diff --git a/contracts/script/Deploy.s.sol b/contracts/script/Deploy.s.sol
index 68f0689..8d6b318 100644
--- a/contracts/script/Deploy.s.sol
+++ b/contracts/script/Deploy.s.sol
@@ -1,13 +1,13 @@
 // SPDX-License-Identifier: UNLICENSED
 pragma solidity >=0.8.19 <=0.9.0;
 
-import { Foo } from "../src/Foo.sol";
+import { ScKeystore } from "../src/ScKeystore.sol";
 import { BaseScript } from "./Base.s.sol";
 import { DeploymentConfig } from "./DeploymentConfig.s.sol";
 
 contract Deploy is BaseScript {
-    function run() public returns (Foo foo, DeploymentConfig deploymentConfig) {
+    function run() public returns (ScKeystore scKeystore, DeploymentConfig deploymentConfig) {
         deploymentConfig = new DeploymentConfig(broadcaster);
-        foo = new Foo();
+        scKeystore = new ScKeystore();
     }
 }
diff --git a/contracts/src/Foo.sol b/contracts/src/Foo.sol
deleted file mode 100644
index d69be05..0000000
--- a/contracts/src/Foo.sol
+++ /dev/null
@@ -1,8 +0,0 @@
-// SPDX-License-Identifier: UNLICENSED
-pragma solidity >=0.8.19;
-
-contract Foo {
-    function id(uint256 value) external pure returns (uint256) {
-        return value;
-    }
-}
diff --git a/contracts/src/ScKeystore.sol b/contracts/src/ScKeystore.sol
new file mode 100644
index 0000000..89266ca
--- /dev/null
+++ b/contracts/src/ScKeystore.sol
@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity 0.8.24;
+
+import {IScKeystore, UserInfo, KeyPackage} from "./IScKeystore.sol";
+
+error UserAlreadyExists();
+error MalformedKeyPackage();
+error MalformedUserInfo();
+error UserDoesNotExist();
+
+contract ScKeystore is IScKeystore {
+    mapping(address user => UserInfo userInfo) private users;
+
+    function userExists(address user) public view returns (bool) {
+        return users[user].signaturePubKey.length > 0;
+    }
+
+    function addUser(UserInfo calldata userInfo) external {
+        if(userInfo.signaturePubKey.length == 0) revert MalformedUserInfo();
+        if(userInfo.keyPackages.length != 1) revert MalformedUserInfo();
+        if(userExists(msg.sender)) revert UserAlreadyExists();
+
+        users[msg.sender] = userInfo;
+    }
+
+    function getUser(address user) external view returns (UserInfo memory) {
+        return users[user];
+    }
+
+    function addKeyPackage(KeyPackage calldata keyPackage) external {
+        if(keyPackage.data.length == 0) revert MalformedKeyPackage();
+        if(!userExists(msg.sender)) revert UserDoesNotExist();
+
+        users[msg.sender].keyPackages.push(keyPackage);
+    }
+
+    function getAvailableKeyPackage(address user) external view returns (KeyPackage memory) {
+        return users[user].keyPackages[users[user].keyPackages.length - 1];
+    }
+}
diff --git a/contracts/test/Foo.t.sol b/contracts/test/Foo.t.sol
deleted file mode 100644
index 6b15158..0000000
--- a/contracts/test/Foo.t.sol
+++ /dev/null
@@ -1,26 +0,0 @@
-// SPDX-License-Identifier: UNLICENSED
-pragma solidity >=0.8.19 <0.9.0;
-
-import { Test, console } from "forge-std/Test.sol";
-
-import { Deploy } from "../script/Deploy.s.sol";
-import { DeploymentConfig } from "../script/DeploymentConfig.s.sol";
-import { Foo } from "../src/Foo.sol";
-
-contract FooTest is Test {
-    Foo internal foo;
-    DeploymentConfig internal deploymentConfig;
-
-    address internal deployer;
-
-    function setUp() public virtual {
-        Deploy deployment = new Deploy();
-        (foo, deploymentConfig) = deployment.run();
-    }
-
-    function test_Example() external {
-        console.log("Hello World");
-        uint256 x = 42;
-        assertEq(foo.id(x), x, "value mismatch");
-    }
-}
diff --git a/contracts/test/ScKeystore.t.sol b/contracts/test/ScKeystore.t.sol
new file mode 100644
index 0000000..f0269cd
--- /dev/null
+++ b/contracts/test/ScKeystore.t.sol
@@ -0,0 +1,56 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity >=0.8.19 <0.9.0;
+
+import { Test } from "forge-std/Test.sol";
+import { Deploy } from "../script/Deploy.s.sol";
+import { DeploymentConfig } from "../script/DeploymentConfig.s.sol";
+import "../src/ScKeystore.sol"; // solhint-disable-line
+
+contract ScKeystoreTest is Test {
+    ScKeystore internal s;
+    DeploymentConfig internal deploymentConfig;
+    address internal deployer;
+
+    function setUp() public virtual {
+        Deploy deployment = new Deploy();
+        (s, deploymentConfig) = deployment.run();
+    }
+
+    function addUser() internal {
+        KeyPackage[] memory keyPackages = new KeyPackage[](1);
+        keyPackages[0] = KeyPackage({data: new bytes[](0)});
+        UserInfo memory userInfo = UserInfo({
+            signaturePubKey: "0x",
+            keyPackages: keyPackages
+        });
+        s.addUser(userInfo);
+    }
+
+    function test__userExists__returnsFalse__whenUserDoesNotExist() public view {
+        assert(!s.userExists(address(this)));
+    }
+
+    function test__addUser__reverts__whenUserInfoIsMalformed() public {
+        UserInfo memory userInfo;
+        vm.expectRevert(MalformedUserInfo.selector);
+        s.addUser(userInfo);
+    }
+
+    function test__addUser__reverts__whenUserAlreadyExists() public {
+        addUser();
+        vm.expectRevert(UserAlreadyExists.selector);
+        addUser();
+    }
+
+    function test__addUser__addsUser__whenUserInfoIsValid() public {
+        addUser();
+        assert(s.userExists(address(this)));
+    }
+
+    function test__getUser__returnsUserInfo__whenUserExists() public {
+        addUser();
+        UserInfo memory userInfo = s.getUser(address(this));
+        assert(userInfo.signaturePubKey.length == 2);
+        assert(userInfo.keyPackages.length == 1);
+    }
+}