>>718
訂正。
#define NUM_BYTES 123
unsigned char a[NUM_BYTES];
...
unsigned char prev4bits = (a[NUM_BYTES - 1] >> 4);
for (int i = 0; i < NUM_BYTES; i++) {
unsigned char b = (a[i] >> 4);
a[i] <<= 4;
a[i] |= prev4bits;
prev4bits = b;
}