MIME-encode words that contain non-ASCII values in text header fields

(e.g. to, from, subject) ... when transmitting to other hosts (e.g. POP3
clients or other SMTP hosts).

For messages that contain CP437 or UTF-8 chars in these fields and were *not*
imported into the BBS via SMTP or POP3, these header fields would be
transmitted to other hosts with the raw CP437 or UTF-8 chars, thus violating
POP3 and SMTP protocols and likely resulting in garbage displayed in message
readers.

The fix is to encode (using MIME "encoded words", per RFC 2407) where
necessary.

Since moderm mail clients (e.g. Thunderbird) don't see to support CP437
charset in MIME encoded header fields, always translate to UTF-8 first.

We probably should be translating message body text to UTF-8 as well, for
maximum compatibilty with modern mail readers, but this commit doesn't address
body text issues with CP437-encoded content. That'll come later.

src/sbbs3/mailsrvr.c

@@ -674,6 +674,46 @@ int compare_addrs(const char* addr1, const char* addr2)
 	return strcmp(angle_bracket(tmp1, sizeof(tmp1), addr1), angle_bracket(tmp2, sizeof(tmp2), addr2));
 }
 
+// MIME-Encode a word (Q-encoded UTF-8, only)
+size_t mime_encode_word(char* buf, size_t size, const char* src)
+{
+	size_t len = snprintf(buf, size - 1, "=?utf-8?q?");
+	// RFC 2047: "An 'encoded-word' may not be more than 75 characters long"
+	for(const char* p = src; *p != '\0' && len < (size - 1) && len < 71; ++p) {
+		if(*p >= 0 && *p != '=' && *p != ' ') { // US-ASCII char, represented without Q-encoding
+			buf[len++] = *p;
+			continue;
+		}
+		len += snprintf(buf + len, size - (len + 1), "=%02X", (unsigned char)*p);
+	}
+	len += snprintf(buf + len, size - (len + 1), "?=");
+	return len;
+}
+
+// MIME-Encode words in string, when necessary
+static char* encode_header_field(const char* src, char* buf, size_t size, const smbmsg_t* msg)
+{
+	if(str_is_ascii(src))
+		return (char*)src;
+	char tmp[256];
+	if(!(msg->hdr.auxattr & MSG_HFIELDS_UTF8))
+		cp437_to_utf8_str(src, tmp, sizeof tmp, /* min-char-val: */'\x80');
+	else
+		SAFECOPY(tmp, src);
+	size_t len = 0;
+	char* state = NULL;
+	for(char* p = strtok_r(tmp, " \t", &state); p != NULL && len < (size - 1); p = strtok_r(NULL, " \t", &state)) {
+		if(len)
+			buf[len++] = ' ';
+		if(str_is_ascii(p))
+			len += snprintf(buf + len, size - (len + 1), "%s", p);
+		else
+			len += mime_encode_word(buf + len, size - (len + 1), p);
+	}
+	buf[len] = '\0';
+	return buf;
+}
+
 static ulong sockmimetext(SOCKET socket, const char* prot, CRYPT_SESSION sess, smbmsg_t* msg, char* msgtxt, ulong maxlines
 						  ,str_list_t file_list, char* mime_boundary)
 {
@@ -681,6 +721,7 @@ static ulong sockmimetext(SOCKET socket, const char* prot, CRYPT_SESSION sess, s
 	char		msgid[256];
 	char		tmp[256];
 	char		date[64];
+	char		encoded_text[256];
 	char*		p;
 	char*		np;
 	int			i;
@@ -708,7 +749,9 @@ static ulong sockmimetext(SOCKET socket, const char* prot, CRYPT_SESSION sess, s
 		char fromname[256];
 		char fromaddr[256];
 		smtp_netmailaddr(&scfg, msg, fromname, sizeof(fromname), fromaddr, sizeof(fromaddr));
-		s = sockprintf(socket,prot,sess,"From: %s %s", fromname, angle_bracket(tmp, sizeof(tmp), fromaddr));
+		s = sockprintf(socket,prot,sess,"From: %s %s"
+			,encode_header_field(fromname, encoded_text, sizeof encoded_text, msg)
+			,angle_bracket(tmp, sizeof(tmp), fromaddr));
 	}
 	if(!s)
 		return(0);
@@ -719,12 +762,12 @@ static ulong sockmimetext(SOCKET socket, const char* prot, CRYPT_SESSION sess, s
 	} else {
 		if(msg->from_org!=NULL || msg->from_net.type==NET_NONE)
 			if(!sockprintf(socket,prot,sess,"Organization: %s"
-				,msg->from_org==NULL ? scfg.sys_name : msg->from_org))
+				,encode_header_field(msg->from_org==NULL ? scfg.sys_name : msg->from_org, encoded_text, sizeof encoded_text, msg)))
 				return(0);
 	}
 
 	p = smb_get_hfield(msg, RFC822SUBJECT, NULL);
-	if(!sockprintf(socket,prot,sess,"Subject: %s", p == NULL ? msg->subj : p))
+	if(!sockprintf(socket,prot,sess,"Subject: %s", p == NULL ? encode_header_field(msg->subj, encoded_text, sizeof encoded_text, msg) : p))
 		return(0);
 
 	if((p = smb_get_hfield(msg, RFC822TO, NULL)) != NULL)
@@ -732,19 +775,20 @@ static ulong sockmimetext(SOCKET socket, const char* prot, CRYPT_SESSION sess, s
 	else if((p = msg->to_list) != NULL)
 		s=sockprintf(socket,prot,sess,"To: %s", p);	/* use original RFC822 header field */
 	else {
+		const char* to = encode_header_field(msg->to, encoded_text, sizeof encoded_text, msg);
 		if(strchr(msg->to,'@')!=NULL || msg->to_net.addr==NULL)
-			s=sockprintf(socket,prot,sess,"To: %s",msg->to);	/* Avoid double-@ */
+			s=sockprintf(socket,prot,sess,"To: %s",to);	/* Avoid double-@ */
 		else if(msg->to_net.type==NET_INTERNET || msg->to_net.type==NET_QWK) {
 			if(strchr((char*)msg->to_net.addr,'<')!=NULL)
 				s=sockprintf(socket,prot,sess,"To: %s",(char*)msg->to_net.addr);
 			else
-				s=sockprintf(socket,prot,sess,"To: \"%s\" <%s>",msg->to,(char*)msg->to_net.addr);
+				s=sockprintf(socket,prot,sess,"To: \"%s\" <%s>",to,(char*)msg->to_net.addr);
 		} else if(msg->to_net.type==NET_FIDO) {
 			char faddrbuf[64];
-			s=sockprintf(socket,prot,sess,"To: \"%s\" (%s)",msg->to, smb_faddrtoa((fidoaddr_t*)msg->to_net.addr, faddrbuf));
+			s=sockprintf(socket,prot,sess,"To: \"%s\" (%s)",to, smb_faddrtoa((fidoaddr_t*)msg->to_net.addr, faddrbuf));
 		} else {
 			usermailaddr(&scfg,toaddr,msg->to);
-			s=sockprintf(socket,prot,sess,"To: \"%s\" <%s>",msg->to,toaddr);
+			s=sockprintf(socket,prot,sess,"To: \"%s\" <%s>",to,toaddr);
 		}
 	}
 	if(!s)