hash - Python unhash value

Question

I am a newbie to the python. Can I unhash, or rather how can I unhash a value. I am using std hash() function. What I would like to do is to first hash a value send it somewhere and then unhash it as such:

#process X
hashedVal = hash(someVal)
#send n receive in process Y
someVal = unhash(hashedVal)
#for example print it
print someVal

Thx in advance

Answer 1

It can't be done.

A hash is not a compressed version of the original value, it is a number (or something similar ) derived from the original value. The nature of hash implementations is that it is possible (but statistically unlikely if the hash algorithm is a good one) that two different objects produce the same hash value.

This is known as the Pigeonhole Principle which basically states that if you have N different items, and want to place them into M different categories, where the N number is larger than M (ie. more items than categories), you're going to end up with some categories containing multiple items. Since a hash value is typically much smaller in size than the data it hashes, it follows the same principles.

As such, it is impossible to go back once you have the hash value. You need a different way of transporting data than this.

For instance, an example (but not a very good one) hash algorithm would be to calculate the number modulus 3 (ie. the remainder after dividing by 3). Then you would have the following hash values from numbers:

1 --> 1  <--+- same hash number, but different original values
2 --> 2     |
3 --> 0     |
4 --> 1  <--+

Are you trying to use the hash function in this way in order to:

• Save space (you have observed that the hash value is much smaller in size than the original data)
• Secure transportation (you have observed that the hash value is difficult to reverse)
• Transport data (you have observed that the hash number/string is easier to transport than a complex object hierarchy)

... ?

Knowing why you want to do this might give you a better answer than just "it can't be done".

For instance, for the above 3 different observations, here's a way to do each of them properly:

• Compression/Decompression, for instance using gzip or zlib (the two typically available in most programming languages/runtimes)
• Encryption/Decryption, for instance using RSA, AES or a similar secure encryption algorithm
• Serialization/Deserialization, which is code built to take a complex object hierarchy and produce either a binary or textual representation that later on can be deserialized back into new objects