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copied!<p>The point of a hash like sha256 is that it is supposed to be a one way function (although the existence of true one way functions is still an open question, see <a href="http://en.wikipedia.org/wiki/One-way_function" rel="noreferrer">http://en.wikipedia.org/wiki/One-way_function</a>).</p> <p>Note <a href="http://en.wikipedia.org/wiki/Cryptographic_hash_function" rel="noreferrer">http://en.wikipedia.org/wiki/Cryptographic_hash_function</a>:</p> <pre><code>The ideal cryptographic hash function has four main properties: 1. it is easy to compute the hash value for any given message 2. it is infeasible to generate a message that has a given hash 3. it is infeasible to modify a message without changing the hash 4. it is infeasible to find two different messages with the same hash. </code></pre> <p>If you could reverse it then you'd be breaking rule 2. These rules allow one to tell another party that they have some information (such as a password), without revealing the information. For example, see wikipedia: <a href="http://en.wikipedia.org/wiki/Cryptographic_hash_function#Illustration" rel="noreferrer">http://en.wikipedia.org/wiki/Cryptographic_hash_function#Illustration</a></p> <p>If you need invertibility see <a href="https://stackoverflow.com/questions/2490334/simple-way-to-encode-a-string-according-to-a-password">Simple way to encode a string according to a password?</a>, you can use something weak like Vignere, but there is also an example using PyCrypto:</p> <pre><code>from Crypto.Cipher import AES import base64 cipher = AES.new(secret_key,AES.MODE_ECB) # never use ECB in strong systems obviously encoded = base64.b64encode(cipher.encrypt(msg_text)) # ... decoded = cipher.decrypt(baes64.b64decode(msg_text)) </code></pre> <p>If you want a reversible hash function, see <a href="https://stackoverflow.com/questions/4273466/reversible-hash-function">Reversible hash function?</a></p>

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