Align SSL_get_shared_ciphers() with OpenSSL

SSL_get_shared_ciphers() has been quite broken forever (see BUGS).
What's maybe even worse than those bugs is that it only ever returned
the string representing the client's ciphers which happen to fit into
buf. That's kind of odd, given its name.

This commit brings it in line with OpenSSL's version which changed
behavior almost three years ago.

reviewed and stupid bug caught by schwarze
ok beck inoguchi jsing

  commit a216df599a6076147c27acea6c976fb11f505b1a
  Author: Matt Caswell <matt@openssl.org>
  Date:   Fri Apr 27 11:20:52 2018 +0100

    Fix SSL_get_shared_ciphers()

    The function SSL_get_shared_ciphers() is supposed to return
    ciphers shared by the client and the server. However it only
    ever returned the client ciphers.

    Fixes #5317

    Reviewed-by: Richard Levitte <levitte@openssl.org>
    (Merged from https://github.com/openssl/openssl/pull/6113)

1 files changed, 18 insertions, 10 deletions

diff --git a/lib/libssl/ssl_lib.c b/lib/libssl/ssl_lib.c
index 69628b48df7..0537cf0e463 100644
--- a/lib/libssl/ssl_lib.c
+++ b/lib/libssl/ssl_lib.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssl_lib.c,v 1.239 2020/12/01 07:46:01 tb Exp $ */
+/* $OpenBSD: ssl_lib.c,v 1.240 2021/01/09 10:34:29 tb Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -1484,22 +1484,30 @@ SSL_set_ciphersuites(SSL *s, const char *str)
 char *
 SSL_get_shared_ciphers(const SSL *s, char *buf, int len)
 {
-	STACK_OF(SSL_CIPHER) *ciphers;
+	STACK_OF(SSL_CIPHER) *client_ciphers, *server_ciphers;
 	const SSL_CIPHER *cipher;
 	size_t curlen = 0;
 	char *end;
 	int i;
 
-	if (s->session == NULL || s->session->ciphers == NULL || len < 2)
-		return (NULL);
+	if (!s->server || s->session == NULL || len < 2)
+		return NULL;
 
-	ciphers = s->session->ciphers;
-	if (sk_SSL_CIPHER_num(ciphers) == 0)
-		return (NULL);
+	if ((client_ciphers = s->session->ciphers) == NULL)
+		return NULL;
+	if ((server_ciphers = SSL_get_ciphers(s)) == NULL)
+		return NULL;
+	if (sk_SSL_CIPHER_num(client_ciphers) == 0 ||
+	    sk_SSL_CIPHER_num(server_ciphers) == 0)
+		return NULL;
 
 	buf[0] = '\0';
-	for (i = 0; i < sk_SSL_CIPHER_num(ciphers); i++) {
-		cipher = sk_SSL_CIPHER_value(ciphers, i);
+	for (i = 0; i < sk_SSL_CIPHER_num(client_ciphers); i++) {
+		cipher = sk_SSL_CIPHER_value(client_ciphers, i);
+
+		if (sk_SSL_CIPHER_find(server_ciphers, cipher) < 0)
+			continue;
+
 		end = buf + curlen;
 		if (strlcat(buf, cipher->name, len) >= len ||
 		    (curlen = strlcat(buf, ":", len)) >= len) {
@@ -1511,7 +1519,7 @@ SSL_get_shared_ciphers(const SSL *s, char *buf, int len)
 	/* remove trailing colon */
 	if ((end = strrchr(buf, ':')) != NULL)
 		*end = '\0';
-	return (buf);
+	return buf;
 }
 
 /*