Re: Questions: adacrypt


"AdaCrypt" <austein.obyrne@xxxxxxxxxxxxxx> wrote in message
news:1188564961.268407.137250@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

1 Is there a word for how much memory each plaintext uses when it is
encrypted into ciphertext in cryptography - like say 'specific volume
of memory' of the ciphertext - memory per item?- memory per character?
- memory per block? of the original plaintext character.

Most stream ciphers encrypt 1:1; one plaintext byte is encrypted to one
ciphertext byte.

Block ciphers encrypt plaintext in units of the block size, implying that
plaintext must be padded to a multiple of the block size. Depending on the
padding algorithm, ciphertext may be as much as one full block longer than
the plaintext.

Note that in both cases, some form of authentication code usually
accompanies the message, expanding it further. While this authentication
has nothing to do with the cipher, it's presence is essential to ensure that
the enciphered message is secure and hasn't been tampered with.


2) How important is this from a design point of view given that memory
has become very cheap now.

A very large subset of applications of cryptography involve transmitting the
ciphertext through a hostile environment. The transmission medium is often
a shared resource so to whatever extent the encipherment expands the
message, some other user of the shared resource is denied access. It seems
to me that transmission resources are almost always more expensive than
storage resources.


3) Which cipher has the better rating from this point of view.

From this point of view, all stream applications of ciphers are equivalent.
With block ciphers, the amount wasted in padding is application-dependent.
Let's say you were only enciphering disk blocks in units of 4096 bytes.
Almost all credible block ciphers have block sizes that are submultiples of
this number. Consequently, your application will experience no waste of
space due to padding (unless your algorithm for padding removal requires
that the last plaintext block consist of padding characters). Other
applications may incur padding overhead that averages-out to half of the
block size.

So, block ciphers with smaller block sizes may incur less padding overhead
at the expense of increased vulnerability to attack.


Thanks in anticipation - adacrypt



.

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