bug#6789: MD5 is broken

[Bruno Haible]

Simon Josefsson wrote:
> MD5 is broken, and
> finding collisions takes just seconds on normal PC.  See:
> http://en.wikipedia.org/wiki/MD5#Security

Here is a suggested patch to improve the awareness of this issue in
coreutils.
  - The documentation of md5sum currently says "modifying a file
    so as to retain its MD5 [is] considered infeasible at the moment",
    but the research results of 2008 mentioned in
    <http://en.wikipedia.org/wiki/MD5#Security> showed how to manipulate
    a digital certificate so that the validity of its MD5 signature can
    be retained.
  - The documentation of md5sum says "For more secure hashes, consider
    using SHA-1 or SHA-2." Well, researchers have already discovered
    security weaknesses in <http://en.wikipedia.org/wiki/SHA-1>, therefore
    it does not seem adequate to recommend SHA-1 any more.
  - The 'md5sum --help' output and, with it, the manual page are silent
    about the security problems.

Here is a proposed patch to make this clearer.


2010-08-08  Bruno Haible  <address@hidden>

        md5sum: Put more emphasis on security weaknesses.
        * doc/coreutils.texi (md5sum invocation): Mention currently known
        security problems. Don't recommend SHA-1 as alternative.
        * src/md5sum.c (usage): Mention that MD5 is not secure. Recommend
        SHA-2 as alternative.
        Reported by Simon Josefsson <address@hidden>.

--- doc/coreutils.texi.orig     Sun Aug  8 15:13:06 2010
+++ doc/coreutils.texi  Sun Aug  8 15:10:26 2010
@@ -3414,14 +3414,13 @@
 Note: The MD5 digest is more reliable than a simple CRC (provided by
 the @command{cksum} command) for detecting accidental file corruption,
 as the chances of accidentally having two files with identical MD5
-are vanishingly small.  However, it should not be considered truly
-secure against malicious tampering: although finding a file with a
-given MD5 fingerprint, or modifying a file so as to retain its MD5 are
-considered infeasible at the moment, it is known how to produce
-different files with identical MD5 (a ``collision''), something which
-can be a security issue in certain contexts.  For more secure hashes,
-consider using SHA-1 or SHA-2.  @xref{sha1sum invocation}, and
address@hidden utilities}.
+are vanishingly small.  However, it should not be considered secure
+against malicious tampering: although finding a file with a given MD5
+fingerprint is considered infeasible at the moment, it is known how
+to modify certain files, including digital certificates, so that they
+appear valid when signed with an MD5 digest.  (See
address@hidden://en.wikipedia.org/wiki/MD5#Security} for details.)
+For more secure hashes, consider using SHA-2.  @xref{sha2 utilities}.
 
 If a @var{file} is specified as @samp{-} or if no files are given
 @command{md5sum} computes the checksum for the standard input.
--- src/md5sum.c.orig   Sun Aug  8 15:13:06 2010
+++ src/md5sum.c        Sun Aug  8 14:48:57 2010
@@ -196,6 +196,15 @@
 a line with checksum, a character indicating type (`*' for binary, ` ' for\n\
 text), and name for each FILE.\n"),
               DIGEST_REFERENCE);
+#if HASH_ALGO_MD5
+      printf (_("\
+\n\
+The MD5 algorithm should not be used any more for security related purposes,\n\
+see <%s>.\n\
+Instead, better use an SHA-2 algorithm, implemented in the programs\n\
+sha224sum, sha256sum, sha384sum, sha512sum.\n"),
+              "http://en.wikipedia.org/wiki/MD5#Security";);
+#endif
       emit_ancillary_info ();
     }