From f52c6cb6df11a3fee98608c44e1f907b7470c507 Mon Sep 17 00:00:00 2001
From: scratko <m@scratko.xyz>
Date: Fri, 10 May 2024 17:58:36 +0300
Subject: Initial commit

---
 encryption.c | 77 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 77 insertions(+)
 create mode 100644 encryption.c

diff --git a/encryption.c b/encryption.c
new file mode 100644
index 0000000..972414c
--- /dev/null
+++ b/encryption.c
@@ -0,0 +1,77 @@
+#include <sys/mman.h>
+#include <stdio.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <sys/stat.h>
+#include <stdlib.h>
+
+union key_type {
+    unsigned int number;
+    char bytes[sizeof(int)];
+};
+
+static void fill_in_some_byte(int remaining_bytes, unsigned int *data,
+                              union key_type key)
+{
+    char *start_data = (char*) data;
+    switch(remaining_bytes) {
+    case 3:
+       start_data[2] ^= key.bytes[2];
+    case 2:
+       start_data[1] ^= key.bytes[1];
+    case 1:
+       *start_data ^= key.bytes[0];
+    }
+}
+
+static int calculate_memory(int block_counter, int size, int pos)
+{
+    return block_counter*size + (pos+1)*sizeof(int);
+}
+
+int main(int argc, char **argv)
+{
+    int fd, size, pgs, offset, pos, block_counter, next_used_memory;
+    unsigned int *data;
+    union key_type key;
+    struct stat sb;
+    size = 4096;
+    offset = 0;
+    block_counter = 0;
+    key.number = ~ (unsigned int) strtol(argv[2], NULL, 10);
+    pgs = getpagesize();
+    size = ((size-1) / pgs+1) * pgs;
+    if(argc != 3) {
+        perror("enter only three params");
+        return 1;
+    }
+    fd = open(argv[1], O_RDWR);
+    if(fd == -1) {
+        perror("can't open file");
+        return 2;
+    }
+    fstat(fd, &sb);
+    while(offset < sb.st_size-1) {
+        data = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, offset);
+        if(data == MAP_FAILED) {
+            perror("mapping error");
+            return 3;
+        }
+        pos = 0;
+        next_used_memory = calculate_memory(block_counter, size, pos);
+        while(next_used_memory <= sb.st_size && next_used_memory <= size) {
+            data[pos] ^= key.number;
+            ++pos;
+            next_used_memory = calculate_memory(block_counter, size, pos);
+        }
+        if(next_used_memory > sb.st_size && next_used_memory <= size) {
+            fill_in_some_byte(sizeof(int) - (next_used_memory - sb.st_size),
+                              data+pos, key);
+        }
+        munmap(data, size);
+        offset += size;
+        ++block_counter;
+    }
+    close(fd);
+    return 0;
+}
-- 
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