Cryptography Reference
In-Depth Information
Observe that error propagation has nothing to do with preventing errors
occurring in the first place. Neither is error propagation anything to do with
correcting errors when they do occur. This requires the use of special error-
correcting codes, which are not regarded as cryptographic primitives (see
Section 1.4.4).
4.2.4
Properties of stream ciphers
Streamciphers have a number of attractive properties thatmake themthe favoured
encryption mechanism in a number of important applications:
No error propagation
. Since stream ciphers encrypt the plaintext bit by bit,
a 1-bit transmission error will only result in a 1-bit error in the plaintext.
For this reason, stream ciphers are popular in communications applications,
especially where the communication channel is of poor quality and errors are
inevitable. For example, stream ciphers are often used in the protection of
mobile communications (see Section 12.3). Stream ciphers are also used to
protect private mobile radio systems, such as those used by emergency services
and taxi operators.
Speed
. The simple encryption process (based on XOR) results in stream
ciphers being extremely fast to operate, which makes them very attractive
for applications that require real-time encryption of data, which is again the
case inmobile telecommunications. Their simplicity alsomakes streamciphers
relatively straightforward to implement.
On-the-fly encryption
. Bitwise encryption means that large chunks of plaintext
do not sit around in registers before being encrypted. This makes stream
ciphers attractive to applications such as secure terminals, where individual
keystrokes should be protected immediately on entry.
The main problem with stream ciphers is:
Need for synchronisation
: Since stream ciphers process data bitwise, it is vital
that the sender and receiver keep their keystream in perfect synchronisation.
The consequences of a 1-bit transmission loss are potentially disastrous,
since subsequent decryptions will be conducted using the wrong bits of the
keystream. Procedures are thus required to allow periodic resynchronisation
of the keystream. There are many different ways in which this could be done.
One method is to regularly restart the keystream using a fresh key. This could
either be done after a fixed period of time, or after the sending of a special
resynchronisation sequence. Some modern stream ciphers are designed to
have special resynchronisation properties, but we will not discuss these here.
As we will see in Section 4.6, block ciphers are often deployed in modes of
operation that essentially convert them into stream ciphers in order to benefit
from some of the above properties.
