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I've often heard that hash functions cant be reversed with the analogy of :take a number and add all the digits together, ex: 412 => 7 but you cant get your original value (412) back from 7. While this does make sense wouldnt it also imply that there are multiple inputs that give the same output?

clement pochart
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1 Answers1

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A hash function is a versatile one-way cryptographic algorithm that maps an input of any size to a unique output of a fixed length of bits. The resulting output, which is known as a hash digest, hash value, or hash code, is the resulting unique identifier.

So answering your question: Yes, multiple inputs can give the same output if and only if the inputs are exactly the same. The analogy(take a number and add all the digits together) does not satisfy the part of the definition that states, that even if the pattern of characters/numbers is changed the hash created will be completely different.

Even if something tiny changed in an input — you capitalize a letter instead of using one that’s lowercase, or you swap an exclamation mark where there was a period — it’s going to result in the generation of an entirely new hash value and that’s the whole idea here — no matter how big or small a change, a completely different hash value gets created.

Att Coder
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  • *...A hash function is a versatile one-way cryptographic algorithm...* There are non-cryptographic hashes. *...Yes, multiple inputs can give the same output if and only if the inputs are exactly the same...* No.No.No. – President James K. Polk May 30 '22 at 10:55
  • President James K. Polk what no no no? Do you have any explanation? – Att Coder May 30 '22 at 11:23
  • Except for very special cases like perfect hash functions, hash functions are many-to-one: there are many distinct inputs that map to a given output. – President James K. Polk May 30 '22 at 13:19
  • For hash functions with an output smaller than the input you‘ll always have the possibility of hash collisions. – seb May 30 '22 at 15:21