Add section for Sapphire gas padding

docs/dapp/sapphire/security.md

@@ -36,8 +36,9 @@ You can also trace a particular transaction, once you know its hash.
 npx hardhat trace --hash 0xTransactionHash
 ```
 
-For both [gas] usage and confidentiality, non-unique data sizes are
-recommended. E.g. 64-byte value will still be distinct from a 128-byte value.
+For both [gas] usage and confidentiality purposes, we recommend using
+non-unique data size. E.g. 64-byte value will still be distinct from a
+128-byte value.
 
 :::caution Inference based on access patterns
 
@@ -48,7 +49,7 @@ transaction.
 
 ## Order of Operations
 
-With regards to errors, gas usage patterns not only can reveal the code path
+When handling errors, gas usage patterns not only can reveal the code path
 taken, but sometimes the balance of a user as well (in the case of a diligent
 attacker using binary search).
 
@@ -73,5 +74,41 @@ function transferFrom(address who, address to, uint amount)
 }
 ```
 
+## Gas Padding
+
+To prevent leaking information about a particular transaction, Sapphire
+provides a [precompile] for dApp developers to pad the amount of gas used
+in a transaction.
+
+```solidity
+contract GasExample {
+    bytes32 tmp;
+
+    function constantMath(bool doMath, uint128 padGasAmount) external {
+        if (doMath) {
+            bytes32 x;
+
+            for (uint256 i = 0; i < 100; i++) {
+                x = keccak256(abi.encodePacked(x, tmp));
+            }
+
+            tmp = x;
+        }
+
+        Sapphire.padGas(padGasAmount);
+    }
+}
+```
+
+Both contract calls below should use the same amount of gas. Sapphire also
+provides the precompile to return the gas [used] by the current transaction.
+
+```typescript
+await contract.constantMath(true, 100000);
+await contract.constantMath(false, 100000);
+```
+
 [gas]: https://docs.soliditylang.org/en/latest/internals/layout_in_storage.html
-[hardhat-tracer]: https://www.npmjs.com/package/hardhat-tracer
+[hardhat-tracer]: https://www.npmjs.com/package/hardhat-tracer
+[precompile]: https://github.com/oasisprotocol/sapphire-paratime/blob/08b5fa24783ea576207dacde20ff4d20a1c7dded/contracts/contracts/Sapphire.sol#L28-L29
+[used]: https://github.com/oasisprotocol/sapphire-paratime/blob/08b5fa24783ea576207dacde20ff4d20a1c7dded/contracts/contracts/Sapphire.sol#L26-L27