Add two fuzzing targets for discovery v5.1

tests/fuzzing/discoveryv5.1/fuzz_decode_message.nim

@@ -0,0 +1,29 @@
+import
+  testutils/fuzzing, stew/byteutils,
+  eth/rlp, eth/p2p/discoveryv5/[encodingv1, typesv1]
+
+test:
+  block:
+    let decoded = decodeMessage(payload)
+
+    if decoded.isOK():
+      let message = decoded.get()
+      var encoded: seq[byte]
+      case message.kind
+      of unused: break
+      of ping: encoded = encodeMessage(message.ping, message.reqId)
+      of pong: encoded = encodeMessage(message.pong, message.reqId)
+      of findNode: encoded = encodeMessage(message.findNode, message.reqId)
+      of nodes: encoded = encodeMessage(message.nodes, message.reqId)
+      of talkreq: encoded = encodeMessage(message.talkreq, message.reqId)
+      of talkresp: encoded = encodeMessage(message.talkresp, message.reqId)
+      of regtopic, ticket, regconfirmation, topicquery:
+        break
+
+      # This will hit assert because of issue:
+      # https://github.com/status-im/nim-eth/issues/255
+      # if encoded != payload:
+      #   echo "payload: ", toHex(payload)
+      #   echo "encoded: ", toHex(encoded)
+
+      #   doAssert(false, "re-encoded result does not equal original payload")

tests/fuzzing/discoveryv5.1/fuzz_decode_packet.nim

@@ -0,0 +1,37 @@
+import
+  testutils/fuzzing, stew/shims/net, stew/byteutils,
+  eth/p2p/discoveryv5/[encodingv1, enr, sessions, node]
+
+init:
+  const
+    nodeAKey = "0xeef77acb6c6a6eebc5b363a475ac583ec7eccdb42b6481424c60f59aa326547f"
+    nodeBKey = "0x66fb62bfbd66b9177a138c1e5cddbe4f7c30c343e94e68df8769459cb1cde628"
+  let
+    rng = newRng()
+    privKeyA = PrivateKey.fromHex(nodeAKey)[] # sender -> encode
+    privKeyB = PrivateKey.fromHex(nodeBKey)[] # receive -> decode
+
+    enrRecA = enr.Record.init(1, privKeyA,
+      some(ValidIpAddress.init("127.0.0.1")), Port(9000),
+      Port(9000)).expect("Properly intialized private key")
+    nodeA = newNode(enrRecA).expect("Properly initialized record")
+
+    enrRecB = enr.Record.init(1, privKeyB,
+      some(ValidIpAddress.init("127.0.0.1")), Port(9000),
+      Port(9000)).expect("Properly intialized private key")
+    nodeB = newNode(enrRecB).expect("Properly initialized record")
+
+  var codecB = Codec(localNode: nodeB, privKey: privKeyB,
+    sessions: Sessions.init(5))
+
+test:
+  # It is not the best idea to generate extra data and encrypt data but we do
+  # it like this as the decodeHeader proc does decrypt + decode + decrypt.
+  # There is no separate decrypt step that can be skipped because of this.
+  var iv: array[ivSize, byte]
+  brHmacDrbgGenerate(rng[], iv)
+  let maskedHeader = encryptHeader(nodeB.id, iv, payload)
+
+  let decoded = decodePacket(codecB, nodeA.address.get(), @iv & maskedHeader)
+  if decoded.isErr():
+    debug "Error occured", error = decoded.error