In cryptography, the goal of the attacker is to break the secrecy of the encryption and learn the secret message and, even better, the secret key. There are dozens of different types of attacks that have been developed against different types of cryptosystems with varying levels of effectiveness. Some are easily understandable while others may require an advanced degree in mathematics to comprehend. In this post, we'll be discussing some of the more common attacks and why they may or may not work against different types of ciphers.

Brute-Force Attack

The simplest attack on a cipher is the brute force attack. In this attack, an attacker simply tries to decrypt the message with each possible secret key and checks the result of the decryption to see if it makes sense. Given enough time and computational resources, this attack is guaranteed to work since the true secret key has to be within the set of possible secret keys and the attacker will eventually try it and (hopefully) realize that the resulting plaintext is the correct one.

Modern ciphers protect themselves against brute forc...

Steganography ("covered writing") is the science of hiding information "in plain sight". Unlike cryptography, the goal of steganography is to completely obscure the existence of information rather than conceal its content. Currently, the most common usage of steganography is to hide one computer file inside of another computer file.

A historic usage of steganography was pioneered by Histaeius of Milan to allow him to communicate secretly with the Greeks. Histaeius would shave the head of a slave, tattoo a message on the slave's scalp, and then wait for the hair to grow back, concealing the message. The slave would then be sent to the Greeks, who would shave the slave's head and read the hidden message.

Blowfish is a symmetric encryption algorithm developed by Bruce Schneier to replace Data Encryption Standard (DES). At the time of its development, most encryption algorithms were protected by patents, government secrecy, or company intellectual property. Schneier placed Blowfish in the public domain making it freely available for anyone to use.

Blowfish is a 16-round Feistel cipher. It's block size is 64-bit and key sizes range from 32 to 448 bits. In this article, we'll first take a look at the Blowfish algorithm (encryption, decryption and key schedule), and then discuss its advantages and disadvantages.

Encryption with Blowfish

Encryption with Blowfish has two main stages: sixteen iterations of the round function and an output operation.

What is Cryptography?

Cryptography deals with a set of methods which enable us to store and transmit information while safeguarding it from intruders. That is, we can use cryptography methods to keep information private (say documents, passwords, etc on your computer), and to communicate in a way such that only the intended recipient can read the message.

Cryptography achieves this by converting data into a different form which is incomprehensible (often called ciphertext or code). The process of converting data to ciphertext is called encryption, and the process of converting retrieving data from ciphertext is known as decryption. Performing encryption and decryption requires secret information such as passwords (or keys).

While many ciphers have been created based upon the Feistel structure, the most famous of these is the Data Encryption Standard (DES). DES was based off of the original Lucifer cipher developed by Feistel and Coppersmith and submitted as an entry to the US National Bureau of Standards as a candidate for the US official encryption standard. After some modification (to improve security against differential cryptanalysis), it was selected and published as a standard in 1977.

Encryption with DES

The DES algorithm is a 16-round Feistel cipher. It takes as input a 64-bit input and a 64-bit secret key, and consists of three main stages:

• The initial permutation
• The round function (repeated 16 times)
• The final permutation

A diagram of how these stages fit together with the ...

Rijndael is a family of block ciphers developed by Belgian cryptographers Vincent Rijmen and Joen Daemen. It was submitted as an entry to the National Institute of Standards and Technology's (NIST) competition to select an Advanced Encryption Standard (AES) to replace Data Encryption Standard (DES). In 2001, Rijndael won the competition and the 128, 192, and 256-bit versions of Rijndael were officially selected as the Advanced Encryption Standard.

The three variants of AES are based on different key sizes (128, 192, and 256 bits). In this article, we will focus on the 128-bit version of the AES key schedule, which provides sufficient background to understand the 192 and 256 bit variants as well. At the end, we'll include a note the other variants, and how they differ from the 128-bit version.

Encryption with AES

The encryption phase of AES can be broken into three...

The one-time pad is famous as being the only completely unbreakable cipher. Assuming that the secret pad is randomly generated, not-reused (hence "one-time pad"), and not leaked, it is impossible to learn a single bit of the plaintext of a message from a ciphertext.

The one-time-pad is one of the best cryptography protocols when the work must be done by hand, without the aid of a computer. This made it important in the pre-computer era, and is still useful in situations where possession of a computer is illegal or incriminating or where trustworthy computers are not available.

Encryption and decryption with the One-Time Pad

Encryption with the one-time pad is based on the exclusive-or (XOR) operation. The exclusive-or of two identical bits (two zeros or two ones) produces a zero and the exclusive-or of two different bits (a zero and a one) produces a one.

The secret in one-time pad encryption is a secret of...

What is a Block Cipher?

There are two main types of ciphers: block and stream ciphers. In a stream cipher (which are discussed in a previous post), the plaintext is encrypted one bit at a time. In a block cipher, the plaintext is broken into blocks of a set length and the bits in each block are encrypted together.

Notable Block Ciphers

Many well-known encryption algorithms are block ciphers. A few of the most popular block ciphers are DES/3DES, AES, Blowfish, and Twofish.

The Data Encryption Cipher (DES) is an algorithm developed by IBM as a submission to the US National Bureau of Standards (precursor to National Institute of Standards and Technology) for a contest to select a government-approved block cipher. DES is a Feistel c...

History of the Playfair Cipher

The Playfair cipher was created in 1854 by Charles Wheatstone. It was championed by Lord Playfair (hence the name) and used by Britain in the Boer War and both World Wars and also by Australia and New Zealand in World War II. It is said that it was rejected for use by the British Foreign Office due to its perceived complexity.

The first solution to the Playfair cipher was published by Lieutenant Joseph Mauborgne in 1914. It is no longer used since it is insecure.

Description of the Playfair Cipher

Key and Matrix

The Playfair cipher uses a 5x5 matrix of letters for...