Adds JWT-based auth with refresh tokens

src/main/kotlin/net/halfbinary/scavengerhuntapi/config/JwtUtil.kt

@@ -0,0 +1,62 @@
+package net.halfbinary.scavengerhuntapi.config
+
+import io.jsonwebtoken.JwtException
+import io.jsonwebtoken.Jwts
+import jakarta.annotation.PostConstruct
+import org.springframework.beans.factory.annotation.Value
+import org.springframework.stereotype.Component
+import java.util.Date
+import javax.crypto.SecretKey
+
+@Component
+class JwtUtil {
+    @Value($$"${jwt.secret}")
+    private val jwtSecret: String? = null
+
+    @Value($$"${jwt.expiration}")
+    private val jwtExpirationMs = 0
+
+    private var key: SecretKey? = null
+
+    // Initializes the key after the class is instantiated and the jwtSecret is injected,
+    // preventing the repeated creation of the key and enhancing performance
+    @PostConstruct
+    fun init() {
+        this.key = Jwts.SIG.HS256.key().build()
+    }
+
+    // Generate JWT token
+    fun generateToken(email: String): String {
+        return Jwts.builder()
+            .subject(email)
+            .issuedAt(Date())
+            .expiration(Date(System.currentTimeMillis() + jwtExpirationMs))
+            .signWith(key)
+            .compact()
+    }
+
+    // Get username from JWT token
+    fun getUsernameFromToken(token: String): String {
+        return Jwts.parser()
+            .verifyWith(key)
+            .build()
+            .parseSignedClaims(token)
+            .payload
+            .subject
+    }
+
+    // Validate JWT token
+    fun validateJwtToken(token: String?): Boolean {
+        try {
+            Jwts.parser().verifyWith(key).build().parseSignedClaims(token)
+            return true
+        } catch (e: SecurityException) {
+            println("Invalid JWT signature: " + e.message)
+        } catch (e: JwtException) {
+            println("Invalid JWT token: " + e.message)
+        } catch (e: IllegalArgumentException) {
+            println("JWT claims string is empty: " + e.message)
+        }
+        return false
+    }
+}