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CTF笔记 01 - Base64编码隐写术
Tsccai Lv2
  2025-02-15 21:26:55   2025-02-16 00:07:52

看《CTF那些事》，17页的一个例题，记录一下。这是一道杂项题，主要考察Base64编码的隐写术。
由于Base64编码是每6个bit编码为一个可打印字符，与ASCII码编码不同。当且仅当待编码的ASCII字符占用字节为3的倍数时，Base64编码不会产生填充字符=。而当Base64编码后的字符末尾有1个=时，该行最后一个非=字符可产生2bit的隐写空间；当编码后的字符末尾有2个=时，则可产生4bit的隐写空间。那么，若使用Base64隐写术来藏flag，则需要大量的Base64编码文本，这是这类题的一个特征。
Base64的解码并不关心末尾的填充bit位的具体取值，它只从高位开始读取2或者4bit，并入前面的6或者4bit，组成一个8bit字节再进行解码，这才给隐写留下了操作空间。

原始文本 ASCII码（二进制） Base64编码 Base64编码（二进制） Base64解码后的文本
f 01100110 Zg== 011001 10 0000 f
f 01100110 Zv== 011001 10 1111 f

以下是这个例题所使用的编码后的文本，共109行。

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// base64.txt
U3RlZ2Fub2dyYXBoeSBpcyB0aGUgYXJ0IGFuZCBzY2llbmNlIG9m
IHdyaXRpbmcgaGlkZGVuIG1lc3NhZ2VzIGluIHN1Y2ggYSB3YXkgdGhhdCBubyBvbmV=
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YWdlLCBhIGZvcm0gb2Ygc2VjdXJpdHkgdGhyb3VnaCBvYnNjdXJpdHkuIFS=
aGUgd29yZCBzdGVnYW5vZ3JhcGh5IGlzIG9mIEdyZWVrIG9yaWdpbiBhbmQgbWVhbnMgImNvbmNlYW==
bGVkIHdyaXRpbmciIGZyb20gdGhlIEdyZWVrIHdvcmRzIHN0ZWdhbm9zIG1lYW5pbmcgImNv
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需要指出的是，Base64编码隐写术只能在末尾有填充字符=的地方进行，但并不意味着但凡出现=的地方都有隐藏的信息，虽然本题确实将所有能利用的隐写空间都利用起来了。若命题者足够变态，甚至可以在含=的行中隔行进行隐写。
以下为提取隐写内容的代码（NodeJS）。

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// base64_steg.js
const fs = require('fs');

// 标准 Base64 字符集
const base64Chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';

// 构建Base64字符集与其编码值的映射Map
const base64CharToCodeMap = {};
for (let i = 0; i < base64Chars.length; i++) {
    const char = base64Chars[i];
    base64CharToCodeMap[char] = i;
}

let buffer = '';
let hidden_msg = '';
//  从命令行参数中读取编码后的文本文件，并按行拆分。
const lines = fs.readFileSync(process.argv[2], { encoding: 'utf-8' }).split('\r\n');
for (let origin of lines) {
    if (!origin.endsWith('=')) {
        // 略去末尾不是'='的行
        continue;
    }
    let letter = '';
    // 注意理解下面的代码：
    // 首先将该行末尾的非'='字符取出，从编码映射表中获取到字符的码值，并取低4位或低2位（依填充的=数量而定）
    // 将取出的二进制位先转为二进制表示的字符串，再按其隐写空间在高位补0，补足2位或4位，以字符串形式存入buffer
    if (origin.endsWith('==')) {
        letter = origin.substring(origin.indexOf('==') - 1)[0];
        buffer += (base64CharToCodeMap[letter] & 0b1111).toString(2).padStart(4, '0');
    }
    else {
        letter = origin.substring(origin.indexOf('=') - 1)[0];
        buffer += (base64CharToCodeMap[letter] & 0b11).toString(2).padStart(2, '0');
    }

    // 当buffer中二进制字符数量达到或超过8时，取出buffer中前8个二进制字符，并转换为ASCII字符
    if (buffer.length >= 8) {
        let char = buffer.substring(0, 8);
        hidden_msg += String.fromCharCode(parseInt(char, 2));
        // 剩余的字符仍留在buffer中，与后续字符使用。Base64编码中，此处buffer中剩余的字符数只可能是0或者2
        buffer = buffer.slice(8);
    }
}
// 最终输出隐写的信息和buffer中剩余的字符（通常buffer中的剩余字符并无意义）
console.log(`hidden message: "${hidden_msg}", buffer remain: ${buffer}`);
CTF笔记 01 - Base64编码隐写术
2025/02/15/ctf-notes-01-base64-steganography/
作者
Tsccai
发布于
2025-02-15 21:26
许可
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