ccna after feb 2020

ccna after feb 2020.7.1 Cryptographic Services 191 successful. The goal of modern cryptographers is to make the key space large enough to make the time and money required to complete a brute-force attack very expensive. A few years ago, a DES cracking machine was used to find out a 56-bit DES key in 22 hours using a brute-force attack. It is estimated that it would take 149 trillion years to break the Advanced Encryption Standard (AES) using the same method on the same machine.Ciphertext-OnlyAttack )In a ciphertext-only attack, the attacker has the ciphertext of multiple messages that are all encrypted using the same encryption algorithm, but the attacker does not know what the plaintext behind the ciphertext is. The attacker's job is to recover the ciphertext of as many messages as possible. Even better for the attacker is to infer the key or keys used to encrypt the messages in order to decrypt other messages encrypted using the same key. An attacker can use statistical analysis to infer the key. However, this type of attack is no longer practical because modern algorithms generate pseudo-random output that cannot be handled by statistical analysis.Known-Plaintext Attack In a known-plaintext attack, the attacker knows the ciphertext of multiple messages and also knows the plaintext behind some of the ciphertexts. Knowing the underlying protocol, the file type, or some characteristic string displayed in these plaintexts, the attacker uses a brute force attack to try each key until a meaningful result is generated by decrypting it with the correct key. This attack is probably the most practical attack because if an attacker can only capture the ciphertext, they can usually assume some features of its plaintext. Modern algorithms that use huge key spaces make this type of attack unlikely to succeed because, on average, an attacker must search at least half of the key space to succeed.Chosen-Plaintext Attack )In a Chosen-Plaintext Attack, the attacker selects some data to be encrypted by an encryption device and observes the output ciphertext. Chosen-plaintext attacks are more powerful than known-plaintext attacks because the selected plaintext may yield more information about the key. This attack is not very practical because unless the trusted network has been breached and the attacker has access to confidential information, it will be difficult or impossible to capture both ciphertext and plaintext.Chosen-Ciphertext Attack )In a Chosen-Ciphertext Attack, an attacker can choose a different ciphertext to decrypt, and can access the decrypted plaintext. With the ciphertext and plaintext pairs, the attacker can search the key space to determine which key is used to decrypt the selected ciphertext in the captured plaintext. For example, an attacker is able to access a tamper-resistant encryption device with an embedded key. The attacker must infer the embedded key by sending data to the device. This attack is similar to the selected plaintext attack, but like the selected plaintext attack, it is not very practical. Unless the trusted network has been breached and the attacker has access to confidential information, it will be difficult or impossible to capture both the ciphertext and the plaintext.Meet-in-the-MiddleA meet-in-the-middle attack is a known plaintext attack. The attacker knows part of the plaintext and the corresponding ciphertext. The plaintext is encrypted using every possible key and the result is saved. The ciphertext is then decrypted with each key until one of the results matches one of the saved values.