feat: finish 20

problems/p20/20.txt

@@ -0,0 +1,60 @@
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problems/p20/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "p20"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+base64 = { workspace = true }

problems/p20/src/main.rs

@@ -0,0 +1,60 @@
+use base64::{Engine as _, engine::general_purpose::STANDARD};
+use std::fs::File;
+use std::io::{BufRead, BufReader};
+
+fn read_and_decode() -> Vec<Vec<u8>> {
+    let file = File::open("problems/p20/20.txt").expect("Failed to open file");
+    let reader = BufReader::new(file);
+    reader
+        .lines()
+        .map(|line| line.expect("Failed to read line"))
+        .filter(|line| !line.trim().is_empty())
+        .map(|line| {
+            STANDARD
+                .decode(line.trim())
+                .expect("Failed to decode base64")
+        })
+        .collect()
+}
+
+fn is_printable(byte: u8) -> bool {
+    byte.is_ascii_graphic() || byte == b' '
+}
+
+fn automated_attack(ciphers: &[Vec<u8>]) -> Vec<u8> {
+    let min_len = ciphers.iter().map(|c| c.len()).min().unwrap_or(0);
+    let mut key_stream = vec![0u8; min_len];
+
+    for pos in 0..min_len {
+        for key_byte in 0u8..=255u8 {
+            let all_printable = ciphers
+                .iter()
+                .all(|cipher| pos < cipher.len() && is_printable(cipher[pos] ^ key_byte));
+
+            if all_printable {
+                key_stream[pos] = key_byte;
+                break;
+            }
+        }
+    }
+
+    // 输出解密结果
+    println!("Decrypted plaintexts:");
+    for (i, cipher) in ciphers.iter().enumerate() {
+        let plaintext: Vec<u8> = cipher
+            .iter()
+            .zip(key_stream.iter())
+            .map(|(&c, &k)| c ^ k)
+            .collect();
+        let plaintext_str = String::from_utf8_lossy(&plaintext[..min_len.min(cipher.len())]);
+        println!("{}: {}", i, plaintext_str);
+    }
+
+    key_stream
+}
+
+fn main() {
+    let ciphers = read_and_decode();
+    let _key_stream = automated_attack(&ciphers);
+}
+