"Inveniam viam aut faciam" : I will either
find a way, or I shall make one
CRYPTOGRAPHY - A GENERAL
What is it?
Cryptography is a common, widespread and fundamental aspect of
It's most basic definition is that it is the practise of keeping
secrets by encrypting the data, it also encompasses cryptanalysis which
is the art of decrypting
secret data. Both are opposite sides of the same coin. People who make
are known as cryptographers, and those who break ciphers are known
It is also important to understand the difference between codes and
are words or symbols which are used to represent something ("cool" is
often a code word
for "looking good"), but are not meant to ensure confidentiality,
whereas ciphers are designed to hide the true meaning of the message.
Cryptography takes plaintext -the unencrypted data-, transforms it into
ciphertext -the encrypted data- by using a cipher -an encryption
algorithm or method- and a key -a value used in conjunction with a
cipher to create ciphertext. Most cryptosystems obey a fundamental idea
regarding cryptosystems called Kerchoff's
Principle. This states that the
strength of a cryptosystem depends on the strength of the key, as well
as the encryption algorithm.
Any encryption algorithm also needs to ensure that certain aims are
met. First is
this is where the relationship between the plaintext and the key
is made complicated enough so that if an attacker has plaintext and the
they cannot make changes to the plaintext to see the resultant
ciphertext and thus
deduce the code. Second is diffusion,
this means that any changes in the plaintext results in multiple
changes spread throughout the ciphertext. Third is the Claude
Shannon Characteristics of a Good Cipher,
Finally, any encryption algorithm should be chosen because;
amount of secrecy needed should determine the amount of work needed for
encryption and decrytption of the data.
set of keys and the enciphering algorithm should be free from
implementation of the enciphering process should be as simple as
errors in the enciphering should not propagate and cause corruption of
size of the ciphertext should be no larger than the text of the
is encrypted in order to make it extremely difficult for an
person to access the data, even if they know the algorithm used and
encrypted data. The goal of many cryptosystems is to make their
computationally infeasible to crack. Take for example a cryptosystem
that used a
key consisting of purely lowercase alphabetic characters and 10
characters long. That means that the attacker would have to try 10 to
the power of 25 possible combinations.
Assume that they could try 10,000,000 combinations a second, I will
leave it as an exercise to the reader to do the maths regarding the
length of time it could take
to crack the key.
is based upon sound mathematics
has been analyzed by competent experts and is sound
has "stood the test of time"
So we see that cryptography can provide confidentiality in it's hiding
secret data, it can provide integrity as it can be used to detect any
which may have happened to the data, and it can provide accountability
as it can
be used to verify the origin of the data. Although please bear in mind
that not all
cryptosystems are built to accomplish all three of these goals.
Types of Cryptosystems..
Cryptosystems are used to encipher data using a cipher, and can be
broken down into a couple of broad categories;
different types sometimes can and do overlap, and some are specifically
designed to meet some or all of the goals of confidentiality, integrity
- Symmetric encryption
- Asymmetric encryption
- Physical encryption
- Hashing encryption
- Quantum encryption
In this type of cryptosystem only one key is used. That means that
there is only
one key for both the encryption of the plaintext and the decryption of
the resultant ciphertext. So if person A used a symmetric cryptosystem,
with a key
of X, to send an encrypted message to person B. Than person B would
need to know, and use, the key of X as well as the same cryptosystem in
order to decrypt the secret message. This type of encryption and
decryption is very fast.
The weak point in the system is also it's strength, the key. If the key
used is compromised then the entire process is compromised, so every
person using the cryptosystem who wants to securely exchange data would
have to keep the key secret. Now with group of a 100 people, each
person would need a copy of the secret key, each of the 100 would need
to securely receive their copy of the key, and then also securely use
and keep it. Statistically the chances of one of 100 copies being
compromised is much greater then a smaller group of 10.
