ソルトとペッパーの理解
パスワードハッシングにおけるソルトとペッパーの役割と、両者の補完関係を学びます。
问题
ソルトとペッパーをそれぞれいつ使うべきか、それぞれが異なる攻撃ベクトルからどう守るのかを理解したい状況です。
前提条件
- パスワードハッシングの理解
- レインボーテーブルと総当たり攻撃に関する知識
ステップ
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1. Understand the salt
A salt is a random value generated per password and stored alongside the hash. It ensures that identical passwords produce different hashes, defeating rainbow tables and multi-target attacks. The salt is not secret; its value comes from being unique.
入力user A: salt=a1 | user B: salt=b2输出same password → different hashes -
2. Understand the pepper
A pepper is a secret value shared across all passwords but not stored with the hash. It is stored in a separate secure location (e.g., environment variable or HSM). If the database is leaked but the pepper is not, the attacker cannot crack any password without also obtaining the pepper.
入力pepper: secret env var输出hash = KDF(password + salt + pepper) -
3. Combine both for defense in depth
Use a per-password salt (stored) plus a global pepper (secret). The salt defends against rainbow tables and identical-password detection. The pepper adds a second layer: even with the full database, passwords cannot be cracked without the pepper. Keep the pepper in a separate secrets manager.
入力salt + pepper输出layered protection
期待される結果
パスワードごとのソルトとシークレットペッパーの両方で保護され、多層防御を実現するパスワードハッシュ。
よくあるエラー
- Storing the pepper in the database
- The pepper must be stored separately from the hashes, otherwise it provides no additional protection if the database is leaked.
- Using the same salt for all passwords
- Generate a unique random salt per password. A shared salt defeats the purpose of salting.
トラブルシューティング指引
- If you lose the pepper, all passwords become unverifiable. Back up the pepper securely.
- If migrating, you can re-hash with a new pepper by verifying with the old one and re-hashing.
セキュリティ上の制限
- The pepper is only effective if it remains secret. Store it in an HSM or secrets manager, not in source code.
- All hashing is local.