How to shift bits across array of ints
Alright, I'll keep this plain & simple.
I'm working on a brute implementation of SHA-256 in Java, and to keep the class methods conducive to other classes in the app, I've decided to I/O the data as an array of 32-bit integers (int). Obviously, these kinds of uses rely heavily on use of bitwise operators, so how do I accomplish bitwise shifting across an array of ints, as such:
Given an array of data, suppose:
int[16] data;
where the first 14 integers' worth of data is filled with 0x0000000, for the purposes of conversation, but the lowest two are filled respectively:
data[14] = 0x12345678;
data[15] = 0x01ABCDEF;
How do I accomplish a leftwise shift so that if shifted left by two,
data[13] = 0x00000012;
data[14] = 0x34567801;
data[15] = 0xABCDEF00;
Anyone got any ideas?
1) Write a helper method to convert your int[16] data to byte[64] bytedata .
2) Create a BigInteger using the constructor BigInteger(byte[] val)
3) Bitshift using shiftLeft(int n) and shiftRight(int n)
4) Write a helper method to convert your byte[64] bytedata back to int[16] data . See byte array to Int Array here
Other useful methods in BigInteger include clearBit(int n) , setBit(int n) , flipBit(int n) , and testBit(int n)
For bitwise shift left:
`x << n` is the same as `x *= 2^n`
To see the overflowing bit (MSB): x / (2^32) >= 1 //Since we are using an 32 bit int
Likewise, for bitwise shift right:
`x >> n` is the same as `x /= 2^n`
To see the overflowing bit (LSB): x % 2
Append the overflowing bit to the adjacent int by adding or subtracting set values. You should really use a BigInteger like I mentioned in my comment, anything else will require an implementation where you read and append the bits moving between array locations yourself.
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