ofb

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Information on the OFB mode
---------------------------

The OFB uses an initialization vector (z_0 = IV). This vector is encrypted
using an encryption algorithm in ECB mode. The encrypted buffer is then 
XORed with the plaintext to get the ciphertext.

	z_i = e_k(z_i - 1)
	y_i = x_i ^ z_i
	i >= 1

	z_i: This is the stream which is encrypted using the encryption
	     algorithm.
	x_i: Plaintext. 
	y_i: Ciphertext
	e_k: Encryption-Algorithm.

The OFB mode is self-inverse, hence the same function is used for decryption.

Links:
	http://www.rsasecurity.com/rsalabs/faq/2-1-4-5.html
	http://www.iks-jena.de/mitarb/lutz/security/cryptfaq/q83.html