listamsb: support "locked" programs.

1 files changed, 87 insertions, 18 deletions

diff --git a/listamsb.c b/listamsb.c
index 207c3bb..6737243 100644
--- a/listamsb.c
+++ b/listamsb.c
@@ -57,6 +57,10 @@
 	it would overlap the GR.0 display list or the ROMs at $c000 */
 #define MAX_PTR 0xbc1f
 
+/* SAVE "filename" LOCK does 'encryption' by subtracting every byte
+   from this (except the 3-byte header) */
+#define UNLOCK_KEY 0x54
+
 const char *self;
 
 char pipe_command[BUFSIZ + 1] = { "a8cat" };
@@ -66,8 +70,9 @@ int raw_output =  0;           /* -a */
 int check_only =  0;           /* -c */
 int startline =   0;           /* -r */
 int endline   =   MAX_LINENO;  /* -r */
+int unlock_mode = 0;           /* -u */
 
-
+int locked =      0;
 int need_pclose = 0;
 int bytes_read = 0;
 int warnings = 0;
@@ -104,6 +109,25 @@ unsigned char read_byte(void) {
 	return (unsigned char)c;
 }
 
+/* "decrypt" a byte from a "SAVE x LOCK" program. */
+unsigned char unlock_byte(unsigned char b) {
+	return ((UNLOCK_KEY - b) & 0xff);
+}
+
+/* the "encryption" is the same (process is reversible) */
+#define lock_byte(x) unlock_byte(x)
+
+unsigned char read_prog_byte(void) {
+	unsigned char b = read_byte();
+	return locked ? unlock_byte(b) : b;
+}
+
+void unread_prog_byte(unsigned char b) {
+	if(locked) b = lock_byte(b);
+	ungetc(b, infile);
+	bytes_read--;
+}
+
 int read_word(void) {
 	int w;
 
@@ -113,11 +137,23 @@ int read_word(void) {
 	return w;
 }
 
+int read_prog_word(void) {
+	int w;
+
+	w = read_prog_byte();
+	w |= (read_prog_byte() << 8);
+
+	return w;
+}
+
 void read_header(void) {
-	unsigned char b;
+	/* $00 for plain, $01 for SAVE with LOCK */
+	locked = read_byte();
+	if(locked > 1) die2("not an AMSB file: first byte not $00 or $01");
 
-	b = read_byte();
-	if(b) die2("not an AMSB file: first byte not $00");
+	if(verbose && locked) {
+		fprintf(stderr, "program is locked, decrypting\n");
+	}
 
 	proglen = read_word();
 
@@ -159,14 +195,14 @@ int next_line(void) {
 	/* pointer to last token on the line, offset by whatever MEMLO
 		happened to be when the file was SAVEd. 0 means this is the
 		last line. */
-	ptr = read_word();
+	ptr = read_prog_word();
 	if(!ptr) {
 		if(verbose)
 			fprintf(stderr, "end of program\n");
 		return 0;
 	}
 
-	lineno = read_word();
+	lineno = read_prog_word();
 	if(verbose)
 		fprintf(stderr, "found line %d, offset %d, end-of-line %d\n", lineno, offset, ptr);
 
@@ -212,7 +248,7 @@ int next_line(void) {
 
 	/* walk and print the tokens. when we hit a null byte, we're done. */
 	while(1) {
-		byte = read_byte();
+		byte = read_prog_byte();
 
 		if(in_string) {
 			if(byte == 0x00 || byte == '|') {
@@ -231,11 +267,10 @@ int next_line(void) {
 			if(printing) putc(byte, outfile);
 		} else if(byte == ':') {
 			/* don't print the colon if the next token is a ! or ' for a comment */
-			unsigned char next = read_byte();
+			unsigned char next = read_prog_byte();
 			if( !(next == TOK_SQUOTE || next == TOK_BANG) )
 				if(printing) putc(byte, outfile);
-			ungetc(next, infile);
-			bytes_read--;
+			unread_prog_byte(next);
 		} else if(byte == '"') {
 			/* strings start but *don't end* with a double-quote */
 			in_string = 1;
@@ -300,12 +335,37 @@ int next_line(void) {
 	return 1;
 }
 
+/* when this gets called, input and output are open, read_header()
+   has already run. "locking" and "unlocking" are the same
+   transform, so this function does both. */
+void unlock_program(void) {
+	int c;
+
+	fprintf(stderr, "%s: program is %slocked, output will be %slocked\n",
+		self, locked ? "" : "un", locked ? "un" : "");
+
+	/* 3-byte header: 0 for unlocked, 1 for locked */
+	fputc(!locked, outfile);
+	/* LSB of program length (not encrypted) */
+	fputc(proglen & 0xff, outfile);
+	/* MSB */
+	fputc((proglen >> 8) & 0xff, outfile);
+
+	/* rest of file, including trailing nulls, is transformed */
+	while( (c = fgetc(infile)) >= 0)
+		fputc(unlock_byte(c & 0xff), outfile);
+	fclose(outfile);
+
+	exit(0);
+}
+
 void print_help(void) {
 	printf("%s v" VERSION " - detokenize Atari Microsoft BASIC files\n", self);
 	puts("By B. Watson <urchlay@slackware.uk>, released under the WTFPL");
-	printf("Usage: %s [-a] [-v] [-h] [-i] [-u] [-t] [-m] [-s] [-r *start,end*] [file]\n", self);
+	printf("Usage: %s [[-l] | [-a] [-v] [-h] [-i] [-u] [-t] [-m] [-s] [-r *start,end*]] [file]\n", self);
 	puts("  -a: raw ATASCII output");
 	puts("  -c: check only (no listing)");
+	puts("  -l: lock or unlock program");
 	puts("  -v: verbose");
 	puts("  -r: only list lines numbered from *start* to *end*");
 	puts("  --help, -h: print this help and exit");
@@ -364,12 +424,13 @@ void parse_args(int argc, char **argv) {
 		}
 	}
 
-	while( (opt = getopt(argc, argv, "r:cvaiutmsh")) != -1) {
+	while( (opt = getopt(argc, argv, "lr:cvaiutmsh")) != -1) {
 		switch(opt) {
-			case 'c': check_only = 1; break;
-			case 'a': raw_output = 1; break;
+			case 'l': unlock_mode = 1; break;
+			case 'c': check_only  = 1; break;
+			case 'a': raw_output  = 1; break;
+			case 'v': verbose =     1; break;
 			case 'h': print_help(); exit(0);
-			case 'v': verbose = 1; break;
 			case 'r': get_line_range(optarg); break;
 			case 'i':
 			case 'u':
@@ -413,9 +474,12 @@ void open_output() {
 		}
 		if(verbose)
 			fprintf(stderr, "using /dev/null for output (check_only)\n");
-	} else if(raw_output) {
-		if(isatty(fileno(stdout)))
-			die("refusing to write raw ATASCII to a terminal");
+	} else if(raw_output || unlock_mode) {
+		if(isatty(fileno(stdout))) {
+			fprintf(stderr, "%s: refusing to write %s to a terminal\n",
+				self, (unlock_mode ? "tokenized BASIC" : "raw ATASCII"));
+			exit(1);
+		}
 		outfile = stdout;
 		if(verbose)
 			fprintf(stderr, "using stdout for output\n");
@@ -442,6 +506,11 @@ int main(int argc, char **argv) {
 
 	read_header();
 
+	if(unlock_mode) {
+		unlock_program();
+		exit(0);
+	}
+
 	while(next_line())
 		linecount++;