Implement KeychainKeyManager (#17)

Sources/Web5/Crypto/KeyManager/Local/KeychainKeyManager.swift

@@ -0,0 +1,69 @@
+import Foundation
+import Security
+
+/// A KeyManager that generates and stores cryptographic keys in the Keychain
+public class KeychainKeyManager: LocalKeyManager {
+
+    public init() {
+        super.init(keyStore: KeychainKeyStore())
+    }
+}
+
+class KeychainKeyStore: LocalKeyStore {
+
+    func getKey(keyAlias: String) throws -> Jwk? {
+        let keychainQuery: [String: Any] = [
+            kSecClass as String: kSecClassKey,
+            kSecAttrKeyClass as String: kSecAttrKeyClassPrivate,
+            kSecAttrApplicationLabel as String: keyAlias,
+            kSecReturnData as String: kCFBooleanTrue!,
+            kSecMatchLimit as String: kSecMatchLimitOne
+        ]
+
+        var item: CFTypeRef?
+        let status = SecItemCopyMatching(keychainQuery as CFDictionary, &item)
+
+        guard status == errSecSuccess else {
+            if status == errSecItemNotFound {
+                return nil
+            } else {
+                throw NSError(
+                    domain: NSOSStatusErrorDomain,
+                    code: Int(status),
+                    userInfo: [
+                        NSLocalizedDescriptionKey: "Failed to fetch key for alias \(keyAlias) from KeychainKeyStore"
+                    ]
+                )
+            }
+        }
+
+        guard let data = item as? Data else {
+            return nil
+        }
+
+        let jwk = try JSONDecoder().decode(Jwk.self, from: data)
+        return jwk
+    }
+
+    func setKey(_ privateKey: Jwk, keyAlias: String) throws {
+        let dataToStore = try JSONEncoder().encode(privateKey)
+
+        let keychainQuery: [String: Any] = [
+            kSecClass as String: kSecClassKey,
+            kSecAttrKeyClass as String: kSecAttrKeyClassPrivate,
+            kSecAttrApplicationLabel as String: keyAlias,
+            kSecValueData as String: dataToStore
+        ]
+
+        let status = SecItemAdd(keychainQuery as CFDictionary, nil)
+        guard status == errSecSuccess else {
+            throw NSError(
+                domain: NSOSStatusErrorDomain,
+                code: Int(status),
+                userInfo: [
+                    NSLocalizedDescriptionKey: "Failed to store key for alias \(keyAlias) in KeychainKeyStore"
+                ]
+            )
+        }
+    }
+}