security - Is there a hashing technique that works both ways?

Question

TLDR;

The hashing function generates a different hash every time for the same piece of data, but it can determine if a particular hash was generated with the piece of data or not.

Eg:

hash_func(xyz): abc123
hash_func(xyz): jhg342 // different hash, even if the data was same.

decode_hash(jhg324) == xyz
This gives true, because the hash function determined that jhg324 is indeed the hash of xyz

The Question

For an Open Source website, I want to store the email in hashed form (because all the users will be public), but the site needs to know if an email was used to register for another account so that it can ensure one account per email.

However, all the emails are from one organization only. This means, they all look exactly like uid@org_name.com. This means anyone can run through all the UIDs and find out which hash belongs to which email, and thus, which person.

Therefore, is there a way to hash the email such that the hash knows which email it belongs to, but hashing the same email does not generate the same hash.

P.S. Please note that I cannot use Salting as the site will be Open Source and the salt will be publicly available.

question from:https://stackoverflow.com/questions/65934866/is-there-a-hashing-technique-that-works-both-ways

Answer 1

This doesn't make sense - you're conflating hashing and encryption in a very strange way. What you're describing wouldn't really be a cryptographically secure hash function. By definition, cryptographically secure hash functions are one way. In fact, if you could reverse it, there would be little point to using it at all because it would no longer be secure. This would make it possible to brute-force passwords and would "break" passwords that were used in multiple places.

Also, why would you want it to hash to different values each time? That's what you use a salt for.

If you want to be able to reverse it later, just use an encryption algorithm like AES. Even better, many databases even offer features for securely storing sensitive information; see, for example, SQL Server's Always Encrypted feature.