Re: spec compliance: header CRC?

[Paul Eggert]

On 08/13/10 07:58, Greg Roelofs wrote:

>  - the "header16 != crc16" error message in gzip.c would look nicer with
>    a pair of "%04x" instead of "%x".

Thanks, I changed it to 0x%04x (see patch below).

>  - algorithm.doc needs updating:

Thanks, I updated that too (see 2nd patch below).

> Also, are you aware of _any_ gzip utilities that support encryption?

As far as I know, gzip has never supported it.  I removed discussion of
that in algorithm.doc while I was at it.

>From c133a2dc08348a5041aeb3f87dfe8817ee7467c0 Mon Sep 17 00:00:00 2001
From: Paul Eggert <address@hidden>
Date: Sun, 15 Aug 2010 15:50:12 -0700
Subject: [PATCH] gzip: Use 0x%04x instead of %x when printing 16-bit checksums

* gzip.c (get_method): Use 0x%04x, not %x, to print 16-bit checksums.
Inspired by a suggestion of Greg Roelofs in
http://lists.gnu.org/archive/html/bug-gzip/2010-08/msg00004.html
---
 gzip.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/gzip.c b/gzip.c
index 65f5897..d42bfa8 100644
--- a/gzip.c
+++ b/gzip.c
@@ -1420,7 +1420,7 @@ local int get_method(in)
             if (header16 != crc16)
               {
                 fprintf (stderr,
-                         "%s: %s: header checksum %x != computed checksum 
%x\n",
+                         "%s: %s: header checksum 0x%04x != computed checksum 
0x%04x\n",
                          program_name, ifname, header16, crc16);
                 exit_code = ERROR;
                 if (force <= 1)
-- 
1.7.2


>From a481d8ea1ff607765edb6bc119abebf1c0d47d3b Mon Sep 17 00:00:00 2001
From: Paul Eggert <address@hidden>
Date: Sun, 15 Aug 2010 16:27:06 -0700
Subject: [PATCH] algorithm.doc: mention Internet RFC 1952 and modernize a bit

* algorithm.doc: Update to mention header-CRC and Internet RFC 1952.
Also, remove the crypto stuff, which never worked.
Inspired by that same suggestion of Greg Roelofs.
---
 algorithm.doc |   35 ++++++++++++++++++-----------------
 1 files changed, 18 insertions(+), 17 deletions(-)

diff --git a/algorithm.doc b/algorithm.doc
index a62800c..c8c3a92 100644
--- a/algorithm.doc
+++ b/algorithm.doc
@@ -1,7 +1,7 @@
 1. Algorithm
 
-The deflation algorithm used by zip and gzip is a variation of LZ77
-(Lempel-Ziv 1977, see reference below). It finds duplicated strings in
+The deflation algorithm used by zip and gzip is a variation of
+Lempel-Ziv 1977 [LZ77].  It finds duplicated strings in
 the input data.  The second occurrence of a string is replaced by a
 pointer to the previous string, in the form of a pair (distance,
 length).  Distances are limited to 32K bytes, and lengths are limited
@@ -57,6 +57,10 @@ but saves time since there are both fewer insertions and 
fewer searches.
 
 2. gzip file format
 
+The gzip file format was standardized in Internet RFC 1952 [RFC1952].
+This section briefly describes the format and comments on some
+implementation details.
+
 The pkzip format imposes a lot of overhead in various headers, which
 are useful for an archiver but not necessary when only one file is
 compressed. gzip uses a much simpler structure. Numbers are in little
@@ -68,12 +72,11 @@ following structure:
 1 byte   compression method (0..7 reserved, 8 = deflate)
 1 byte   flags
             bit 0 set: file probably ascii text
-            bit 1 set: continuation of multi-part gzip file
+            bit 1 set: header CRC-16 present
             bit 2 set: extra field present
             bit 3 set: original file name present
             bit 4 set: file comment present
-            bit 5 set: file is encrypted
-            bit 6,7:   reserved
+            bit 5,6,7: reserved
 4 bytes  file modification time in Unix format
 1 byte   extra flags (depend on compression method)
 1 byte   operating system on which compression took place
@@ -83,7 +86,7 @@ following structure:
 ? bytes  optional extra field
 ? bytes  optional original file name, zero terminated
 ? bytes  optional file comment, zero terminated
-12 bytes optional encryption header
+2 bytes  optional 16-bit header CRC
 ? bytes  compressed data
 4 bytes  crc32
 4 bytes  uncompressed input size modulo 2^32
@@ -128,7 +131,7 @@ compression format, regardless of the actual size of the 
compressed
 data. If the compressed data cannot fit in one file (in particular for
 diskettes), each part starts with a header as described above, but
 only the last part has the crc32 and uncompressed size. A decompressor
-may prompt for additional data for multipart compressed files. It is
+may prompt for additional data for multi-part compressed files. It is
 desirable but not mandatory that multiple parts be extractable
 independently so that partial data can be recovered if one of the
 parts is damaged. This is possible only if no compression state is
@@ -145,20 +148,18 @@ a few bytes for the gzip file header, plus 5 bytes every 
32K block,
 or an expansion ratio of 0.015% for large files. Note that the actual
 number of used disk blocks almost never increases.
 
-The encryption is that of zip 1.9. For the encryption check, the
-last byte of the decoded encryption header must be zero. The time
-stamp of an encrypted file might be set to zero to avoid giving a clue
-about the construction of the random header.
-
 Jean-loup Gailly
 address@hidden
 
 References:
 
 [LZ77] Ziv J., Lempel A., "A Universal Algorithm for Sequential Data
-Compression", IEEE Transactions on Information Theory", Vol. 23, No. 3,
-pp. 337-343.
+Compression", IEEE Transactions on Information Theory, Vol. 23, No. 3,
+May 1977, pp. 337-343.
+
+[RFC1952] Deutsch P., "GZIP file format specification version 4.3",
+Internet RFC 1952, May 1996, <http://www.ietf.org/rfc/rfc1952.txt>.
 
-APPNOTE.TXT documentation file in PKZIP 1.93a. It is available by
-ftp in ftp.cso.uiuc.edu:/pc/exec-pc/pkz193a.exe [128.174.5.59]
-Use "unzip pkz193a.exe APPNOTE.TXT" to extract (note: unzip, not gunzip).
+APPNOTE.TXT documentation file in PKZIP 1.93a (October 1991).  This
+version no longer seems to be available online; the latest version is
+in <http://www.pkware.com/documents/casestudies/APPNOTE.TXT>.
-- 
1.7.2