diff --git a/README.md b/README.md
index caeb7d0..532d63a 100644
--- a/README.md
+++ b/README.md
@@ -135,7 +135,7 @@ Seeded once from the OS, then a deterministic AEGIS keystream: each call increme
 
 ### Random-access encrypted files (aeg.raf)
 
-pread/pwrite-style access to files split into independently encrypted chunks, with optional Merkle tree integrity. Storage is pluggable (file path, aeg.raf.FileStorage, aeg.raf.BytesIOStorage, or a custom Storage).
+pread/pwrite-style access to files split into independently encrypted chunks, with optional Merkle tree integrity. Storage is pluggable: a file path, io.BytesIO, aeg.raf.FileStorage / BytesIOStorage / StreamStorage, or a custom Storage.
 
 - raf.create(path_or_storage, key, cipher, chunk_size=65536, merkle=False) / raf.open(...)
 - f.read(size, offset), f.pread(size, offset), f.write(data, offset), f.pwrite(data, offset)
diff --git a/src/aeg/raf.py b/src/aeg/raf.py
index a0399d9..07b56c2 100644
--- a/src/aeg/raf.py
+++ b/src/aeg/raf.py
@@ -4,8 +4,8 @@ Provides pread/pwrite-style access to encrypted files. Files are divided into
 fixed-size chunks, each independently encrypted with a fresh nonce, enabling
 efficient random access without decrypting the whole file.
 
-Use create() / open() with a cipher module (from aeg.cipher()) and either a
-path or a Storage instance.
+Use create() / open() with a cipher module (from aeg.cipher()) and a path,
+an io.BytesIO, or a Storage instance.
 
 Portions of the storage/callback/Merkle/file-like machinery are adapted from
 pyaegis (https://github.com/jedisct1/pyaegis) by Frank Denis, MIT license.
@@ -15,6 +15,7 @@ from __future__ import annotations
 
 import errno as errno_module
 import hashlib
+import io as io_module
 import os
 import threading
 from typing import NamedTuple, Protocol, runtime_checkable
@@ -33,6 +34,7 @@ __all__ = [
     "Storage",
     "FileStorage",
     "BytesIOStorage",
+    "StreamStorage",
     "MerkleHasher",
     "SHA256MerkleHasher",
     "RafInfo",
@@ -218,6 +220,65 @@ class BytesIOStorage:
         pass
 
 
+class StreamStorage:
+    """Storage adapter for seek-based file-like objects (io.BytesIO, open files).
+
+    Uses a lock plus seek/read/write, mirroring FileStorage's fallback path
+    for platforms without os.pread/os.pwrite. The stream is not closed by
+    Raf; closing it remains the caller's responsibility.
+    """
+
+    def __init__(self, stream):
+        self._stream = stream
+        self._lock = threading.Lock()
+
+    def read_at(self, buf: bytearray, offset: int) -> None:
+        """Read exactly len(buf) bytes at offset into buf."""
+        with self._lock:
+            self._stream.seek(offset)
+            view = memoryview(buf)
+            done = 0
+            while done < len(view):
+                chunk = self._stream.read(len(view) - done)
+                if not chunk:
+                    raise OSError(f"Short read: expected {len(view)}, got {done}")
+                view[done : done + len(chunk)] = chunk
+                done += len(chunk)
+
+    def write_at(self, data: bytes, offset: int) -> None:
+        """Write exactly len(data) bytes at offset."""
+        with self._lock:
+            self._stream.seek(offset)
+            view = memoryview(data)
+            done = 0
+            while done < len(view):
+                done += self._stream.write(view[done:])
+
+    def get_size(self) -> int:
+        """Return current stream size."""
+        with self._lock:
+            pos = self._stream.tell()
+            try:
+                return self._stream.seek(0, os.SEEK_END)
+            finally:
+                self._stream.seek(pos)
+
+    def set_size(self, size: int) -> None:
+        """Resize the stream (truncate or extend with zeros)."""
+        self._stream.truncate(size)
+
+    def sync(self) -> None:
+        """Flush the stream if it supports flushing."""
+        if hasattr(self._stream, "flush"):
+            self._stream.flush()
+
+    def __enter__(self) -> StreamStorage:
+        return self
+
+    def __exit__(self, *args) -> None:
+        pass
+
+
 @runtime_checkable
 class MerkleHasher(Protocol):
     """Protocol for Merkle tree hash functions."""
@@ -507,6 +568,8 @@ def probe(storage: Storage) -> RafInfo:
 def _as_storage(storage_or_path, mode: str) -> tuple[Storage, bool]:
     if isinstance(storage_or_path, (str, bytes, os.PathLike)):
         return FileStorage(storage_or_path, mode), True
+    if isinstance(storage_or_path, io_module.BytesIO):
+        return StreamStorage(storage_or_path), False
     return storage_or_path, False
 
 
diff --git a/tests/test_raf.py b/tests/test_raf.py
index faf9618..984a820 100644
--- a/tests/test_raf.py
+++ b/tests/test_raf.py
@@ -25,6 +25,26 @@ def test_roundtrip_bytesio(c):
         assert f.read() == data
 
 
+@pytest.mark.parametrize("c", CIPHERS, ids=cipher_id)
+def test_roundtrip_io_bytesio(c):
+    """A bare io.BytesIO is auto-wrapped in StreamStorage."""
+    import io
+
+    key = c.random_key()
+    data = bytes(range(256)) * 20
+    bio = io.BytesIO()
+    with raf.create(bio, key, c, chunk_size=CHUNK) as f:
+        f.write(data)
+        f.pwrite(b"XY", 1000)  # unaligned write across a chunk boundary
+        data = data[:1000] + b"XY" + data[1002:]
+        f.truncate(2000)
+    with raf.open(bio, key, c) as f:
+        assert f.size == 2000
+        assert f.pread(6, 999) == data[999:1005]
+        assert f.read() == data[:2000]
+    bio.close()
+
+
 @pytest.mark.parametrize("c", CIPHERS, ids=cipher_id)
 def test_roundtrip_filestorage(c, tmp_path):
     key = c.random_key()