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1 files changed, 226 insertions, 0 deletions

diff --git a/torrenthash.c b/torrenthash.c
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+++ b/torrenthash.c
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+/*
+  Copyright (c) 2017, Matthias Schiffer <mschiffer@universe-factory.net>
+  All rights reserved.
+
+  Redistribution and use in source and binary forms, with or without
+  modification, are permitted provided that the following conditions are met:
+
+    1. Redistributions of source code must retain the above copyright notice,
+       this list of conditions and the following disclaimer.
+    2. Redistributions in binary form must reproduce the above copyright notice,
+       this list of conditions and the following disclaimer in the documentation
+       and/or other materials provided with the distribution.
+
+  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+  DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+  FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+  DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+  SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+  CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+  OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+  OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+*/
+
+
+#include <fcntl.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/mman.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <openssl/sha.h>
+
+
+static const char * skip_bcode(const char *data, const char *end);
+
+
+/** Prints a (printf-style) error message and exits */
+#define exit_error(...) do { fprintf(stderr, __VA_ARGS__); exit(1); } while (0)
+
+/** Prints an error messages about unexpected EOF and exits */
+__attribute__((noreturn))
+static void eof(void) {
+	exit_error("unexptected EOF\n");
+}
+
+
+/** Returns the length of the file assiciated with file descriptor fd */
+static size_t flen(int fd) {
+	struct stat statbuf;
+	if (fstat(fd, &statbuf) < 0)
+		exit_error("fstat: %m\n");
+
+	return statbuf.st_size;
+}
+
+
+/**
+	Returns a pointer after the current integer
+
+	An integer has the form "i42e". 'i' is already skipped by skip_bcode.
+*/
+static const char * skip_int(const char *data, const char *end) {
+	if (data >= end)
+		eof();
+
+	const char *e = memchr(data, 'e', end-data);
+	if (!e)
+		eof();
+
+	return e+1;
+}
+
+/**
+	Returns a pointer after the current byte string
+
+	A byte string has the form length:data (length is a decimal number
+	and data can be arbitrary binary data).
+*/
+static const char * skip_string(const char *data, const char *end) {
+	if (data >= end)
+		eof();
+
+	const char *colon = memchr(data, ':', end-data);
+	if (!colon)
+		eof();
+
+	char *endp;
+	size_t len = (size_t)strtoull(data, &endp, 10);
+	if (endp != colon)
+		exit_error("syntax error\n");
+
+	const char *ret = colon + 1 + len;
+	if (ret >= end || ret <= colon /* Check for overflow */)
+		eof();
+
+	return ret;
+}
+
+/**
+	Returns a pointer after the current dictionary
+
+	A dictionary has the form "dkeyvaluekeyvalue...e", where keys are
+	byte strings and values are arbitrary bcode objects. 'd' is already
+	skipped by skip_bcode.
+*/
+static const char * skip_dict(const char *key, const char *end) {
+	while (key < end) {
+		if (key[0] == 'e')
+			return key+1;
+
+		const char *value = skip_string(key, end);
+		key = skip_bcode(value, end);
+	}
+
+	eof();
+}
+
+/**
+	Returns a pointer after the current list
+
+	A dictionary has the form "lvaluevalue...e", where values are arbitrary
+	bcode objects. 'l' is already skipped by skip_bcode.
+*/
+static const char * skip_list(const char *data, const char *end) {
+	while (data < end) {
+		if (data[0] == 'e')
+			return data+1;
+
+		data = skip_bcode(data, end);
+	}
+
+	eof();
+}
+
+/**
+	Returns a pointer after the current bencoded object
+
+	Possible object types are integers, byte strings, dictionaries and
+	lists.
+*/
+static const char * skip_bcode(const char *data, const char *end) {
+	if (data >= end)
+		eof();
+
+	switch(data[0]) {
+	case 'i':
+		return skip_int(data+1, end);
+	case '0' ... '9':
+		return skip_string(data, end);
+	case 'd':
+		return skip_dict(data+1, end);
+	case 'l':
+		return skip_list(data+1, end);
+	default:
+		exit_error("unexpected character '%c'\n", data[0]);
+	}
+}
+
+
+/**
+	Prints the SHA1 hash of the data block bounded by the data and end
+	arguments.
+*/
+static void print_hash(const char *data, const char *end) {
+	SHA_CTX ctx;
+	unsigned char hash[20];
+
+	SHA1_Init(&ctx);
+	SHA1_Update(&ctx, (unsigned char *)data, end-data);
+	SHA1_Final(hash, &ctx);
+
+	for (size_t i = 0; i < 20; i++)
+		printf("%02x", hash[i]);
+
+	printf("\n");
+}
+
+
+int main(int argc, char *argv[]) {
+	if (argc != 2) {
+		fprintf(stderr, "Usage: torrenthash <file>\n");
+		return 1;
+	}
+
+	int fd = open(argv[1], O_RDONLY);
+	if (fd < 0)
+		exit_error("open: %m\n");
+	size_t len = flen(fd);
+	if (len == 0)
+		exit_error("empty file\n");
+
+	/* Map the file into memory */
+	const char *data = mmap(NULL, len, PROT_READ, MAP_PRIVATE, fd, 0);
+	if (data == MAP_FAILED)
+		exit_error("mmap: %m\n");
+
+	const char *end = data + len;
+
+	/* The file must have a dictionary on the top level */
+	if (data[0] != 'd')
+		exit_error("unexpected character '%c'\n", data[0]);
+
+	/* Iterate through the dictionary entries */
+	for (const char *key = data+1; key < end;) {
+		if (key[0] == 'e')
+			exit_error("key 'info' not found\n");
+
+		const char *value = skip_string(key, end);
+		const char *next_key = skip_bcode(value, end);
+
+		/* Check for "info" key */
+		if ((value-key) == 6 && memcmp(key, "4:info", 6) == 0) {
+			print_hash(value, next_key);
+			return 0;
+		}
+
+		key = next_key;
+	}
+
+	eof();
+}