SHA-256 hashing in oracle 11g
Unfortunately DBMS_CRYPTO in Oracle 11.2 only supports SHA1 (documentation link), which is 160-bit. .

DBMS_CRYPTO in Oracle 12.1 supports SHA2 HASH_SH256 (documentation link), which does what you require.

There are some free implementations of SHA2 just a google away. This blog post, for example.

As for decrypting a hashed password? I don't think you understand hashing. Hashing is 1-way, unless you brute-force it, or use Rainbow Tables. To check if a password is correct, you would hash it and compare it to the stored hash.