Write Code in C To Rearrange the Order of Bits Assignment Solution

Instructions

Objective

Write a program to rearrange the order of bits in C language.

Requirements and Specifications

Objective

Practice in bit manipulation of integers in the C programming language.

Submitting Your Work

Complete your C assignment program file as bits.c and submit it and your report via the Lab Exercise 3 Submission item on the course web site. You must submit your program by 11:59 PM two days after your scheduled lab session.

Introduction

Your task for this lab exercise is to complete several function definitions which solve puzzles, using only a limited set of operators in the C language. In addition, you must write a main() function which calls each of your solution functions to demonstrate that each function performs as specified.

Screenshots of output

Source Code

#include /* Function prototypes */ int func1(int x, int y); int func2(int x, int y); int func3(int x); int func4(int x); int func5(int highbit, int lowbit); int func6(int x, int m, int n); int main() { printf("Testing func1 function:\n"); printf("func1(5,6) = 0x%X, (expected = 0x%X)\n", func1(5,6), 5 & 6); printf("func1(0xFF,0x1234) = 0x%X, (expected = 0x%X)\n", func1(0xFF, 0x1234), 0xFF & 0x1234); printf("\nTesting func2 function:\n"); printf("func2(4,5) = 0x%X, (expected = 0x%X)\n", func2(4,5), 4 ^ 5); printf("func2(5,6) = 0x%X, (expected = 0x%X)\n", func2(5,6), 5 ^ 6); printf("func2(0xFF,0x1234) = 0x%X, (expected = 0x%X)\n", func2(0xFF, 0x1234), 0xFF ^ 0x1234); printf("\nTesting func3 function:\n"); printf("func3(5) = 0x%X, (expected = 0x%X)\n", func3(5), 0); printf("func3(7) = 0x%X, (expected = 0x%X)\n", func3(7), 1); printf("func3(0x100) = 0x%X, (expected = 0x%X)\n", func3(0x100), 0); printf("func3(0x255) = 0x%X, (expected = 0x%X)\n", func3(0x255), 1); printf("\nTesting func4 function:\n"); printf("func4(0xFFFFFFFD) = 0x%X, (expected = 0x%X)\n", func4(0xFFFFFFFD), 0); printf("func4(0xAAAAAAAA) = 0x%X, (expected = 0x%X)\n", func4(0xAAAAAAAA), 1); printf("func4(0xABEABEAB) = 0x%X, (expected = 0x%X)\n", func4(0xABEABEAB), 1); printf("func4(0xFFFF) = 0x%X, (expected = 0x%X)\n", func4(0xFFFF), 0); printf("\nTesting func5 function:\n"); printf("func5(5,3) = 0x%X, (expected = 0x%X)\n", func5(5,3), 0x38); printf("func5(31,16) = 0x%X, (expected = 0x%X)\n", func5(31,16), 0xFFFF0000); printf("func5(1,2) = 0x%X, (expected = 0x%X)\n", func5(1,2), 0); printf("\nTesting func6 function:\n"); printf("func6(0x12345678,1,3) = 0x%X, (expected = 0x%X)\n", func6(0x12345678,1,3), 0x56341278); printf("func6(0xDEADBEEF,0,2) = 0x%X, (expected = 0x%X)\n", func6(0xDEADBEEF,0,2), 0xDEEFBEAD); printf("\n"); return 0; } /* * func1 returns x & y */ int func1(int x, int y) { /* We use the DeMorgan's theorem to remove the and: x*y = ~(~x + ~y) */ return ~((~x) | (~y)); } /* * func2 returns x ^ y */ int func2(int x, int y) { /* We use the boolean expression for xor = ~x*y + x*~y and we remove the or using the DeMorgan's theorem x+y = ~(~x*~y) */ return ~(~(~x & y) & ~(x & ~y)); } /* * func3 returns 1 if an odd numbered bit in x is 1 */ int func3(int x) { /* Tests the odd bits in each of the 4 bytes of the 32bit int, test the result logically using double ! (any non zero becomes 1) then ors all of the results if any is 1 returns 1 */ int b1 = !!(x & 0xAA); /* test bits in lowest byte */ int b2 = !!((x >> 8) & 0xAA); /* test bits in second byte */ int b3 = !!((x >> 16) & 0xAA); /* test bits in third byte */ int b4 = !!((x >> 24) & 0xAA); /* test bits in highest byte */ return b1 | b2 | b3 | b4; /* if any byte has an odd 1, return 1 */ } /* * func4 returns 1 if all the odd numbered bits in x is 1 */ int func4(int x) { /* Tests the odd bits in each of the 4 bytes of the 32bit int, then ands all of them if all bits are set the result must be 0xAA so it's xored with 0xAA and if zero returns 1 by making a logical inversion with ! */ int b1 = (x & 0xAA); /* test bits in lowest byte */ int b2 = ((x >> 8) & 0xAA); /* test bits in second byte */ int b3 = ((x >> 16) & 0xAA); /* test bits in third byte */ int b4 = ((x >> 24) & 0xAA); /* test bits in highest byte */ return !((b1 & b2 & b3 & b4) ^ 0xAA); /* if all bytes have odd ones, return 1 */ } /* * func5 returns a mask with ones between highbit and lowbit */ int func5(int highbit, int lowbit) { /* Makes a bitmask from the high bit down, then makes a mask from the low bit up and returns the and of both */ int maskhi = ((1 << highbit) + (~0)) | (1 << highbit); /* mask up to high bit */ int masklow = (~0) << lowbit; /* mask up to lowbit */ return maskhi & masklow; /* make mask combining both */ } /* Swaps the mth byte and the nth byte of an integer */ int func6(int x, int m, int n) { /* saves the initial bytes shifting them and masking them, then clears the old positions and ors the swapped values */ int byte1 = (x >> (m << 3)) & 0xFF; /* move byte at position m*8 down, mask 8 bits */ int byte2 = (x >> (n << 3)) & 0xFF;/* move byte at position n*8 down, mask 8 bits */ int mask1 = ~(0xFF << (m << 3)); /* move 0xFF up at position m*8, invert to get zeros there */ int mask2 = ~(0xFF << (n << 3)); /* move 0xFF up at position n*8, invert to get zeros there */ x = (x & mask1) | (byte2 << (m<<3)); /* replace first value */ x = (x & mask2) | (byte1 << (n<<3)); /* replace second value */ return x; }

