set1, challenges 1-3

.gitignore

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+# ---> C
+# Prerequisites
+*.d
+
+# Object files
+*.o
+*.ko
+*.obj
+*.elf
+
+# Linker output
+*.ilk
+*.map
+*.exp
+
+# Precompiled Headers
+*.gch
+*.pch
+
+# Libraries
+*.lib
+*.a
+*.la
+*.lo
+
+# Shared objects (inc. Windows DLLs)
+*.dll
+*.so
+*.so.*
+*.dylib
+
+# Executables
+*.exe
+*.out
+*.app
+*.i*86
+*.x86_64
+*.hex
+
+# Debug files
+*.dSYM/
+*.su
+*.idb
+*.pdb
+
+# Kernel Module Compile Results
+*.mod*
+*.cmd
+.tmp_versions/
+modules.order
+Module.symvers
+Mkfile.old
+dkms.conf

set1/01_base64.c

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+#include <stdio.h>
+#include <ctype.h>
+#include <stdlib.h>
+
+#include "utils.h"
+#define INPUT_LEN 200
+
+const char b64[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+
+int main() {
+       char *input = (char*)malloc(INPUT_LEN); 
+       char *output = (char*)malloc((INPUT_LEN / 3 + INPUT_LEN % 3) * 4);
+       char buf[3];
+       char sixtet;
+       int i; int j;
+       int n = read_bytes(input, INPUT_LEN);
+
+       // number of byte triples (n_triples * 4 - output size)
+	   int n_triples = n / 3;
+       // if n is not divided by 3 then we should add one more triple, we will fill remaining bytes with zeroes
+       if (n % 3)
+               n_triples++;
+
+       // i - starting number of triple, j - starting number of quadriple
+       for (i = 0, j = 0; i < 3 * n_triples; i += 3, j += 4) {
+            // if it's the last triple (maybe not full)
+            if (i == 3 * n_triples - 1) {
+                buf[0] = input[i];
+                if (n % 3 == 1)
+                        buf[1] = buf[2] = 0;
+                else if (n % 3 == 2) {
+                        buf[1] = input[i + 1];                        
+                        buf[2] = 0;
+                }
+                else {
+                        buf[1] = input[i + 1];
+                        buf[2] = input[i + 2];
+                }
+            }
+            else {
+                buf[0] = input[i]; buf[1] = input[i+1]; buf[2] = input[i+2];
+            }
+    	  	sixtet = (buf[0] & 0b11111100) >> 2;
+			output[j] = b64[(int)sixtet];
+			sixtet = ((buf[0] & 0b00000011) << 4) | ((buf[1] & 0b11110000) >> 4);
+			output[j+1] = b64[(int)sixtet];
+			sixtet = ((buf[1] & 0b00001111) << 2) | ((buf[2] & 0b11000000) >> 6);
+			output[j+2] = b64[(int)sixtet];
+			sixtet = buf[2] & 0b00111111;
+			output[j+3] = b64[(int)sixtet]; 
+       }
+	for (int k = 0; k < n_triples * 4; ++k) {
+		printf("%c", output[k]);
+	}
+	printf("\n");
+
+}

set1/02_fixed_xor.c

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+#include <stdio.h>
+#include <stdlib.h>
+#include "utils.h"
+
+#define INPUT_LEN 200
+int main() {
+    char *a = (char*)malloc(INPUT_LEN);
+    char *b = (char*)malloc(INPUT_LEN);
+    char *output = (char*)malloc(INPUT_LEN); 
+
+    int n = read_bytes(a, INPUT_LEN);
+    read_bytes(b, INPUT_LEN);
+    
+    for(int i = 0; i < n; ++i) {
+        output[i] = a[i] ^ b[i];
+    } 
+    print_bytes(output, n); 
+}

set1/03_single_character_xor.c

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+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <ctype.h>
+#include <float.h>
+
+#include "utils.h"
+
+// 'a'-'z', last value is a frequency of non-alphaber symbols
+const double eng_freq[] = {0.08167,0.01492,0.02782,0.04253,0.12702,0.02228,0.02015,0.06094,0.06966,0.00153,0.00772,0.04025,0.02406,0.06749,0.07507,0.01929,0.00095,0.05987,0.06327,0.09056,0.02758,0.00978,0.02360,0.00150,0.01974,0.00074,0};
+
+void freq(char *str, int n, double* freqs, char min, char max) {
+       for (int i = 0; i < n; ++i) {
+               if (str[i] >= min && str[i] <= max)
+                    freqs[str[i] - min]++;
+               else
+                    freqs[max - min + 1]++;
+       }
+    
+       for (int j = 0; j < max - min + 2; ++j)
+            freqs[j] /= n;
+}
+
+double std(double *a, const double *b, int n) {
+        double result = 0;
+        for (int i = 0; i < n; ++i)
+            result += (a[i] - b[i]) * (a[i] - b[i]); 
+        result /= n;
+        return result;
+}
+
+double freq_score(char *str, int n) {
+        double score = 0;
+        char* str_copy = (char*)malloc(n); 
+        strncpy(str_copy, str, n);
+
+        // last value is a frequency of non-alphaber symbols
+        double* freqs = (double*)malloc(('z' - 'a' + 2) * sizeof(double));
+
+        for (int i = 0; i < n; ++i)
+            if (str_copy[i] >= 'A' && str_copy[i] <='Z')
+                str_copy[i] = tolower(str[i]);  
+
+        freq(str_copy, n, freqs, 'a', 'z');
+
+        score = std(freqs, eng_freq, 'z' - 'a' + 2); 
+        free(str_copy);
+        free(freqs);
+        return score;
+}
+
+int main() {
+        char *input = (char*)malloc(34); 
+        read_bytes(input, 34);
+
+        char *result = (char*)malloc(35);
+        char *buf = (char*)malloc(34);
+        double score = 0; double min_score = DBL_MAX;
+
+        for (unsigned char i = 0; i < 255; ++i) {
+           for (int j = 0; j < 34; ++j)
+                  buf[j] = input[j] ^ i; 
+           score = freq_score(buf, 34);
+           if (score < min_score) {
+                min_score = score;
+                strncpy(result, buf, 34);
+           }
+        }
+        result[34] = '\0';
+        printf("%s\n", result);
+
+        free(input); free(result); free(buf);
+}

set1/utils.c

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+#include <stdio.h>
+#include <string.h>
+
+int read_bytes(char *out_byte_array, int max_n) {
+       char buf[2];
+       int i = 0;
+       while(((buf[0] = (char)getchar())!= EOF) && (buf[0] != '\n') && (i < max_n)) {
+               buf[1] = getchar();
+               if (buf[1] == EOF)
+                       return -1;
+               sscanf(buf, "%hhx", out_byte_array + i);
+               i++;
+       }
+       // number of read characters
+       return i;
+}
+void print_bytes(char *byte_array, int n) {
+    for (int i = 0; i < n; ++i) {
+        printf("%hhx", byte_array[i]);
+    }
+}
+
+int print_str_bytes(char *str, int n) {
+    for (int i = 0; i < n; ++i) {
+        printf("%hhx", str[i]);
+    }
+
+}
+/*int main() {
+    char meow[10];
+    int n = read_bytes(meow, 10);
+    print_bytes(meow, n);
+    char *nya = "meow meow ctagirl uwu";
+    print_str_bytes(nya, strlen(nya));
+}*/

set1/utils.h

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+int read_bytes(char *output, int max_n);
+
+void print_bytes(char *byte_array, int n);
+
+int print_str_bytes(char *str, int n);