java - 在 Java 中创建 'mpint' 值(安全外壳 (SSH) 协议(protocol)架构 - RFC 4251)

Question

我正在尝试使用 RFC4251 中指定的 BigInteger 创建 mpint 字符串:

mpint

Represents multiple precision integers in two's complement format, stored as a string, 8 bits per byte, MSB first. Negative numbers have the value 1 as the most significant bit of the first byte of the data partition. If the most significant bit would be set for a positive number, the number MUST be preceded by a zero byte. Unnecessary leading bytes with the value 0 or 255 MUST NOT be included. The value zero MUST be stored as a string with zero bytes of data.

By convention, a number that is used in modular computations in Z_n SHOULD be represented in the range 0 <= x < n.

Examples:

     value (hex)        representation (hex)
     -----------        --------------------
     0                  00 00 00 00
     9a378f9b2e332a7    00 00 00 08 09 a3 78 f9 b2 e3 32 a7
     80                 00 00 00 02 00 80
     -1234              00 00 00 02 ed cc
     -deadbeef          00 00 00 05 ff 21 52 41 11

一切都差不多清楚了，但如何解释“不得包含值为 0 或 255 的不必要的前导字节。”？

第二个问题是关于这一行的:“按照惯例，Z_n 中模计算中使用的数字应该在 0 <= x < n 范围内表示。”。怎么解释呢？

编辑:

我的第一个建议是:

/**
 * Write 'mpint' to output stream including length.
 *
 * @param dos output stream
 * @param bi the value to be written
 */
public static void writeMPInt(DataOutputStream dos, BigInteger bi) throws IOException {

    byte[] twos = bi.toByteArray();

    dos.writeInt(twos.length);
    dos.write(twos);
}

根据上述规则，该方法是否有效？

Answer 1

Unnecessary leading bytes with the value 0 or 255 MUST NOT be included.

请勿在数字前面添加额外的 00或ff字节。

1. 80存储为00 00 00 03 00 00 80有一个额外的前导 00字节。

2. -deadbeef存储为00 00 00 06 ff ff 21 52 41 11有一个额外的前导 ff字节。

在这两种情况下，数字在技术上都是正确的，但具有不必要的前导字节。

<小时/>

By convention, a number that is used in modular computations in Z_n SHOULD be represented in the range 0 <= x < n.

Z_n 是书写 integers modulo n 的粗体数学符号的 ASCII 方式。 。 (see Modular arithmetic)

这意味着，您不应该存储号码 x大于模数n待使用，或小于零。

1. 您希望存储号码 123。

2. 您知道该数字肯定会以 100 为模。即123 % 100 .

3. 您应该存储 23。

<小时/>

Does this method is valid according mentioned above rules?

不，write()不检查字节数组中的值是否符合上述规则。