semantic security
Semantic security refers to the property of a encryption scheme in which the meaning of the ciphertext is indistinguishable from random noise, even if the encryption key is known. The security of a semantic security scheme is based on the computational difficulty of distinguishing the ciphertext from random data. This property makes it difficult for an attacker who has obtained the encryption key to determine the meaning of the ciphertext without additional information.
Ciphertext is the output of an encryption algorithm that transforms plaintext into an unreadable form. Ciphertext is typically created by applying a mathematical function (such as a substitution or permutation) to the plaintext, using a secret key. The process of converting ciphertext back into plaintext is known as decryption, which typically involves applying the inverse of the encryption function to the ciphertext, using the same or a related secret key.
Semantic security types
here are two types of semantic security:
Indistinguishability under chosen plaintext attack (IND-CPA): This type of semantic security guarantees that it is computationally infeasible for an attacker to determine the encryption key or distinguish the ciphertext from random noise, even if they are able to choose the plaintext that is used as input to the encryption algorithm.
Indistinguishability under chosen ciphertext attack (IND-CCA): This type of semantic security guarantees that it is computationally infeasible for an attacker to determine the encryption key or recover the plaintext, even if they are able to choose the ciphertext that is used as input to the decryption algorithm.
It is worth mentioning that IND-CCA2 is a stronger notion of semantic security. It is required for many modern cryptographic protocols
Here's an example:
Input | 'sec': | 01110011 01100101 01100011 |
Key | ⊕ | 11001100 01110001 00011000 |
Result | 10111111 00010100 01111011 |
Example 2
KEY 11100000 01100001 01101101
Result 10101101 00011000 01001101
The XOR cipher is a simple encryption algorithm that operates on a bit-by-bit basis. It takes two inputs, the plaintext and a key, and produces the ciphertext by performing an exclusive-or (XOR) operation between each bit of the plaintext and the corresponding bit of the key.
For example, let's say the plaintext is "HELLO" and the key is "WORLD". To encrypt the plaintext using the XOR cipher, we first convert the plaintext and the key to their binary representations:
Plaintext: "HELLO" (01001000 01000101 01001100 01001100 01001111) Key: "WORLD" (01110111 01101111 01110010 01101100 01100100)
We then perform the XOR operation on each corresponding bit of the plaintext and the key to produce the ciphertext:
Ciphertext: (00001111 00101010 00111011 00111011 00110000)
Note that to decrypt the ciphertext, we simply need to perform the XOR operation again, using the same key.
It's worth noting that XOR cipher, like the one-time pad, is considered semantically secure, but it has its own weaknesses. If the same key is used more than once, the pattern in the plaintext can be observed by attacker and thus, the key can be found out. Additionally, the size of the key must be the same as the plaintext, otherwise, you will get a truncated or padded result.
Ciphertext example
An example of a simple ciphertext is the Caesar Cipher. It's a mono-alphabetic substitution cipher where each letter of a given text is replaced by a letter with a fixed number of positions down the alphabet.
For example, if the key is 3 then A will be replaced by D, B will be replaced by E, and so on. Therefore, the plaintext "HELLO WORLD" will be encrypted as "KHOOR ZRUOG" if the key is 3.
This is the encryption process for Caesar Cipher:
Plaintext: HELLO WORLD
Key: 3
Ciphertext: KHOOR ZRUOG
It's worth mentioning that this encryption method is relatively easy to break as frequency analysis can be used to determine the key. However, this is just an example of a simple ciphertext, There are many other advanced encryption algorithms that are more secure and suitable for real-world use cases.
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