Security - Telenor

14
K
m i
Stream
cipher
algorithm
Figure 2 Principles for
synchronous stream cipher
Figure 3 Principles for
block cipher
K
k i
c i
Stream
cipher
algorithm
system (also called public key system), it is
impossible to find K D from knowledge of K E
and the encryption key can be a public parameter
belonging to the entity B. In this case there is a
need for an authenticated key channel from B to
A. Current systems often solve this distribution
problem using certificates and a Public Key
Infrastructure (PKI).
For symmetric systems, the algorithm defining
the encryption/decryption transformation can
either be a block cipher or a stream cipher depending
on the use of internal memory in the
encrypting device.
A stream cipher breaks the plaintext message
into successive characters or bits m 1 , m 2 , ... m n
and encrypts each m i with the i’th element of a
key stream k 1 , k 2 , ... k n derived from the basic
key K and optionally an additional message key.
Most stream ciphers operate bitwise by adding
the message bit and the key stream bit modulo 2,
i.e. c i = m i ⊕ k i . The receiver can now recover
the plaintext bit m i from the ciphertext bit c i by
adding with the corresponding key stream bit k i .
Note that this requires full bit synchronisation
between the encrypting and decrypting devices.
The general principles for a stream cipher are
shown in Figure 2.
A block cipher breaks the plaintext message into
successive blocks M 1 , M 2 , ..., M n and encrypts
each block using the same key K. A typical
block length is 64 or 128 bits. For a fixed key
K, the cryptographic transformation can then be
seen as a permutation on the set of all possible
blocks. The US Data Encryption Standard DES
[3] is a well-known example of a block cipher
using a 56 bits key and operating on 64 bits data
blocks. Note that a block cipher can operate in
M
Block
cipher
C
Block
cipher
M
encryption
decryption
K
K
k i
m i
different modes as defined by the international
standard ISO/IEC IS 10116 [4]. By operating a
block cipher in Cipher Feedback Mode (CBC) or
Output Feedback Mode (OFB), a block cipher
behaves like a stream cipher. There are also standardised
methods for using a block cipher to
compute Message Authentication Codes (MAC)
[5] and for hashing operations [6]. This means
that a strong and efficient block cipher is a
handy tool for implementation of several security
services. The general principle for a block
cipher is shown in Figure 3.
The strength of a cryptographic system relies
heavily on the two components, the encryption
algorithm and the key management system; neither
is simple to design. For the encryption algorithm
several security requirements can be summarised
in the following: Without knowledge
of the secret key, it shall be impossible for an
attacker to find the plaintext, given the ciphertext;
or from known plaintext/ciphertext pairs to
find the secret key. This means that the security
of the system does not depend on the secrecy of
the algorithm itself. When an algorithm is analysed,
it is important to assume that the opponent
has detailed information about the specifications.
3 The Strength of
Cryptographic Algorithms
Cryptographic algorithms are fundamentally
different from other algorithms with respect
to their intended goal. Normally an algorithm is
designed to solve a specific problem like error
correction or finding a path through a complex
network. We are then normally able to prove
that the algorithm solves the problem completely,
even if other and more efficient algorithms
may be found. As long as we can meet
operational requirements, the algorithm will
continue to do its job and it will never be
“worn out” or “broken”.
The goal of an encryption algorithm is to protect
information against all possible attacks of known
and unknown enemies. Even if we are able to
prove that an algorithm is resistant to a specific
attack, we can never be sure that it can withstand
a novel attack tomorrow. Perhaps we can design
an algorithm that is secure against the computational
power of a single enterprise. With the current
growth in processing power, the same algorithm
could be vulnerable to governmental agencies
and dedicated hardware in a few years’
time. We see today that some of the most modern
and secure crypto systems will be easy tasks
for new computational models like DNA computing
[7] and quantum computing [8].
A theoretical foundation for cryptographic systems
may be based on information theory or
computational complexity theory, but neither of
Telektronikk 3.2000