chan_sip: On authentication, pick MD5 for sure.

RFC 8760 added new digest-access-authentication schemes. Testing
revealed that chan_sip does not pick MD5 if several schemes are offered
by the User Agent Server (UAS). This change does not implement any of
the new schemes like SHA-256. This change makes sure, MD5 is picked so
UAS with SHA-2 enabled, like the service www.linphone.org/freesip, can
still be used. This should have worked since day one because SIP/2.0
already envisioned several schemes (see RFC 3261 and its augmented BNF
for 'algorithm' which includes 'token' as third alternative; note: if
'algorithm' was not present, MD5 is still assumed even in RFC 7616).

Change-Id: I61ca0b1f74b5ec2b5f3062c2d661cafeaf597fcd

channels/chan_sip.c

@@ -23077,6 +23077,7 @@ static int reply_digest(struct sip_pvt *p, struct sip_request *req, char *header
 	char tmp[512];
 	char *c;
 	char oldnonce[256];
+	int start = 0;
 
 	/* table of recognised keywords, and places where they should be copied */
 	const struct x {
@@ -23091,9 +23092,11 @@ static int reply_digest(struct sip_pvt *p, struct sip_request *req, char *header
 		{ NULL, 0 },
 	};
 
-	ast_copy_string(tmp, sip_get_header(req, header), sizeof(tmp));
-	if (ast_strlen_zero(tmp))
-		return -1;
+	do {
+		ast_copy_string(tmp, __get_header(req, header, &start), sizeof(tmp));
+		if (ast_strlen_zero(tmp))
+			return -1;
+	} while (strcasestr(tmp, "algorithm=") && !strcasestr(tmp, "algorithm=MD5"));
 	if (strncasecmp(tmp, "Digest ", strlen("Digest "))) {
 		ast_log(LOG_WARNING, "missing Digest.\n");
 		return -1;