It is possible to use a hash function to construct a block
cipher with a structure similar to DES? Because a hash function
is one way and a block cipher must be reversible (to decrypt),
how is it possible? migrated from security.stackexchange.com Nov
8 '12 at 12:26 It is possible to build a block cipher out of a
great many things. If you want to use a hash function, the
classic trick is to follow a Feistel structure, which is,
incidentally, the same kind of structure than what DES uses. The
schematics on the Wikipedia page are quite clear; you would use
the hash function for the "F" part, which combines one (sub)key
and one half of the current block, to produce a value which is
to be XORed with the other half of the current block. The beauty
of the scheme is that the "F" function is always invoked in the
same direction, both for encryption and for decryption.
Therefore, it can be a one-way function, like a hash function.
Luby and Rackoff have demonstrated in 1988 that the Feistel
scheme offers remarkable secu