1. 문제Permalink

[Bronze V] Double Crypt 1 - 24218Permalink

문제 링크

성능 요약Permalink

메모리: 30860 KB, 시간: 64 ms

분류Permalink

구현(implementation)

문제 설명Permalink

The Advanced Encryption Standard (AES) involves a new strong encryption algorithm. It works with three blocks of $128$ bits. Given a message block $p$ (plaintext) and a key block $k$, the AES encryption function $E$ returns an encrypted block $c$ (ciphertext):

 $c=E(p,k)$.

The inverse of the AES encryption function $E$ is the decryption function $D$ such that

 $D(E(p,k),k)=p$, $E(D(c,k),k)=c$.

In Double AES, two independent key blocks $k_1$ and $k_2$ are used in succession, first $k_1$, then $k_2$:

 $c_2=E(E(p,k_1),k_2)$.

In this task, an integer $s$ is also given. Only the leftmost $4\times s$ bits of all keys are relevant, while the other bits (the rightmost $128$ minus $4\times s$ bits) are all zero.

You are to recover the encryption key pairs for some messages encrypted by Double AES. You are given both the plaintext $p$ and the corresponding double-encrypted ciphertext $c_2$, and the structure of the encryption keys as expressed by the integer $s$.

You must submit the recovered keys, and not a recovery program.

입력Permalink

You are given ten problem instances in the text files named double1.in to double10.in. Each input file consists of three lines. The first line contains the integer $s$, the second line the plaintext block $p$, and the third line the ciphertext block $c_2$ obtained from $p$ by Double AES encryption. Both blocks are written as strings of 32 hexadecimal digits ('0'..'9', 'A'..'F'). The library provides a routine to convert strings to blocks. All input files are solvable.

출력Permalink

The first line contains the key block $k_1$, and the second line the key block $k_2$, such that

 $c_2=E(E(p,k_1),k_2)$.

Both blocks must be written as strings of 32 hexadecimal digits ('0'..'9', 'A'..'F'). If there are multiple solutions, you need submit only one of them.

출처: 백준, https://https://www.acmicpc.net/

2. 해결방법 시간복잡도Permalink

1. 단순 코딩 O(1)

3. 문제 해결 및 코드Permalink

• 주석을 참고하면서 이해를 돕습니다.Permalink

4. 알고리즘 및 해설Permalink

1. 문제의 핵심은 해당 32자리수를 출력하는 것이다.
2. 그러므로 해당 수를 출력해준다.