DECRYPT PROGRAM

#include /* * Function prototypes */ void get_keys (char *twokeys, unsigned *key1, unsigned *key2); unsigned int get_n_bits (unsigned bits, int width, int index); unsigned int rotate_left3(unsigned bits); void shuffle_nibbles (unsigned *bits); void decode_28bits (unsigned cipher, char *plain, unsigned key1, unsigned key2); /* * Main function */ int main(int argc, char **argv) { char line[10], plaintext[10]; unsigned int bits; unsigned key1, key2; if (argc != 2) /* if there was no command line argument */ { printf("Missing key argument!\n"); printf("Usage:\n %s key\n", argv[0]); return 0; } get_keys (argv[1], &key1, &key2); /* get the 2 keys from the cmd line argument */ /* Get the next 7 characters of input (leaving space for the null at the end) */ while (fgets(line, 8, stdin) != NULL) { if (line[0] != '\n') /* ignore empty lines */ { /* convert to a number */ if (sscanf(line, "%x", &bits) == 1) /* read hexadecimal ciphertext */ { decode_28bits (bits, plaintext, key1, key2); /* decode using keys */ printf("%s", plaintext); /* print result */ } } else printf("\n"); } printf("\n"); return 0; } /* * Extracts two 28=bit keys from 4 bytes */ void get_keys (char *twokeys, unsigned *key1, unsigned *key2) { int i; *key1 = 0; /* initialize keys to zero */ *key2 = 0; for (i = 0; i < 4; i++) /* for the 4 characters used in each key */ { *key1 = (*key1 << 7) | (twokeys[i] & 0x7F); /* add 7 bits of each character of first key */ *key2 = (*key2 << 7) | (twokeys[i + 4] & 0x7F); /* add 7 bits of each character of second key */ } } /* * Returns the integer value of a specified subsequence of 32 bits, * width is the number of bits to be extracted, index*width is the index * of the rightmost bit to extract */ unsigned int get_n_bits (unsigned bits, int width, int index) { return (bits >> (index * width)) & ((1 << width) - 1); /* shift to put selected bits at lowest position, mask using width */ } /* * Given a sequence of 7 bits, rotate 3 bits to the left */ unsigned int rotate_left3(unsigned bits) { int i, msb; for (i = 0; i < 3; i++) { msb = (bits >> 6) & 1; /* get msb bit */ bits = (bits << 1) | msb; /* shift and put msb at lsb to rotate */ } return bits & 0x7F; /* mask to leave only 7 bits */ } /* shuffle nibbles in the 28 bit value given as argument, the shuffle inverts: 3 1 2 0 4 5 6 -> 6 5 4 3 2 1 0 */ void shuffle_nibbles (unsigned *bits) { int shuffled = 0; shuffled |= get_n_bits(*bits, 4, 0); /* locate nibble 6 */ shuffled <<= 4; /* shift left up 4 bits */ shuffled |= get_n_bits(*bits, 4, 1); /* locate nibble 5 */ shuffled <<= 4; /* shift left up 4 bits */ shuffled |= get_n_bits(*bits, 4, 2); /* locate nibble 4 */ shuffled <<= 4; /* shift left up 4 bits */ shuffled |= get_n_bits(*bits, 4, 6); /* locate nibble 3 */ shuffled <<= 4; /* shift left up 4 bits */ shuffled |= get_n_bits(*bits, 4, 4); /* locate nibble 2 */ shuffled <<= 4; /* shift left up 4 bits */ shuffled |= get_n_bits(*bits, 4, 5); /* locate nibble 1 */ shuffled <<= 4; /* shift left up 4 bits */ shuffled |= get_n_bits(*bits, 4, 3); /* locate nibble 0 */ *bits = shuffled; /* update bits with shuffled value */ } /* * Decode a block of 28 bits */ void decode_28bits (unsigned cipher, char *plain, unsigned key1, unsigned key2) { int i; cipher = cipher ^ key2; /* xor ciphertext with key2 */ shuffle_nibbles(&cipher); /* shuffle nibbles */ cipher = cipher ^ key1; /* xor ciphertext with key1 */ for (i = 0; i < 4; i++) /* process 4 septets */ { plain[3- i] = rotate_left3(get_n_bits(cipher, 7, i)); /* get septet and rotate, save as 8 bit char */ } plain[4] = 0; /* insert end of string */ }