密碼學哈希函數

A Hash Function is a mathematical function that converts a numerical value into another compressed numeric value. The input value for the hash functions can be of arbitrary length, but the output text that it will produce will always be of fixed length.

哈希函數是一種數學函數，可將數字值轉換為另一個壓縮數字值。 哈希函數的輸入值可以是任意長度，但是它將產生的輸出文本始終是固定長度。

哈希函數的屬性 (Properties of Hash Functions)

Compressed output

壓縮輸出

Fixed Length Output

定長輸出

Pre-Image Resistance

像前電阻

Second pre-Image Resistance

二次成像前電阻

Collision Resistance

耐碰撞

應用程序和哈希函數 (Applications and of the hash functions)

Password storage:

密碼存儲：

Due to the pre-Image resistance and collision resistance property, the hash functions are ideal to use for storing the passwords. Therefore, the values that are stored in the database are the user-Id in its original form and the hash value of the password. Therefore, whenever you enter the Id and password into a system, it searches for the user-Id, if available, it matches the hash values of the entered password and the password that is already there in the database. If both are the same, then access is granted.

由于具有前圖像抗性和抗碰撞性，因此哈希函數非常適合用于存儲密碼。 因此，存儲在數據庫中的值是原始形式的用戶ID和密碼的哈希值。 因此，每當您將Id和密碼輸入系統時，它都會搜索user-Id(如果可用)，它將與輸入密碼的哈希值和數據庫中已經存在的密碼匹配。 如果兩者相同，則授予訪問權限。

This provides:

提供：

Authentication:

認證方式 ：

Whenever the user enters the id and password, the id is matched with the one with the present in the database, and the password entered is then and there converted into its hash value and is compared with that stored in the database.

每當用戶輸入ID和密碼時，ID就會與數據庫中存在的ID進行匹配，然后將輸入的密碼轉換為哈希值并與數據庫中存儲的哈希值進行比較。

Provides high-level security:

提供高級安全性 ：

The hash function technique provides extreme level security, because even if the hacker or attacker is successful in breaking into the system’s database, all he gets is the user-Id and the hash value of the password, and getting the original password form this hash value is almost impossible.

哈希函數技術提供了最高級別的安全性，因為即使黑客或攻擊者成功闖入系統數據庫，他得到的只是用戶ID和密碼的哈希值，并從該哈希值獲取原始密碼幾乎是不可能的。

Fig. The storage and Authentication methods for Id passwords.
Reference link: https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&ved=2ahUKEwi-vZHc4fXmAhWgIbcAHb5JBfgQjRx6BAgBEAQ&url=https%3A%2F%2Fwww.darkreading.com%2Fsafely-storing-user-passwords-hashing-vs-encrypting%2Fa%2Fd-id%2F1269374&psig=AOvVaw1p7DLjEAMSySYPZaaUqO_c&ust=1578633364016737

圖：Id密碼的存儲和身份驗證方法。
參考鏈接：https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&ved=2ahUKEwi-vZHc4fXmAhWgIbcAHb5JBfgQjRx6BAgBEAQ&url=https%3A%2F%2Fwww.darkreadinglytoring.com -passwordshashing-vs-encrypting％2Fa％2Fd-id％2F1269374＆psig = AOvVaw1p7DLjEAMSySYPZaaUqO_c＆ust = 1578633364016737

Data Integrity check:

數據完整性檢查：

To maintain the data integrity, the sender sends both the message and its hash value to the user. The Receiver then checks whether the hash value of the message is the same as the hash value sent by the sender. This ensures that no modification has been done in the data while being transmitted. This process of integrity checks can be seen in email systems, messaging applications, etc. However, as everything takes place in the back-end, the user cannot identify this process.

為了保持數據完整性，發送方將消息及其哈希值發送給用戶。 然后，接收方檢查消息的哈希值是否與發送方發送的哈希值相同。 這樣可以確保在發送時未對數據進行任何修改。 完整性檢查過程可以在電子郵件系統，消息傳遞應用程序等中看到。但是，由于一切都在后端進行，因此用戶無法識別此過程。

Digital Signature:

電子簽名：

A signature is usually used to bind signatory to the message. The digital signature is thus a technique that binds a person or the entity to the digital data. This binding ensures that the person sending the data is solely responsible for being for it and this binding can be verified by the receiver and the third party. This digital signature is also stored using hash functions so that no one’s signature can be stolen and be misused.

簽名通常用于將簽名者綁定到消息。 因此，數字簽名是將個人或實體綁定到數字數據的技術。 此綁定確保發送數據的人對此負全責，并且此綁定可以由接收方和第三方驗證。 此數字簽名也使用哈希函數存儲，因此任何人的簽名都不會被盜用或濫用。

翻譯自: https://www.includehelp.com/cryptography/applications-of-hash-function.aspx

密碼學哈希函數

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