python compression function which returns 32 digit string?

Question

Hi,

I am using the md5 function to hash a string into a 32 digit string.

str_to_encode = 'this is a test string which I want to encode'
encoded = hashlib.md5(str_to_encode).hexdigest() 

I want to be able to decode this string (i.e. encoded in the example above) back to its original value. I don't think this is possible using md5 (but if it is please let me know), but is there a compressionn function which I can use which will give me a 32 digit string at the end but which can be reverted?

EDIT: The string being encoded is a url so will only be a couple of hundred characters max although in most cases it will be a lot less.

Thanks

Answer 1

You seem to want two things that can't coexist:

• Any string of any length is converted to exactly 32-bytes, even if it started as 4gb
• The encoded string is decodable without loss of information

There's only so many bits in an MD5 hash, so by the pigeonhole principle it's impossible to reverse it. If it were reversible you could use a hash to compress information infinitely. Furthermore, irreversibility is the main point of a hash; they're intended to be one-way functions. Encryption algorithms are reversible, but require more bytes to store the ciphertext since decodability means they must be collision-free (two plaintexts can't encode to the same ciphertext, or the decode function wouldn't know which plaintext to output given that ciphertext)

Answer 2

It seems to me that you aren't looking for a hash or encryption, you are looking for compression. Try zlib and base64 encoding:

s = 'Hello, world'
encoded = zlib.compress(s).encode('base64')

The length of the encoded data will grow as the input grows, but it may work for you.

Even restricting yourself to URLs, there's no way to reversibly map them to 32-character strings, there are just too many possible URLs.