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In this interview, David Bombal sits down with Dr. Mike Pound (Computerphile) to clear up one of the biggest crypto misconceptions on the Internet: hashing is not encryption, and hash functions are not reversible.

In this video you’ll learn what a hash function actually does (a deterministic, fixed-length, “random-looking” summary of data) and why the whole point is that you cannot take a hash and reconstruct the original file. Dr Mike explains the key properties of secure hashing, including the avalanche effect (tiny input change, massive output change), and why older algorithms like MD5 and SHA-1 became unsafe due to collisions. We also cover what “collisions” really mean, why they must exist in theory (the pigeonhole principle) and why they can appear sooner than expected (the birthday paradox).

Then we tackle the YouTube-comments classic: rainbow tables. If hashes are one-way, how do attackers “crack” passwords? The answer: they don’t reverse hashes. They guess passwords, hash them forward, and match the results. Mike breaks down how rainbow tables speed this up with precomputed hashes, and why salting makes those precomputations far less effective by forcing attackers to redo work per user.

Finally, we zoom out into modern cryptography: why SHA-2 is widely used today, why SHA-3 exists as a structurally different backup option, what length extension attacks are, and what quantum computing changes (and doesn’t change) for hashing and encryption. We also touch on how hashes power digital signatures, file integrity checks (like verifying an ISO download), and why AES dominates symmetric encryption.

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// YouTube Video REFERENCE //

SHA: Secure Hashing Algorithm: • SHA: Secure Hashing Algorithm - Computerphile

Birthday Paradox: • Hash Collisions & The Birthday Paradox - C...

The Next Big SHA? SHA3 Sponge Function Explained: • The Next Big SHA? SHA3 Sponge Function Exp...

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// MENU //

0:00 - Coming up

01:09 - DeleteMe sponsored segment

02:54 - Hashing is not Encryption // Encryption and Hashing explained

09:47 - Hash functions are irreversible

15:22 - How hashing works

17:23 - Why MD5 is bad

20:09 - Recommended hashing function

21:47 - Birthday paradox explained

23:39 - Rainbow table explained

29:44 - Salting explained

33:35 - Pigeon Hole principle explained

36:35 - SHA-2 is the answer

37:17 - SHA-3 vs SHA-2

40:42 - The effect of quantum computing

42:47 - Quick summary

43:52 - Sign-In with private key

45:21 - Avalanche effect explained

49:10 - Where to learn more about hash functions

50:27 - Conclusion

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Disclaimer: This video is for educational purposes only.

#cryptography #hashing #encryption