字节数组的快速移位-CMAC子项 [英] Fast bit shift of a byte array - CMAC subkeys

问题描述

我需要在 JavaCard 中尽快实现16字节数组的左移.

I need to implement as fast as possible left bit shift of a 16-byte array in JavaCard.

我尝试了以下代码:

private static final void rotateLeft(final byte[] output, final byte[] input) {
         short carry = 0;
         short i = (short) 16;
         do {
             --i;
             carry = (short)((input[i] << 1) | carry);
             output[i] = (byte)carry;
             carry = (short)((carry >> 8) & 1);
         } while (i > 0);
}

任何想法如何提高性能?我当时在考虑一些Util.getShort(...)和Util.setShort(...)魔术，但是我没有设法使其比上面的实现更快地工作.

Any ideas how to improve the performace? I was thinking about some Util.getShort(...) and Util.setShort(...) magic, but I did not manage to make it work faster then the implementation above.

这是CMAC子键计算的一部分，不幸的是，它经常执行.如果您知道一些更快的方法来计算CMAC子项(一个循环中的两个子项或类似的东西)，请告诉我.

This is one part of CMAC subkeys computation and it is done quite often, unfortunately. In case you know some faster way to compute CMAC subkeys (both subkeys in one loop or something like that), please, let me know.

推荐答案

当涉及到速度，已知长度时，硬编码版本是最快的(但很丑陋).如果需要移位多个位，请确保相应地更新代码.

When it comes for speed, known length, hard-coded version is the fastest (but ugly). If you need to shift more than one bit, ensure to update the code accordingly.

output[0] = (byte)((byte)(input[0] << 1) | (byte)((input[1] >> 7) & 1));
output[1] = (byte)((byte)(input[1] << 1) | (byte)((input[2] >> 7) & 1));
output[2] = (byte)((byte)(input[2] << 1) | (byte)((input[3] >> 7) & 1));
output[3] = (byte)((byte)(input[3] << 1) | (byte)((input[4] >> 7) & 1));
output[4] = (byte)((byte)(input[4] << 1) | (byte)((input[5] >> 7) & 1));
output[5] = (byte)((byte)(input[5] << 1) | (byte)((input[6] >> 7) & 1));
output[6] = (byte)((byte)(input[6] << 1) | (byte)((input[7] >> 7) & 1));
output[7] = (byte)((byte)(input[7] << 1) | (byte)((input[8] >> 7) & 1));
output[8] = (byte)((byte)(input[8] << 1) | (byte)((input[9] >> 7) & 1));
output[9] = (byte)((byte)(input[9] << 1) | (byte)((input[10] >> 7) & 1));
output[10] = (byte)((byte)(input[10] << 1) | (byte)((input[11] >> 7) & 1));
output[11] = (byte)((byte)(input[11] << 1) | (byte)((input[12] >> 7) & 1));
output[12] = (byte)((byte)(input[12] << 1) | (byte)((input[13] >> 7) & 1));
output[13] = (byte)((byte)(input[13] << 1) | (byte)((input[14] >> 7) & 1));
output[14] = (byte)((byte)(input[14] << 1) | (byte)((input[15] >> 7) & 1));
output[15] = (byte)(input[15] << 1);

并使用RAM字节数组！

And use RAM byte array!

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