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Block ciphers use one of the following four encryption modes:
itemization(
itb(CBC (Cipher Block Chaining))
The first block is XOR-ed by the initialization vector and then
encrypted using the specified method. Subsequent blocks are XOR-ed by the
encrypted version of the preceding block. Due to the initialization vector
dictionary attacks are infeasible, as long as the initialization vector is
truly random.
itb(ECB (Electronic Code Book))
Each block is encrypted by itself, using the specified encryption
method. Although an em(initialization vector) may be specified, it is not
used. This method is susceptible to dictionary attacks and should therefore be
avoided, unless you know what you're doing.
itb(CFB (Cipher Feednack))
This method allows a block cipher to be used as a stream cipher. It
uses an initialization vector, which should be unique and random for each new
stream of data that is encrypted using the method. Encryption can only start
after the first data block has been received.
itb(OFB (Output Feednack))
This is an alternative way to use a block cipher as a stream
cipher. It is somewhat more susceptible to traditional data manipulation
attacks, which can usually be thwarted when a message authentication code is
added to the information as well. Like CFB it uses an initialization vector,
which should again be unique and random for each new stream of data that is
encrypted.
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