implement approvedToLog check

implementation/contracts/DepositLog.sol

@@ -1,5 +1,7 @@
 pragma solidity ^0.5.10;
 
+import {TBTCDepositToken} from "./system/TBTCDepositToken.sol";
+
 
 contract DepositLog {
     /*
@@ -10,6 +12,10 @@ contract DepositLog {
       Everyone should be able to ENTIRELY rely on log messages
     */
 
+    // `TBTCDepositToken` mints a token for every new Deposit.
+    // If a token exists for a given ID, we know it is a valid Deposit address.
+    TBTCDepositToken tbtcDepositToken;
+
     // This event is fired when we init the deposit
     event Created(
         address indexed _depositContractAddress,
@@ -99,15 +105,26 @@ contract DepositLog {
     //
 
     /// @notice             Checks if an address is an allowed logger
-    /// @dev                Calls the system to check if the caller is a Deposit
+    /// @dev                checks tbtcDepositToken to see if the caller represents
+    ///                     an existing deposit.
     ///                     We don't require this, so deposits are not bricked if the system borks
     /// @param  _caller     The address of the calling contract
     /// @return             True if approved, otherwise false
     /* solium-disable-next-line no-empty-blocks */
-    function approvedToLog(address _caller) public pure returns (bool) {
-        /* TODO: auth via system */
-        _caller;
-        return true;
+    function approvedToLog(address _caller) public view returns (bool) {
+        return tbtcDepositToken.exists(uint256(_caller));
+    }
+
+    /// @notice               Sets the tbtcDepositToken contract.
+    /// @dev                  The contract is used by `approvedToLog` to check if the
+    ///                       caller is a Deposit contract. This should only be called once.
+    /// @param  _tbtcDepositTokenAddress  The address of the tbtcDepositToken.
+    function setTbtcDepositToken(address _tbtcDepositTokenAddress) public {
+        require(
+            address(tbtcDepositToken) == address(0),
+            "tbtcDepositToken is already set"
+        );
+        tbtcDepositToken = TBTCDepositToken(_tbtcDepositTokenAddress);
     }
 
     //

implementation/contracts/system/TBTCSystem.sol

@@ -94,6 +94,7 @@ contract TBTCSystem is Ownable, ITBTCSystem, DepositLog {
             _keepThreshold,
             _keepSize
         );
+        setTbtcDepositToken(_tbtcDepositToken);
         _initialized = true;
         allowNewDeposits = true;
     }

implementation/contracts/test/deposit/TBTCSystemStub.sol

@@ -26,9 +26,4 @@ contract TBTCSystemStub is TBTCSystem {
     function requestNewKeep(uint256, uint256, uint256) external payable returns (address _keepAddress) {
         return keepAddress;
     }
-
-    // override parent
-    function approvedToLog(address) public pure returns (bool) {
-        return true;
-    }
 }