This is repeated with b2 and s2 b3 and s3 and the

the S-box is retrieved and stored away. This is repeated with B[2] and S[2], B[3] and S[3], and
the others up to B[8] and S[8]. At this point, you now have 8 4-bit numbers, which when strung
together one after the other in the order of retrieval, give a 32-bit result.
5. The result from the previous stage is now passed into the P Permutation.
6. This number is now XORed with L[I-1], and moved into R[I]. R[I-1] is moved into L[I].
7. At this point we have a new L[I] and R[I]. Here, we increment I and repeat the core function
until I = 17, which means that 16 rounds have been executed and keys K[1]-K[16] have all been
used.
When L[16] and R[16] have been obtained, they are joined back together in the same fashion they
were split apart (L[16] is the left-hand half, R[16] is the right-hand half), then the two halves are
swapped, R[16] becomes the left-most 32 bits and L[16] becomes the right-most 32 bits of the
pre-output block and the resultant 64-bit number is called the pre-output.
The method described above will encrypt a block of plaintext and return a block of ciphertext. In
order to decrypt the ciphertext and get the original plaintext again, the procedure is simply
repeated but the subkeys are applied in reverse order, from K[16]-K[1]. That is, stage 2 of the
Core Function as outlined above changes from R[I-1] XOR K[I] to R[I-1] XOR K[17-I]. Other
than that, decryption is performed exactly the same as encryption.
Modes of Operation
ECB (Electronic Code Book)
This is the regular DES algorithm, exactly as described above. Data is divided into 64-bit
blocks and each block is encrypted one at a time. Separate encryptions with different
blocks are totally independent of each other. This means that if data is transmitted over a
network or phone line, transmission errors will only affect the block containing the error.
It also means, however, that the blocks can be rearranged, thus scrambling a file beyond
recognition, and this action would go undetected. ECB is the weakest of the various
modes because no additional security measures are implemented besides the basic DES
algorithm. However, ECB is the fastest and easiest to implement, making it the most
common mode of DES seen in commercial applications. This is the mode of operation
used by Private Encryptor.
CBC (Cipher Block Chaining)

In this mode of operation, each block of ECB encrypted ciphertext is XORed with the
next plaintext block to be encrypted, thus making all the blocks dependent on all the
previous blocks. This means that in order to find the plaintext of a particular block, you
need to know the ciphertext, the key, and the ciphertext for the previous block. The first
block to be encrypted has no previous ciphertext, so the plaintext is XORed with a 64-bit
number called the Initialization Vector, or IV for short. So if data is transmitted over a
network or phone line and there is a transmission error (adding or deleting bits), the error
will be carried forward to all subsequent blocks since each block is dependent upon the
last. If the bits are just modified in transit (as is the more common case) the error will