
+    });
+  });
+
+  describe('.get()', () => {
+    test('can retrieve existing blob', async () => {
+      const blob = b('hello world');
+      const { cas } = setup();
+      const hash = await cas.put(blob);
+      const blob2 = await cas.get(hash);
+      expect(blob2).toStrictEqual(blob);
+    });
+
+    test('throws if blob does not exist', async () => {
+      const blob = b('hello world 2');
+      const { cas } = setup();
+      const hash = await cas.put(blob);
+      const [, err] = await of(cas.get('2aae6c35c94fcfb415dbe95f408b9ce91ee846ed'));
+      expect(err).toBeInstanceOf(DOMException);
+      expect((<any>err).name).toBe('BlobNotFound');
+    });
+  });
+
+  describe('.info()', () => {
+    test('can retrieve existing blob info', async () => {
+      const blob = b('hello world');
+      const { cas } = setup();
+      const hash = await cas.put(blob);
+      const info = await cas.info(hash);
+      expect(info.size).toBe(11);
+    });
+
+    test('throws if blob does not exist', async () => {
+      const blob = b('hello world 2');
+      const { cas } = setup();
+      const hash = await cas.put(blob);
+      const [, err] = await of(cas.info('2aae6c35c94fcfb415dbe95f408b9ce91ee846ed'));
+      expect(err).toBeInstanceOf(DOMException);
+      expect((<any>err).name).toBe('BlobNotFound');
+    });
+  });
+
+  describe('.del()', () => {
+    test('can delete an existing blob', async () => {
+      const blob = b('hello world');
+      const { cas } = setup();
+      const hash = await cas.put(blob);
+      const info = await cas.info(hash);
+      await cas.del(hash);
+      const [, err] = await of(cas.info(hash));
+      expect(err).toBeInstanceOf(DOMException);
+      expect((<any>err).name).toBe('BlobNotFound');
+    });
+
+    test('throws if blob does not exist', async () => {
+      const blob = b('hello world 2');
+      const { cas } = setup();
+      const hash = await cas.put(blob);
+      const [, err] = await of(cas.del('2aae6c35c94fcfb415dbe95f408b9ce91ee846ed'));
+      expect(err).toBeInstanceOf(DOMException);
+      expect((<any>err).name).toBe('BlobNotFound');
+    });
+
+    test('does not throw if "silent" flag is provided', async () => {
+      const blob = b('hello world 2');
+      const { cas } = setup();
+      const hash = await cas.put(blob);
+      await cas.del('2aae6c35c94fcfb415dbe95f408b9ce91ee846ed', true);
+    });
+  });
+});
diff --git a/src/crud-to-cas/__tests__/__snapshots__/CrudCas.test.ts.snap b/src/crud-to-cas/__tests__/__snapshots__/CrudCas.test.ts.snap
new file mode 100644
index 000000000..48f1ceea7
--- /dev/null
+++ b/src/crud-to-cas/__tests__/__snapshots__/CrudCas.test.ts.snap
@@ -0,0 +1,7 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`CrudCas .put() can store a blob 1`] = `
+Object {
+  "/ed/46/2aae6c35c94fcfb415dbe95f408b9ce91ee846ed": "hello world",
+}
+`;
diff --git a/src/crud-to-cas/util.ts b/src/crud-to-cas/util.ts
new file mode 100644
index 000000000..e12ab1cab
--- /dev/null
+++ b/src/crud-to-cas/util.ts
@@ -0,0 +1,9 @@
+import type {FsLocation} from "../fsa-to-node/types";
+
+export const hashToLocation = (hash: string): FsLocation => {
+  if (hash.length < 20) throw new TypeError('Hash is too short');
+  const lastTwo = hash.slice(-2);
+  const twoBeforeLastTwo = hash.slice(-4, -2);
+  const folder = [lastTwo, twoBeforeLastTwo];
+  return [folder, hash];
+};
diff --git a/src/crud/types.ts b/src/crud/types.ts
index 05b51e398..e75cb8c03 100644
--- a/src/crud/types.ts
+++ b/src/crud/types.ts
@@ -7,7 +7,7 @@ export interface CrudApi {
    * @param data Blob content of the resource.
    * @param options Write behavior options.
    */
-  put: (collection: CrudCollection, id: string, data: Uint8Array, options: CrudPutOptions) => Promise<void>;
+  put: (collection: CrudCollection, id: string, data: Uint8Array, options?: CrudPutOptions) => Promise<void>;
 
   /**
    * Retrieves the content of a resource.