如何修改文件SHA1哈希，使不同内容的两个文件哈希值一致？

Great question—let’s break this down clearly based on how SHA1 works and the current state of cryptographic research:

Key Background #

First, SHA1 is a cryptographic hash function originally built with two core security guarantees:

• Collision resistance: It should be extremely hard to find two distinct files that produce the same SHA1 hash.
• Second-preimage resistance: Given a file and its SHA1 hash, it should be nearly impossible to craft a different file that outputs that exact same hash.

What’s Feasible Today? #

Let’s split this into two critical scenarios:

1. Targeted collision (matching a specific existing hash) #

If you have File A (with SHA1 hash H) and File B (with a different hash H'), and you want to modify File B so its SHA1 equals H—this is still practically infeasible for most real-world use cases. While SHA1’s collision resistance was broken back in 2017 (via Google’s SHAttered attack), that attack only lets you generate new pairs of files that collide (same SHA1, different content). It doesn’t let you take an existing hash and build a file to match it—that’s the second-preimage resistance property, which hasn’t been effectively cracked for SHA1 yet.

2. Arbitrary collision (creating any two colliding files) #

If you just need two different files with the same SHA1 hash (not tied to a specific existing file’s hash), this is absolutely possible. The SHAttered attack proved this publicly by generating two distinct PDF files that share an identical SHA1 hash. Tools and research exist to create such collision pairs, but again—this isn’t about modifying an existing file to match another’s precomputed hash.

Practical Takeaways #

• If your goal is to forge a file that mimics the SHA1 of a specific existing file, current technology can’t pull that off reliably.
• SHA1 is no longer considered secure for cryptographic use cases (like digital signatures or critical file integrity checks) precisely because collision attacks are now feasible. Modern systems should use SHA-256 or SHA-3 instead.

内容的提问来源于stack exchange，提问作者Br3zzly