### Vigenère Cipher - Decoder, Encoder, Solver, Translator

Oct 19, By Kevin W. This came up a few times during the last round of security reports we at Denim have been writing, so I wanted ensure everyone. In cryptography, ciphertext or cyphertext is the result of encryption performed on plaintext using an algorithm, called a cipher. Ciphertext is also known as encrypted or encoded information because it contains a form of the original plaintext that is unreadable by a human or computer without the proper cipher to decrypt it. . The keys are unknown, but the relationship. When encrypting, the key is added to the plain text to get encrypted.

Organizational focus on physical security programs are key to avoid weaknesses in an encryption system due to poor physical security practices. Simply improving the physical security of the trash is not sufficient to guard against theft of plaintext. As a result, implementing local procedures to guard against unauthorized removal of portable media and proper disposal of legacy disk drives and discarded computers is another key to preventing theft and possibly compromise of plaintext data that may contain sensitive or privileged information.

### Data Encryption and Decryption - Windows applications | Microsoft Docs

A continual problem is the storage of classified plaintext information on laptop computers in the United States and British governments. All major agencies have had more than one instance of laptops that contain classified information stolen from employees. Unfortunately, in most cases proper laptop disk encryption procedures were either not employed or not yet improved before the thefts occurred.

As a result, potentially sensitive information in plaintext form was exposed to unauthorized personnel. Even in cases where the information stored on a host computer is encrypted, portable media that is used to transfer data between system is not encrypted and represents another threat to compromise of sensitive plaintext data. What are the Types of Ciphers? Today, historical ciphers are not normally used for standalone encryption since they can be cracked easily through modern brute force attacks or by analyzing simply the ciphertext.

Modern-day methods have improved to the point that all methods but the one-time pad can be cracked with only access to the ciphertext. Substitution cipher In the substitution cipher, plaintext characters or units are replaced directly with cipher text. Famous instances of this cipher include the One-time pad and the Caesar cipher. Transposition Cipher In the transposition cipher, the resulting cipher text is a permutation of the plaintext input.

A famous example of this type of cipher is the Rail fence cipher. Polyalphabetic substitution Cipher In the polyalphabetic substitution cipher, multiple substitution alphabets are used to create ciphertext from the plaintext input. Permutation cipher The permutation cipher is a transposition cipher where the key to the ciphertext is a permutation.

How Do Modern Ciphers Work? Current-day ciphers are significantly more secure than the classic cipher techniques. They are created to be able to withstand a variety of attacks on the ciphertext. Attackers are not able to determine what the key s used in modern ciphers are even with access to a significant amount of plaintext and corresponding ciphertext. The modern day cipher techniques include: Private-key Cryptography In private key cryptography which uses a symmetric key algorithm, the same key is used to both encrypt and decrypt information.

Public-key Cryptography In public key cryptography that uses an asymmetric key, there are two key used for encryption and decryption. For ciphers that leverage symmetric key algorithms including AES and DESboth the sending and receiving computer and host has to have a shared key setup in advance. The key must be kept secret from all other users in order for the communication sessions to remain private.

The sending computer will use the secret key for encryption and the receiving computer uses the same key to decrypt the ciphertext to plaintext. For asymmetric key ciphers such as RSAthere are two separate keys used for operations. A public key is promulgated that allows any sending computer to encrypt data. The private key is only held by the receiving party and allows only that person, computer, or authorized user to decrypt the information. For symmetric key ciphers, the two variants in use today are stream and block ciphers.

Stream ciphers encrypt information or plaintext data one unit or character at a time while processing a continual stream of data.

**What Is Meant By Plain Text And Cipher Text?**

The transformation of the information will vary during the encryption process based on the specific encryption process being employed. For block ciphers, fixed-length groupings of data called blocks are processed using a transformation calculation. Cryptanalysis Attack Models There are a number of cryptanalysis attack models in use today for attempting to crack or break cipher texts.

Some of the most common include: Continue with the next letter of the plaintext, and the next letter of the key. When arrived at the end of the key, go back to the first letter of the key. NGMNI is the ciphertext. Locate the first letter of the plaintext message in the first line of the table and the first letter of the key on the left column.

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- Vigenere Encoder
- What is Plaintext?

The cipher letter is at the intersection. Locate the letter D on the first row, and the letter K on the first column, the ciphered letter is the intersection cell N. How to decrypt Vigenere cipher?

Decryption requires a key and an alphabet. As for encryption, two ways are possible. Decryption of Vigenere by subtracting letters Example: To decrypt, take the first letter of the ciphertext and the first letter of the key, and subtract their value letters have a value equals to their position in the alphabet starting from 0. Continue with the next letters of the message and the next letters of the key, when arrived at the end of the key, go back the the first key of the key.

## Vigenere Cipher

DCODE is the plain text. Locates the first letter of the key in the left column, and locates on the row the first letter of the ciphered message. Then go up in the column to read the first letter, it is the corresponding plain letter.

Locate the letter K on the first column, and on the row of it, find the cell of the letter N, the name of its column is D, it is the first letter of the plain message. How to recognize Vigenere ciphertext? Following a Vigenere encryption, the message has a coincidence index which decreases between 0. How to decipher Vigenere without knowing the key? Most common techniques uses statistical methods in order to find the key length, then a simple frequency analysis allow to find the key.

Kasiski test Kasiski test consists in finding repeating sequences of letters in the ciphertext.

## Data Encryption and Decryption

By analyzing the gaps between two identical redunding sequences, an attacker can find multiples of the key length. By analyzing each gaps in term of number of letters, and by calculating divisorsan attacker can deduct with a high probability the size of the key. Positions of ABC are 0, 6 et 18, gaps are 6, 12 and 18 letters length, their most common divisors are 2, 3 and 6, so the key has an high probability to be 2, 3 or 6 letters long.

Index of coincidence test The test using the index of coincidence consists in taking one letter out of n in the ciphertext and calculate the IC. The higher it is, the higher the probability n is the key size. Indeed, taking one letter every n where n is the key-length, ends with a sequence of letters that are always crypted using the same shift.

The index of coincidence is then equals to the one of the plain text. How to find the key when having both cipher and plaintext?