A similar problem is that if you want to securely exchange data with a
separately, you would need to keep track of, securely keep and use, 100
This is also known as "public key" encryption. In this system each user
has two keys,
one private key which is kept secret, and a public key which is shared.
A user can encrypt data with another person's public key to ensure that
only the person with the matching private key can decrypt it. Or they
could encrypt data with their private key and allow anyone with access
to the public key to decrypt it.
This system allows any new users to be easily added as the same key
pair is used, users
can be removed easily, the key only needs to be changed when the
private key has
been compromised, and the distribution of keys is safe as only the
public keys are
distributed. The drawback to this system is the that the encryption and
decryption is slower than other systems.
This method of encryption is what many of the classical encryption
systems fall into.
By classical I mean those systems used by the Greeks, by Caesar, etc. A
general definition of these systems is that they do not encipher the
data using a mathematical
This encryption system uses a mathematical process to perform a
calculation against the data and return a numeric or hash value. Unlike
other encryption systems, hashing
is a one-way system, this means that it is impossible to use the hash
value to deduce
the original data. But the hash value does serve as a highly effective
checksum, meaning that it allows a way of seeing if the data has
changed. If the checksum of
data at the recipient is different to that of the originator, the data
was tampered with in transit.
This system depends on the scientific method known as Heisenberg's
Principle and can only be used
across fibre optic links. The scientific principle in question states
that the act of observing something causes changes in that which
is being observed. So when data is sent across a fibre link, the
sequence and polarity of the photons is used as the key, and if the
flow of photons was somehow monitored,
then the sequence and polarity of the photons would be altered thus
alerting the recipient of possible tampering.
Types of Ciphers..
Lets look at some of the ciphers used by cryptosystems to encode data;
- Substitution cipher
- Transposition cipher
- Steganongraphic cipher
- Block cipher
- Stream cipher
These are ciphers that changes one character or symbol into another
one. The basic
way of doing this is the Monoalphabetic
Substitution Cipher, where a
single alphabet is used to make the substitution, this means that a
character is only changed to one other particular character. This means
"a" would be a "z", so the data of "aaa" would be "zzz". A more
of the substitution cipher is the Polyalphabetic
Substitution Cipher, where
multiple alphabets are used to encrypt the data. An example would be
where the position of a character in a message would indicate that it
would be replaced by
a character that far ahead of it, so "aaa" becomes "bcd".
These are ciphers which involve scrambling the letters in some way, the
generally broken into blocks then each block is scrambled, and the
interchanged in some way. An example would be "attack at five", take
out the spaces
would give "attackatfive", split into 2 blocks of 6 would be "attack
atfive". Now stack
take the 1 letter at the top, then second letter at bottom, and so on
to get "attice",
the remain characters are "atfavk", so the encrypted message is "attice
These ciphers basically hide the secret data inside other data. The
could still look like normal data but the real message is encrypted
inside it. A
very basic example would be "please all gather to meet at seventeen one
one", take every third word and the decrypted message is "gather at
Ciphers like this mathematically encipher data in blocks. The plaintext
data is split
into blocks and then each block is enciphered to produce the total
are multiple ways to do this;
- ECB, Electronic Code Book is where each block is encrypted
- CBC, Code Book Cipher is where the block which is to be
of a mathematical process run against the previous block to help secure
- OFB, Output Feedback Cipher is where individual blocks are not
but the key is
used to generate data the same length as the data to be encrypted, and
then uses a mathematical process against the generated data to securely
encrypt the plaintext.
With this cipher each bit of data is sequentially encrypted using one
bit of the key,
the reverse process is followed to decrypt the data. Stream ciphers can
be made mathematically impossible to break by using different random
keys for the encryption.
One implementation of such a system is One-Time
Pads, where each use of the
encryption process uses a different key.
Well thats it, a general overview of some cryptographic principles. I
hope it was
interesting. It is worth getting to about cryptography as it is heavily
used in our
everyday lives, and the more we understand about it, the better we can
we use day in and day out.