[PATCH 2/3] 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 "@" and to the right of the "." in the
Message-ID with 96 bits of Base64 encoded randomness, we no longer include
this information.
---
 sendlib.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/sendlib.c b/sendlib.c
index 77c14e8f..e1dc3ea3 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)
@@ -2399,17 +2397,18 @@ char *mutt_gen_msgid (void)
   char buf[SHORT_STRING];
   time_t now;
   struct tm *tm;
+  char randomness[17];
   const char *fqdn;
 
   now = time (NULL);
   tm = gmtime (&now);
+  mutt_base64_random96(randomness);
   if (!(fqdn = mutt_fqdn(0)))
     fqdn = NONULL(Hostname);
 
-  snprintf (buf, sizeof (buf), "<%d%02d%02d%02d%02d%02d.G%c%u@%s>",
+  snprintf (buf, sizeof (buf), "<%d%02d%02d%02d%02d%02d.%s@%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;
+           tm->tm_min, tm->tm_sec, randomness, fqdn);
   return (safe_strdup (buf));
 }
 
-- 
2.26.2