Best practice for two way hashing in python?

Question

I want to allow users to validate their email address by clicking on a link. The link would look something like

http://www.example.com/verifyemail?id=some-random-string

When I am sending this email, I want to be able to easily generate this 'some-random-string' from row id of user, an integer. and when user clicks on this link, generate that integer back.

Only requirement is this 'some-random-string' should be as opaque and non-guessable to the user as possible.

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Finally, this is what I settled on

def p3_encrypt_safe(plain, key):
    return base64.urlsafe_b64encode(p3_encrypt(plain, key))

used the nice crypto library from http://www.nightsong.com/phr/crypto/p3.py addition of base64 safe encoding is mine.

Answer 1

Use encryption, that's exactly what it's designed for. Blowfish, AES, even DES3 if you don't need particularly high security.

Alternatively, you could compute an SHA-256 or SHA-512 (or whatever) hash of the email address and store it in a database along with the email address itself. That way you can just look up the email address using the hash as a key.

Answer 2

Your best choice is to generate a hash (one-way function) of some of the user's data. For example, to generate a hash of user's row id, you could use something like:

>>> import hashlib
>>> hashlib.sha1('3').hexdigest()
'77de68daecd823babbb58edb1c8e14d7106e83bb'

However, basing your pseudorandom string only on a row id is not very secure, as the user could easily reverse the hash (try googling 77de68daecd823babbb58edb1c8e14d7106e83bb) of such a short string.

A simple solution here is to "salt" the hashed string, i.e. add the same secret string to every value that is hashed. For example:

>>> hashlib.sha1('3' + '[email protected]' + 'somestringconstant').hexdigest()
'b3ca694a9987f39783a324f00cfe8279601decd3'

If you google b3ca694a9987f39783a324f00cfe8279601decd3, probably the only result will be a link to this answer :-), which is not a proof, but a good hint that this hash is quite unique.