Page 113 - DCAP309_INFORMATION_SECURITY_AND_PRIVACY

Unit 8: Cryptography and Encryption

8.2.3 Integrity in Transmission Notes

Most of the users of communication systems are not as much worried regarding secrecy as about
integrity. In an electronic funds transfer, the sum sent from one account to another is frequently
public knowledge. What the bank concerns about is that only proper transfers can happen. If an
active tapper could bring in a false transfer, funds would be moved illegally. An inaccuracy in a
single bit could factually cause millions of dollars to be incorrectly credited or debited.
Cryptographic methods are broadly used to guarantee that intentional or accidental modification
of transmitted information does not cause flawed actions to occur.
A classic technique for guaranteeing integrity is to carry out a checksum of the information
being transmitted and broadcast the checksum in encrypted form. Once the information and
encrypted checksum are obtained, the information is again checksummed and compared to the
transmitted checksum after decryption. If the checksums agree, there is a high likelihood that
the message is unchanged. Unfortunately, this method is too simple to be of realistic value as it
is easily forged. The trouble is that the checksum of the unique message is straight away apparent
and a plaintext message with the same checksum can be easily bogus. Scheming strong
cryptographic checksums is as a result significant to the guarantee of integrity in systems of this
sort.
The key distribution trouble in a one-key system is as before, but an appealing alternative is
presented by the use of public keys. If we produce a single public-key for the whole system and
throw away the private key that would go with it, we can make the checksum impossible to
decrypt. To verify the original message, we simply produce a new checksum, encrypt with the
public key, and confirm that the encrypted checksum matches. This is called a one-way function
because it is hard to invert.
Definite systems of this sort use high quality cryptographic checksums and complex key
distribution and preservation protocols, but there is a tendency towards the use of public keys
for key protection.

8.2.4 Integrity in Storage

Integrity beside random noise has been the focus of much study in the fields of error tolerant
computing and coding theory, but only lately has the requirement for integrity of stored
information beside intentional attack turn out to be a matter for cryptography.
The main mean of assuring integrity of accumulated information has previously been access
control. Access control involves systems of locks and keys, guards, and other techniques of a
physical or logical nature. The current advent of computer viruses has altered this to a significant
degree, and the use of cryptographic checksums for assuring the integrity of stored information
is now becoming extensive.

As in the case of integrity in transmission, a cryptographic checksum is formed and compared to
expectations, but storage media tends to have dissimilar properties than transmission media.
Transmitted information is normally more broadly obtainable over a shorter period of time,
used for a comparatively low volume of information, and used at a slower rate than stored
information. These parameters cause dissimilar tradeoffs in how cryptosystems are used.

8.2.5 Authentication of Identity

Authenticating the identity of individuals or systems to each other has been a trouble for a very
long time. Simple passwords have been used to prove identity. More compound protocols like
series of keywords exchanged among sets of parties are frequently shown in the movies or on

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