refactor: switch password hashing from argon2 to bcrypt

- hashPassword now emits bcrypt (cost 12) instead of argon2id
- checkLogin migrates legacy md5/argon2id hashes to bcrypt on sign-in
- keep argon2id verification only as a one-time migration path
- replace ARGON2_* env vars with BCRYPT_COST

src/lib/auth/password.ts

@@ -1,16 +1,13 @@
 import { randomBytes } from "node:crypto";
-import { argon2id, argon2Verify, bcryptVerify, md5 } from "hash-wasm";
+import { argon2Verify, bcrypt, bcryptVerify, md5 } from "hash-wasm";
 
 import { env } from "@/env";
 
 export async function hashPassword(password: string): Promise<string> {
-	return await argon2id({
+	return await bcrypt({
 		password,
 		salt: randomBytes(16),
-		parallelism: env.ARGON2_PARALLELISM,
-		iterations: env.ARGON2_ITERATIONS,
-		memorySize: env.ARGON2_MEMORY_KB,
-		hashLength: 32,
+		costFactor: env.BCRYPT_COST,
 		outputType: "encoded",
 	});
 }
@@ -29,6 +26,11 @@ export async function isMd5Of(
 	);
 }
 
+/**
+ * Legacy argon2id verification — kept ONLY so accounts hashed before the
+ * bcrypt switch can still sign in once and be migrated to bcrypt. No new
+ * argon2 hashes are ever produced.
+ */
 export async function isArgon2idOf(
 	password: string,
 	stored: string,
@@ -41,7 +43,6 @@ export async function isArgon2idOf(
 	}
 }
 
-/** Legacy bcrypt support — only kept to verify & auto-upgrade old accounts. */
 export async function isBcryptOf(
 	password: string,
 	stored: string,
@@ -58,9 +59,6 @@ export async function verifyPassword(
 	password: string,
 	stored: string,
 ): Promise<boolean> {
-	if (/^\$argon2id\$/.test(stored)) {
-		return isArgon2idOf(password, stored);
-	}
 	return isBcryptOf(password, stored);
 }
 
@@ -74,14 +72,22 @@ export async function checkLogin(
 	stored: string,
 	opts: { convertPasswords: boolean },
 ): Promise<LoginCheck> {
+	// Legacy argon2id — verify so existing users can sign in, then immediately
+	// rehash to bcrypt so the hash format converges on bcrypt.
+	if (/^\$argon2id\$/.test(stored)) {
+		if (await isArgon2idOf(password, stored)) {
+			return { valid: true, upgradedHash: await hashPassword(password) };
+		}
+		return { valid: false };
+	}
+
+	if (/^\$2[aby]\$/.test(stored)) {
+		return { valid: await isBcryptOf(password, stored) };
+	}
+
 	if (opts.convertPasswords && (await isMd5Of(password, stored))) {
 		return { valid: true, upgradedHash: await hashPassword(password) };
 	}
-	if (opts.convertPasswords && (await isArgon2idOf(password, stored))) {
-		return { valid: true };
-	}
-	if (opts.convertPasswords && (await isBcryptOf(password, stored))) {
-		return { valid: true, upgradedHash: await hashPassword(password) };
-	}
+
 	return { valid: await verifyPassword(password, stored) };
 }