Working reader

Assignment9/src/main.c

@@ -30,10 +30,14 @@ int main(int argc, char *argv[]) {
 		bool is_idat = memcmp(chunks[i]->type, "IDAT", 4) == 0;
 		bool is_iend = memcmp(chunks[i]->type, "IEND", 4) == 0;
 		if (is_idat || is_iend) {
-			printf("Found %s chunk\n", chunks[i]->type);
+			printf("Found %.4s chunk\n", chunks[i]->type);
+			if (is_idat) {
+				xor_data(chunks[i]);
+			}
 		} else {
-			printf("Found unknown: %s\n", chunks[i]->type);
+			printf("Found unknown: %.4s\n", chunks[i]->type);
 		}
 		printf("Chunk size is:%d\n", chunks[i]->length);
 	}
+	// write_png_chunks(path, chunks);
 }

Assignment9/src/png.c

@@ -69,11 +69,6 @@ char *load_file(int fd) {
 		exit(1);
 	}
 	fread(buffer, 1, size, fp);
-	// Iterate over buffer and convert from network byte order to host byte order
-	//for (int i = 0; i < size; i += 4) {
-	//	uint32_t *ptr = (uint32_t *)(buffer + i);
-	//	*ptr = ntohl(*ptr);
-	//}
 	fclose(fp);
 	return buffer;
 }
@@ -111,7 +106,8 @@ png_chunk **get_png_chunks(char *buffer) {
 	int i = 0;
 	while (1) {
 		chunks[i] = get_png_chunk(buffer, offset);
-		offset += chunks[i]->length;
+		// 12 = 4 (length) + 4 (type) + 4 (crc)
+		offset += 12 + chunks[i]->length;
 		if (memcmp(chunks[i]->type, "IEND", 4) == 0) {
 			break;
 		}
@@ -125,18 +121,6 @@ png_chunk **get_png_chunks(char *buffer) {
 	return chunks;
 }
 
-int is_chunk_valid(png_chunk *chunk) {
-	uint32_t crc = htonl(chunk->crc);
-	//uint32_t calculated_crc = crc32(0, chunk->type, 4);
-	//calculated_crc = crc32(calculated_crc, chunk->data, chunk->length);
-	//return crc == calculated_crc;
-	return 1;
-}
-
-int is_chunk_critical(png_chunk *chunk) {
-	return chunk->type[0] >= 'A' && chunk->type[0] <= 'Z';
-}
-
 int get_number_of_chunks(png_chunk **chunks) {
 	int i = 0;
 	while (chunks[i] != NULL) {
@@ -144,3 +128,25 @@ int get_number_of_chunks(png_chunk **chunks) {
 	}
 	return i;
 }
+
+void xor_data(png_chunk *chunk) {
+	for (int i = 0; i < chunk->length; i++) {
+		chunk->data[i] ^= KEY;
+	}
+}
+
+void write_png_chunks(char *path, png_chunk **chunks) {
+	FILE *fp = fopen(path, "wb");
+	if (fp == NULL) {
+		perror("fopen");
+		exit(1);
+	}
+	fwrite(PNG_SIGNATURE, PNG_SIGNATURE_SIZE, 1, fp);
+	for (int i = 0; chunks[i] != NULL; i++) {
+		fwrite(&chunks[i]->length, 4, 1, fp);
+		fwrite(chunks[i]->type, 4, 1, fp);
+		fwrite(chunks[i]->data, chunks[i]->length, 1, fp);
+		fwrite(&chunks[i]->crc, 4, 1, fp);
+	}
+	fclose(fp);
+}

Assignment9/src/png.h

@@ -39,6 +39,9 @@
 // http://www.libpng.org/pub/png/spec/1.2/PNG-Rationale.html#R.PNG-file-signature
 #define PNG_SIGNATURE "\211PNG\r\n\032\n"
 #define PNG_SIGNATURE_SIZE 8
+
+#define KEY 42
+
 typedef struct png_chunk {
 	uint32_t length;
 	char type[4];
@@ -67,8 +70,8 @@ png_chunk **get_png_chunks(char *buffer);
 // Get number of chunks
 int get_number_of_chunks(png_chunk **chunks);
 
-// Check if chunk is critical
+// XOR chunk with key
-int is_chunk_critical(png_chunk *chunk);
+void xor_data(png_chunk *chunk);
 
-// Is chunk valid?
+// Write PNG chunks to file
-int is_chunk_valid(png_chunk *chunk);
+void write_png_chunks(char *path, png_chunk **chunks);