Encrypt / Decrypt password

Question

Hi
I'm new in Java and I need to create a function to encrypt / decrypt passwords using the Blowfish algorithm. I know how to create a key, but I don't know how to recover it to decrypt the password.
Another question, Is it possible to use public/private keys in this case???.
Can you give some links or examples please???
Regards
J.C.

Answer 1

This is typically done either one of two ways:
1) PBE based encryption. This uses a password or pass phrase to derive
a key to use with a symmetric algorithm.
2) Asymmetric using something like RSA. Typically RSA is used to wrap
the actual symmetric key used to do the encryption but for very short
plaintext it can be used directly on the plaintext. Passwords are a
good example of short plaintext.
Obviously symmetric encryption is a great deal faster than asymmetric
encryption. So if your plaintext was large you would want to use
symmetric. Also Asymmetric encryption is length dependant. AKA if your
public key's modulus is 1024 bits then you could encrypt any plaintext
that was 121 bytes or shorter.
PBE takes a salt (a random byte array) and an iteration count and
hashes a passphrase with the salt iteration number of times to generate
a key that can be reproduced over and over again and used with a
symmetric algorithm. The issue here is that your salt/ic either need
to be hard coded and reused or the values for any single encryption
need to be saved along with the ciphertext. Using the same ic/salt for
a large number of plaintext to ciphertext operations can lead to a
weakening of the pass phrase (aka the key) and aids a cryptoanalyst in
breaking the code. Although it is still difficult it becomes easier
with each successive encryption.
Its upto you which route you take but you should note that private keys
used in asymmetric encryption use PBE to keep them private anyway so in
a sense if you use asymmetric encryption you are really using both
asymmetric encryption and PBE...