lib: Harden symmetric-key rotation

Flow crypto signalled rotation with a single phase-parity bit, so a
loss burst that hid an even number of rotations went unnoticed and
wedged the flow for good.

Each packet now carries a small cleartext selector naming its key
directly, so a receiver that falls behind recovers on the next packet
instead of getting stuck.

The selector also serves as the AEAD nonce and is authenticated as
associated data (AAD). Key rotation moves into a new backend-agnostic
keyrot module that rotates sub-keys to bound AEAD usage while
preserving forward secrecy.

Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>

1 files changed, 36 insertions, 2 deletions

diff --git a/cmake/config/lib.cmake b/cmake/config/lib.cmake
index 25130519..2c01b311 100644
--- a/cmake/config/lib.cmake
+++ b/cmake/config/lib.cmake
@@ -87,8 +87,42 @@ set(TPM_DEBUG_ABORT_TIMEOUT 0 CACHE STRING
   "TPM abort process after a thread reaches this timeout (s), 0 disables")
 
 # Encryption
-set(KEY_ROTATION_BIT 20 CACHE STRING
-  "Bit position in packet counter that triggers key rotation (default 20 = every 2^20 packets)")
+set(KEY_LEAF_BITS 20 CACHE STRING
+  "Packets per leaf key as a power of two (2^20 = AEAD-safe default)")
+set(KEY_NODE_BITS 6 CACHE STRING
+  "Leaf keys per node key, power of two (2^6 = 64; leak compartment)")
+set(KEY_NODE_COUNT 128 CACHE STRING
+  "Node keys per batch (N); <= 4096, the 12-bit on-wire node index")
+set(KEY_REKEY_WATERMARK 4 CACHE STRING
+  "Re-key when this many node keys remain; 0 disables the count trigger")
+set(KEY_REPLAY_WINDOW 2048 CACHE STRING
+  "RX replay window in packets; power of two, >= 128")
+if(NOT KEY_REPLAY_WINDOW MATCHES "^[0-9]+$")
+  message(FATAL_ERROR "KEY_REPLAY_WINDOW must be a positive integer")
+endif()
+math(EXPR _krw_p2 "${KEY_REPLAY_WINDOW} & (${KEY_REPLAY_WINDOW} - 1)")
+if(KEY_REPLAY_WINDOW LESS 128 OR NOT _krw_p2 EQUAL 0)
+  message(FATAL_ERROR "KEY_REPLAY_WINDOW must be a power of two >= 128")
+endif()
+
+# Re-key must finish within its lead window - KEY_REKEY_WATERMARK node keys
+# worth of packets - before the batch exhausts and TX fails closed. dev.c only
+# evaluates the watermark once per FLOW_WM_CHECK writes, so a lead below ~2x
+# that leaves a high-rate flow no room to complete the exchange. Production
+# defaults are vast; this guards under-sized (test) geometries.
+if(KEY_REKEY_WATERMARK GREATER 0)
+  set(_rk_wm_check 65536) # FLOW_WM_CHECK in src/lib/dev.c (2^16)
+  math(EXPR _rk_lead
+    "${KEY_REKEY_WATERMARK} << (${KEY_LEAF_BITS} + ${KEY_NODE_BITS})")
+  math(EXPR _rk_min "2 * ${_rk_wm_check}")
+  if(_rk_lead LESS _rk_min)
+    message(WARNING
+      "Re-key lead is ${_rk_lead} packets vs the watermark check interval "
+      "${_rk_wm_check}; a high-rate flow may exhaust its key batch before the "
+      "re-key completes (TX fails closed until it does). Raise KEY_LEAF_BITS, "
+      "KEY_NODE_BITS, or KEY_REKEY_WATERMARK.")
+  endif()
+endif()
 
 # Flow statistics (requires FUSE)
 if(HAVE_FUSE)