firebase_rules.json

{
  "rules": {
    // read is always allowed
    ".read": true,
    "messages": {
    	".write": true
    },

    "snake": {
      "players": {
        "$uid": {
          // only allow writing if this is new data or if this is the user, that created the data
          ".write": "(!data.exists()) || (data.child('verifyName').exists() && data.child('verifyName').val() == $uid)"
        }
      },

      // every player can write food data
      "foods": {
        ".write": true
      }
    }
  }
}