# 1. A chosen plaintext attack cannot be used to break a one-time-pad because of why? 2. If it takes one day to break a 32-bit symmetric cipher key by trying all possible decryption cipher keys, how...

1. A chosen plaintext attack cannot be used to break a one-time-pad because of why?

2. If it takes one day to break a 32-bit symmetric cipher key by trying all possible decryption cipher keys, how long will it take to break a 128-bit cipher key?

3. What data can be hashed using SHA1 that result in a string in it of the first three letters of your name?

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### 1 Answer

**Plaintext** is the data text that the communicator wishes to hide through the process of encryption, which is the process of applying cipher keys to text to disguise important or secret information. Before the age of the computer, plaintext was written text. After the dawn of the computer age, the concept of plaintext was expanded to encompass *electronic representations of written text* and *electronic representations of speech* (e.g., music, images, ATM transactions, credit card information). **Encryption** is the method that allows information to be hidden so that it cannot be accessed without special prior knowledge such as a secret cipher key (secret key) or password.

A **chosen plaintext attack (CPA)** is a hostile action that attempts to gain encrypted information by reducing the security or by calculating the secret key. CPA is a cryptoanalysis model that assume the attacker can choose random plaintexts to be encrypted and obtain corresponding ciphertexts.

A **one-time-pad (OTP)** is a symmetric cipher that is completely random. As long as the cipher key to the encrypted plaintext is kept secret and used only once, the encryptedÂ ciphertext cannot be broken **because** the key is random and secret and corresponds character for character to the plaintext (OTP cipher keys were originally transferred to the decrypter on a paper pad, sometime minutely small, thus were referred to by the metonymy "pad").

A 128-bit symmetric cipher key is the minimum length recommended for symmetric encryption. A 32-bit key is so short that it can be broken by brute force. Symmetric cipher keys use the same cipher key to encrypt the data and to later decrypt the data. A **cryptographic** **cipher key** is a piece of information that allows control over the encryption (hiding) or decryption (exposing) processes. The two kinds of cryptographic cipher key are symmetric and asymmetric. **Symmetric**: uses one key for encrypting and decrypting a symmetric cryptographic algorithm. **Asymmetric**: uses two different keys--each of which can be used only to encrypt data or only to decrypt it--one for encrypting and a different one for decrypting an asymmetric cryptographic algorithm.

A **hash function** is an algorithm that, when applied to equal value data, results in the same shortened encryption each time; it maps data of arbitrary length to data of a fixed length. "Hashing" is the process of applying a hash function to data. "Hashed" refers to data to which a hash function has previously been applied. To hash with the SHA1 means to apply to data the cryptographic hash function that was developed by the NSA. The SHA1 hash function result is usually expressed as a 160-bit hex number.

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