[PATCH 3/3] Change Message-ID generation to be more unique and leak less information

[Remco Rijnders]

A Message-ID should be globally unique. Currently mutt generates this ID
based on the current date and time, followed by ".G", followed by a letter
A to Z (A for the 1st and 27th email sent, Z for the 26th, etc.), followed
by the pid of the active mutt process, followed by "@" and the configured
fqdn.

This can lead to information being leaked as to an users email habits and
activities, which might be undesirable.

By replacing everything left of the "@" in the Message-ID with a Base64
encoded timestamp and 64 bits of randomness, we no longer include this
information.
---
 sendlib.c | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/sendlib.c b/sendlib.c
index 77c14e8f..009f8971 100644
--- a/sendlib.c
+++ b/sendlib.c
@@ -80,8 +80,6 @@ const char B64Chars[64] = {
   '8', '9', '+', '/'
 };
 
-static char MsgIdPfx = 'A';
-
 static void transform_to_7bit (BODY *a, FILE *fpin);
 
 static void encode_quoted (FGETCONV * fc, FILE *fout, int istext)
@@ -2397,19 +2395,27 @@ const char *mutt_fqdn(short may_hide_host)
 char *mutt_gen_msgid (void)
 {
   char buf[SHORT_STRING];
-  time_t now;
-  struct tm *tm;
+  time_t now = time (NULL);
+  u_int32_t randombits[2] = { mutt_random32(), mutt_random32() };
+  char localpart[12]; /* = 32 bit timestamp, plus 64 bit randomness */
+  unsigned char localpart_B64[16+1]; /* = Base64 encoded value of localpart 
plus
+                                        terminating \0 */
   const char *fqdn;
 
-  now = time (NULL);
-  tm = gmtime (&now);
+  /* Convert the four least significant bytes of our timestamp and put it in
+     localpart, with proper endianness (for humans) taken into account. */
+  for (int i = 0; i < 4; i++)
+       localpart[i] = (uint8_t) (now >> (3-i)*8u);
+
+  memcpy(&localpart[4], &randombits[0], 4);
+  memcpy(&localpart[8], &randombits[0], 4);
+
+  mutt_to_base64(localpart_B64, (unsigned char *) localpart, 12, 17);
+
   if (!(fqdn = mutt_fqdn(0)))
     fqdn = NONULL(Hostname);
 
-  snprintf (buf, sizeof (buf), "<%d%02d%02d%02d%02d%02d.G%c%u@%s>",
-           tm->tm_year + 1900, tm->tm_mon + 1, tm->tm_mday, tm->tm_hour,
-           tm->tm_min, tm->tm_sec, MsgIdPfx, (unsigned int)getpid (), fqdn);
-  MsgIdPfx = (MsgIdPfx == 'Z') ? 'A' : MsgIdPfx + 1;
+  snprintf (buf, sizeof (buf), "<%s@%s>", localpart_B64, fqdn);
   return (safe_strdup (buf));
 }
 
-- 
2.26.2