Classic Definition of Cryptography
, or the art of "
", classically meant hiding the
contents or existence of messages from an adversary.
concerned with hiding the existence
of a message
often in plain sight.
science of information security
science of communicating and computing in the presence of an
derived from the Greek
before, it i
ncludes techniques such as microdots, merging words with
now, it is most often associated with scrambling
) then back again (known as
) to plaintext
individuals who practice this field are known as
refers to the study of ciphers,
, or cryptosystems with a view
to finding weaknesses in them that will permit retrieval of the
from the ciphertext, without necessarily knowing the
. This is known as
relies on a knowledge of the encryption algorithm (that for civilian
applications should be in the public domain) and some knowledge of
the possible s
tructure of the plaintext (such as the structure of a
typical interbank financial transaction) for a partial or full
reconstruction of the plaintext from ciphertext
additionally, the goal is to also infer the key for decryption of future
cise methods used for cryptanalysis depend on whether the
“attacker” has just a piece of ciphertext, or pairs of plaintext and ciphertext,
how much structure is possessed by the plaintext, and how much of that
structure is known to the attacker.
of cryptanalysis for classical encryption exploit the fact that some
aspect of the structure of plaintext may survive in the ciphertext.
is a science which incorporates both
is the mathematics, such as number theory, and the
application of formulas and
s, that add force to
Brute force attack
when encryption and decryption algorithms are publicly available, as
they generally are, a brute
force attack means trying every possible
key on a piece of ciphertext until an intelligible transla
plaintext is obtained
adversary tests all possible keys and checks which key
decrypts the message,
this assumes we can identify the correct
plaintext among all plaintexts generated by the attack
|key space| = 26
The adversary tests these 26 keys
and 1 of these keys will give the
the total number of all possible keys that can be used in a
uses a 56
bit key. So, the key space is of size 2
which is approximately the same as 7
is the art and science of writing hidden messages in such a
way that no one, apart from the sender and intended recipient, suspects
the existence of the message, a form of
security through obscurity
Codes replace a specific piece of plaintext with a predefined
Codes are essentially a substitution cipher,
replace strings of
symbols rather than just individual symbols.
"One if by land, two if by sea."
A cryptosystem should be secure even if everything about it is public
knowledge except the secret key.
“Do not re
ly on "security through obscurity".
CRYPTOGRAPHY: ENCRYPTION & DECRYPTION
The sequence of data processing steps that
transform plaintext into ciphertext.
In cryptography for commercial and other civilian applications, the
encryption and decryption algorithms are made public.
A secret key is used to set some or all of the various
parameters used by the encryption algorithm.
The important thing to note is that, in classical cryptography,
secret key is used for encryption and decryption.
It is for this reason that
classical cryptography is also referred to as
symmetric key cryptography
On the other hand, in the more modern cryptographic algorithms,
encryption and decryption
keys are not only different, but also one of them
is placed in the public
Such algorithms are commonly referred to
asymmetric key cryptography
The sequence of data processing steps
back into plaintext.
the process of transforming plaintext using an algorithm (called
) to make it unreadable to anyone except those who has the
key to decrypt it.
ocess by which plaintext is converted into ciphertext
is the plaintext
is the cipher function (encryption algorithm)
is the secret key
is the ciphertext
makes the contents of a message unintelligible to
anyone not possessing some secret information.
the process to make the encrypted information readable again
ecovering plaintext from ciphertext
is the decipher function (decryption algorithm)
is public, the secrecy of
depends totally on the
Usually in cryptography the parties are denoted by:
Alice & Bob
Someone listening is denoted by:
any single scheme for encryption
a cipher means the same thing as a “cryptographic system”
a block cipher processes a block of input data at a time and produces
a ciphertext block of the same size
a stream cipher encrypts data on the fly, usually one
byte at a time.
Secret Key Cryptography or Symmetric Cryptography
Each communicating party needs access to the
This needs to
be known to both sides, but needs to be kept secret
This secret key is used for encryption and decryption
he private key must be kept secret
, while the p
c key can be
an be used for digital
Examples of public key cryptography:
RSA, El Gamal, DSA, various algorithms based on elliptic