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Cryptology

Brute Force Attack
A brute force attack consists of trying every possible code, combination, or password until you find the right one.Determining the Difficulty of a Brute Force Attack

The difficulty of a brute force attack depends on several factors, such as:How long can the key be?How many possible values can each component of the key have?How long will it take to attempt each key?Is there a mechanism which will lock the attacker out after a number of failed attempts?

As an example, imagine a system which...
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Symmetric and Asymmetric ciphers
In a symmetric cipher, both parties must use the same key for encryption and decryption. This means that the encryption key must be shared between the two parties before any messages can be decrypted. Symmetric systems are also known as shared secret systems or private key systems.

Symmetric ciphers are significantly faster than asymmetric ciphers, but the requirements for key exchange make them difficult to use.

In an asymmetric cipher, the encryption key and the decryption keys are...
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Dictionary Attack
A dictionary attack consists of trying "every word in the dictionary" as a possible password for an encrypted message.

A dictionary attack is generally more efficient than a brute force attack, because users typically choose poor passwords.

Dictionary attacks are generally far less successful against systems that use passphrases instead of passwords.Improving Dictionary Attacks

There are two methods of improving the success of a dictionary attack.

The first method of improving the success...
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XOR encryption

XOR encryption is a trivially simple symmetric cipher which is used in many applications where security is not a defined requirement.

The XOR Operator

XOR, also know as Exclusive OR, is a bitwise operator from binary mathematics.

The six bitwise operators, as defined in the C programming language, are:

OperationSymbol
AND&
Inclusive OR|
Exclusive OR (XOR)^
Right Shift>>
Left Shift<<
Complement~

The XOR operator returns a 1 when the value of either the first bit or the second bit is a 1.

The XOR operator returns a 0 when neither or both of the bits is 1.

This is best illustrated in the following chart:

First BitSecond BitResult
000
011
101
110

The XOR operator is used to "flip" bits (zeroes and ones) in a piece of plaintext to create a ciphertext.

Converting Plaintext to Ciphertext with XOR Encryption

The plaintext we will start with is the term "FAQ".

  • ASCII representation of the plaintext: FAQ
  • Hexadecimal representation of the plaintext: 70 65 81
  • Binary representation of the plaintext: 01110000 01100101 1000000

We will XOR the first character of this plaintext into ciphertext using a "V" as the key:

  • ASCII representation of the key: V
  • Hexadecimal representation of the key: 86
  • Binary representation of the key: 10000110
Plaintext 'F'Key 'V'Ciphertext
011
101
101
101
000
011
000
011

Converting Ciphertext to Plaintext with XOR Encryption

XOR encryption is a symmetric algorithm. This means that we can use the encryption key as the decryption key.

Let's decrypt our ciphertext to recreate our original plaintext.

CiphertextKey 'V'Plaintext
110
101
101
101
000
110
000
110

Do the math yourself with the other two characters of plaintext to prove this to yourself.

Many encryption algorithms utilize the XOR operator as part of their operations.

Understanding XOR and the other binary operators is a necessary step on the path to becoming a cryptologist.

XOR Security

XOR encryption is trivially simple to implement and equally trivial to break.

XOR encryption should not be utilized for any data which you would want to protect.
 

PKI Certificate
A PKI certificate, which stands for Public Key Infrastructure certificate, allows someone to combine their digital signature with a public key and something that identifies them, an example being their real life name. This certificate is used to allow computer users to show that they do own the public keys they claim to. In other words, it is a security mechanism for public keys.

As mentioned before, a digital signature is required for the PKI certificate. This signature can either be made...
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Block and Stream Ciphers
Block and Stream Ciphers are two categories of ciphers used in classical cryptography. Block and Stream Ciphers differ in how large a piece of the message is processed in each encryption operation.Block Ciphers

Block ciphers encrypt plaintext in chunks. Common block sizes are 64 and 128 bits.Stream Ciphers

Stream ciphers encrypt plaintext one byte or one bit at a time. A stream cipher can be thought of as a block cipher with a really small block size.Block Ciphers vs. Stream...
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Substitution and Transposition ciphers
Substitution and Transposition Ciphers are two categories of ciphers used in classical cryptography. Substitution and Transposition differ in how chunks of the message are handled by the encryption process.Substitution ciphers

Substition ciphers encrypt plaintext by changing the plaintext one piece at a time.

The Ceasar Cipher was an early substitution cipher. In the Caesar Cipher, each character is shifted three places up. Therefore, A becomes D and B becomes E, etc...

This table shows...
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Message Digest
A message digest is a number which is created algorithmically from a file and represents that file uniquely. If the file changes, the message digest will change.

In addition to allowing us to determine if a file has changed, message digests can also help to identify duplicate files.Creating a Message Digest

On many Unix systems, the `md5` command wll generate a message digest:

bash-2.05a$ md5 /kernelMD5 (/kernel) = cdd6056e13686981c50ca6008782732a

On older legacy Unix systems, the `sum`...
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Cryptographic Algorithms
Symmetric Cryptographic Algorithms

DES (Data Encryption Standard) is perhaps the most widely used cryptographic algorithm. DES is based upon IBM's Lucifer algorithm. The NBS (National Bureau of Standards) and the NSA (National Security Agency) adopted DES as a federal standard in 1977. DES is a symmetric cipher with an effective key length of 56 bits.

Triple-DES is a variant of DES which encrypts the plaintext with the DES algorithm three times.

IDEA (International Data Encryption...
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Cryptology

Cryptology

Cryptology is the science which incorporates both cryptography and cryptanalysis.

Cryptologists

Practitioners of cryptology are known as cryptologists.

Cryptology in Context

A cryptographer will use cryptography to convert plaintext into ciphertext and a cryptanalyst will use cryptanalysis to attempt to turn that ciphertext back into plaintext.

Both the cryptographer and the cryptanalyst are cryptologists. Cryptography and cryptanalysis are the two sides of cryptology.
 
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