diff --git a/docs/proposed/http-storage-node-protocol.rst b/docs/proposed/http-storage-node-protocol.rst
index ade0bc167..3a09ccae0 100644
--- a/docs/proposed/http-storage-node-protocol.rst
+++ b/docs/proposed/http-storage-node-protocol.rst
@@ -45,7 +45,7 @@ Glossary
      (sometimes "slot" is considered a synonym for "storage index of a slot")
 
    storage index
-     a short string which can address a slot or a bucket
+     a 16 byte string which can address a slot or a bucket
      (in practice, derived by hashing the encryption key associated with contents of that slot or bucket)
 
    write enabler
diff --git a/docs/specifications/derive_renewal_secret.py b/docs/specifications/derive_renewal_secret.py
new file mode 100644
index 000000000..75009eda4
--- /dev/null
+++ b/docs/specifications/derive_renewal_secret.py
@@ -0,0 +1,87 @@
+
+"""
+This is a reference implementation of the lease renewal secret derivation
+protocol in use by Tahoe-LAFS clients as of 1.16.0.
+"""
+
+from allmydata.util.base32 import (
+    a2b as b32decode,
+    b2a as b32encode,
+)
+from allmydata.util.hashutil import (
+    tagged_hash,
+    tagged_pair_hash,
+)
+
+
+def derive_renewal_secret(lease_secret: bytes, storage_index: bytes, tubid: bytes) -> bytes:
+    assert len(lease_secret) == 32
+    assert len(storage_index) == 16
+    assert len(tubid) == 20
+
+    bucket_renewal_tag = b"allmydata_bucket_renewal_secret_v1"
+    file_renewal_tag = b"allmydata_file_renewal_secret_v1"
+    client_renewal_tag = b"allmydata_client_renewal_secret_v1"
+
+    client_renewal_secret = tagged_hash(lease_secret, client_renewal_tag)
+    file_renewal_secret = tagged_pair_hash(
+        file_renewal_tag,
+        client_renewal_secret,
+        storage_index,
+    )
+    peer_id = tubid
+
+    return tagged_pair_hash(bucket_renewal_tag, file_renewal_secret, peer_id)
+
+def demo():
+    secret = b32encode(derive_renewal_secret(
+        b"lease secretxxxxxxxxxxxxxxxxxxxx",
+        b"storage indexxxx",
+        b"tub idxxxxxxxxxxxxxx",
+    )).decode("ascii")
+    print("An example renewal secret: {}".format(secret))
+
+def test():
+    # These test vectors created by intrumenting Tahoe-LAFS
+    # bb57fcfb50d4e01bbc4de2e23dbbf7a60c004031 to emit `self.renew_secret` in
+    # allmydata.immutable.upload.ServerTracker.query and then uploading a
+    # couple files to a couple different storage servers.
+    test_vector = [
+        dict(lease_secret=b"boity2cdh7jvl3ltaeebuiobbspjmbuopnwbde2yeh4k6x7jioga",
+             storage_index=b"vrttmwlicrzbt7gh5qsooogr7u",
+             tubid=b"v67jiisoty6ooyxlql5fuucitqiok2ic",
+             expected=b"osd6wmc5vz4g3ukg64sitmzlfiaaordutrez7oxdp5kkze7zp5zq",
+        ),
+        dict(lease_secret=b"boity2cdh7jvl3ltaeebuiobbspjmbuopnwbde2yeh4k6x7jioga",
+             storage_index=b"75gmmfts772ww4beiewc234o5e",
+             tubid=b"v67jiisoty6ooyxlql5fuucitqiok2ic",
+             expected=b"35itmusj7qm2pfimh62snbyxp3imreofhx4djr7i2fweta75szda",
+        ),
+        dict(lease_secret=b"boity2cdh7jvl3ltaeebuiobbspjmbuopnwbde2yeh4k6x7jioga",
+             storage_index=b"75gmmfts772ww4beiewc234o5e",
+             tubid=b"lh5fhobkjrmkqjmkxhy3yaonoociggpz",
+             expected=b"srrlruge47ws3lm53vgdxprgqb6bz7cdblnuovdgtfkqrygrjm4q",
+        ),
+        dict(lease_secret=b"vacviff4xfqxsbp64tdr3frg3xnkcsuwt5jpyat2qxcm44bwu75a",
+             storage_index=b"75gmmfts772ww4beiewc234o5e",
+             tubid=b"lh5fhobkjrmkqjmkxhy3yaonoociggpz",
+             expected=b"b4jledjiqjqekbm2erekzqumqzblegxi23i5ojva7g7xmqqnl5pq",
+        ),
+    ]
+
+    for n, item in enumerate(test_vector):
+        derived = b32encode(derive_renewal_secret(
+            b32decode(item["lease_secret"]),
+            b32decode(item["storage_index"]),
+            b32decode(item["tubid"]),
+        ))
+        assert derived == item["expected"] , \
+            "Test vector {} failed: {} (expected) != {} (derived)".format(
+                n,
+                item["expected"],
+                derived,
+            )
+    print("{} test vectors validated".format(len(test_vector)))
+
+test()
+demo()