[WIP] Securing EVE Logs

pkg/newlog/cmd/fssagg.go

@@ -0,0 +1,95 @@
+// Copyright (c) 2024 Zededa, Inc.
+// SPDX-License-Identifier: Apache-2.0
+
+package main
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+)
+
+// Notes :
+// This is based on wor of Ma and Tsudik:
+// Ma, Di, and Gene Tsudik. "Forward-secure sequential aggregate authentication."
+// IEEE Symposium on Security and Privacy (SP'07). IEEE, 2007.
+// https://eprint.iacr.org/2007/052.pdf
+//
+// This implementation provides forward security, content integrity, stream integrity
+// and truncation detection for each batch of logs *gzipped*. This means logs not
+// yet processed by the newlogd are not protected by this scheme, the risk of an
+// attacker compromising the logs can be lowered by adjusting the batch size or time
+// interval to create batches of logs more frequently.
+//
+// Current implementation lacks the batch (or tail) deletion detection, this is
+// to be implemented in the next version using tpm.
+//
+// Pure golang implementation of FssAgg might not be entirely safe, the initial
+// key, before gc collected, might hang around a bit and swapped into disk if
+// the attacker puts the system under memory pressure. Bear in mid this is a highly
+// unlikly scenario. Unfortunately mlocking might not possible to implement as
+// the underlying golang crypto calls copy data around into not locked memory.
+
+// h must be a collision-resistant one-way hash function (we are using SHA-256,
+// ofcouse given enough universes nothing is collision resistant)
+func h(data []byte) []byte {
+	h := sha256.New()
+	h.Write(data)
+	return h.Sum(nil)
+}
+
+// hsk must be a secure MAC function h (here we are using HMAC with SHA-256 and a secret key)
+func hsk(key, message []byte) []byte {
+	mac := hmac.New(sha256.New, key)
+	mac.Write(message)
+	return mac.Sum(nil)
+}
+
+// fssAggUpd updates key using hash of previous key
+func fssAggUpd(prevKey []byte) []byte {
+	return h(prevKey)
+}
+
+// FssAggSig takes the current key, aggregated signature so far and a message,
+// calculates the message MAC, updated aggregated signature and updates the key
+// for forward security. It returns the aggregated signature plus the updated key.
+func fssAggSig(key, aggMAC, message []byte) ([]byte, []byte) {
+	var aggSig []byte
+
+	// generate MAC for current message and aggregate current MAC with previous MACs
+	curMAC := hsk(key, message)
+	aggSig = h(append(aggMAC, curMAC...))
+
+	// update key for forward security
+	nextKey := fssAggUpd(key)
+
+	return nextKey, aggSig
+}
+
+// FssAggVer verifies aggregated signature over messages, accepting the initial
+// secret key and the aggregate signature, returning a boolean indicating the
+// verification result.
+func fssAggVer(secretKey, aggSig []byte, messages [][]byte) bool {
+	key := secretKey
+	var computedAggSig []byte
+
+	for _, message := range messages {
+		// generate MAC for current message
+		curMAC := hsk(key, message)
+		// aggregate current MAC with previous MACs
+		computedAggSig = h(append(computedAggSig, curMAC...))
+
+		// update key
+		key = fssAggUpd(key)
+	}
+
+	// this compare is constant time, so no time leak and attack.
+	return hmac.Equal(computedAggSig, aggSig)
+}
+
+func evolveKey(secretKey []byte, num uint64) []byte {
+	key := secretKey
+	for i := uint64(0); i < num; i++ {
+		key = fssAggUpd(key)
+	}
+	return key
+}