fix(secret): authenticate bottled-secret encryption (#468)

The per-bottle egress-secret encryption (secret_store.py) was
unauthenticated CTR/XOR: decrypting with the WRONG ENV_VAR_SECRET
produced garbage that decrypt_value only rejected when it wasn't valid
UTF-8. For short token values that garbage is coincidentally valid UTF-8
~5% of the time, so `reprovision_from_secret` would occasionally "succeed"
with a wrong key and inject a garbage egress credential — and
test_reprovision_rejects_missing_rows_and_wrong_key failed ~5% of runs
(flaky CI, surfaced by this stack's unit job).

Switch to authenticated encrypt-then-MAC: append an HMAC-SHA256 tag over
`nonce || ciphertext`, keyed by a domain-separated MAC subkey derived from
the ENV_VAR_SECRET. decrypt_value verifies the tag (constant-time) before
returning any plaintext, so a wrong key or tampered ciphertext is rejected
deterministically. Blob format is now `nonce || ciphertext || tag`
(the stored rows are transient — re-written every launch — so no
migration is needed).

Pre-existing bug on main, unrelated to the transport work, but it blocks
this stack's CI. Tests: wrong key rejected 200/200; tampered ciphertext
rejected; round-trips unchanged. Deterministic now (was ~5% flaky).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/unit/test_orchestrator_secret_store.py

@@ -2,10 +2,12 @@
 
 from __future__ import annotations
 
+import base64
 import unittest
 
 from bot_bottle.orchestrator.store.secret_store import (
     ENV_VAR_SECRET_NAME,
+    _NONCE_BYTES,
     decrypt_value,
     encrypt_value,
     new_env_var_secret,
@@ -65,22 +67,27 @@ class TestDecryptErrors(unittest.TestCase):
     def setUp(self) -> None:
         self.secret = new_env_var_secret()
 
-    def test_wrong_key_raises_value_error(self) -> None:
+    def test_wrong_key_always_raises_value_error(self) -> None:
+        # Deterministic: the authentication tag rejects a wrong key every time,
+        # so reprovision can never inject a garbage token. Repeat across many
+        # random keys (the old unauthenticated scheme let ~5% through when the
+        # garbage happened to decode as valid UTF-8).
+        for _ in range(200):
+            ct = encrypt_value(self.secret, "secret-token")
+            with self.assertRaises(ValueError):
+                decrypt_value(new_env_var_secret(), ct)
+
+    def test_tampered_ciphertext_raises_value_error(self) -> None:
         ct = encrypt_value(self.secret, "secret-token")
-        other_key = new_env_var_secret()
-        # Wrong key produces garbage bytes; decrypt_value raises ValueError
-        # when the result is non-UTF-8 (which is very likely for 12-char data).
-        # We allow it to succeed only if garbage happens to be valid UTF-8, but
-        # the plaintext must not match.
-        try:
-            result = decrypt_value(other_key, ct)
-            self.assertNotEqual("secret-token", result)
-        except ValueError:
-            pass
+        raw = bytearray(base64.urlsafe_b64decode(ct + "=" * (-len(ct) % 4)))
+        raw[_NONCE_BYTES] ^= 0x01  # flip a bit in the ciphertext body → tag mismatch
+        tampered = base64.urlsafe_b64encode(bytes(raw)).rstrip(b"=").decode()
+        with self.assertRaises(ValueError):
+            decrypt_value(self.secret, tampered)
 
     def test_truncated_blob_raises_value_error(self) -> None:
         with self.assertRaises(ValueError):
-            decrypt_value(self.secret, "dG9vc2hvcnQ")  # "tooshort" — under 16 nonce bytes
+            decrypt_value(self.secret, "dG9vc2hvcnQ")  # "tooshort" — under nonce+tag
 
     def test_invalid_base64_raises_value_error(self) -> None:
         with self.assertRaises(ValueError):