# 哈希碰撞

> **最後更新：2026 年 1 月**

RIPE-MD160.\
\
2^160 is 1,461,501,637,330,902,918,203,684,832,716,283,019,655,932,542,976

## 怎麼可能一直有獨特值

你可能已經發現，對[哈希函數](/gei-kai-fa-zhe-de-zhi-nan/cryptography/hash.md)而言，輸入的區間若是無限大，而輸出區間僅是固定長度的編號的話，一定會發現兩個完全不同的輸入得出相同的[哈希值](/qu-kuai-lian-ru-he-da-cheng-gong-shi/wa/ha-xi-zhi.md)。這類情況稱為碰撞，比方說輸入"丁手中"得到哈希值"0x666666"，然而卻發現"顆P"的哈希值竟然剛好也是"0x666666"，如此一來，從哈希值來看，我們就會無法判斷台北的未來到底在誰手中了。

## 碰撞怎麼辦

一個好的哈希函數理論上會努力減低碰撞被發現的可能性。通常偶爾發現的碰撞並不會影響哈希函數的可用性，除非多餘一個碰撞被發現，大家才會開始淘汰使用這個哈希函數。

***

#### 相關條目

* [哈希算法](/gei-kai-fa-zhe-de-zhi-nan/cryptography/hash.md)
* [SHA256](/gei-kai-fa-zhe-de-zhi-nan/cryptography/sha256.md)

#### 參考資料


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