Add SSL_get_pending_cipher()

The existing function SSL_get_current_cipher() queries the
current session for the ciphersuite in use, but there is no way
for application code to determine what ciphersuite has been
negotiated and will be used in the future, prior to ChangeCipherState
(or the TLS 1.3 equivalent) causing the new cipher to take effect and
become visible in the session information.  Expose this information
to appropriate application callbacks to use during the handshake.

The name SSL_get_pending_cipher() was chosen for compatibility with
BoringSSL's routine of that name.

Improve the note on macro implementations in SSL_get_current_cipher.pod
while here.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4070)

4 files changed, 25 insertions, 2 deletions

diff --git a/doc/man3/SSL_get_current_cipher.pod b/doc/man3/SSL_get_current_cipher.pod
index cb7e9994a7..e5b249fd1e 100644
--- a/doc/man3/SSL_get_current_cipher.pod
+++ b/doc/man3/SSL_get_current_cipher.pod
@@ -3,13 +3,15 @@
 =head1 NAME
 
 SSL_get_current_cipher, SSL_get_cipher_name, SSL_get_cipher,
-SSL_get_cipher_bits, SSL_get_cipher_version - get SSL_CIPHER of a connection
+SSL_get_cipher_bits, SSL_get_cipher_version,
+SSL_get_pending_cipher - get SSL_CIPHER of a connection
 
 =head1 SYNOPSIS
 
  #include <openssl/ssl.h>
 
  SSL_CIPHER *SSL_get_current_cipher(const SSL *ssl);
+ SSL_CIPHER *SSL_get_pending_cipher(const SSL *ssl);
 
  const char *SSL_get_cipher_name(const SSL *s);
  const char *SSL_get_cipher(const SSL *s);
@@ -30,14 +32,28 @@ SSL_get_cipher_bits() is a
 macro to obtain the number of secret/algorithm bits used and
 SSL_get_cipher_version() returns the protocol name.
 
+SSL_get_pending_cipher() returns a pointer to an SSL_CIPHER object containing
+the description of the cipher (if any) that has been negotiated for future use
+on the connection established with the B<ssl> object, but is not yet in use.
+This may be the case during handshake processing, when control flow can be
+returned to the application via any of several callback methods.  The internal
+sequencing of handshake processing and callback invocation is not guaranteed
+to be stable from release to release, and at present only the callback set
+by SSL_CTX_set_alpn_select_cb() is guaranteed to have a non-NULL return value.
+Other callbacks may be added to this list over time.
+
 =head1 RETURN VALUES
 
 SSL_get_current_cipher() returns the cipher actually used, or NULL if
 no session has been established.
 
+SSL_get_pending_cipher() returns the cipher to be used at the next change
+of cipher suite, or NULL if no such cipher is known.
+
 =head1 NOTES
 
-These are implemented as macros.
+SSL_get_cipher, SSL_get_cipher_bits, SSL_get_cipher_version, and
+SSL_get_cipher_name are implemented as macros.
 
 =head1 SEE ALSO
 
diff --git a/include/openssl/ssl.h b/include/openssl/ssl.h
index b4c66441c2..04388817b6 100644
--- a/include/openssl/ssl.h
+++ b/include/openssl/ssl.h
@@ -1430,6 +1430,7 @@ __owur int SSL_clear(SSL *s);
 void SSL_CTX_flush_sessions(SSL_CTX *ctx, long tm);
 
 __owur const SSL_CIPHER *SSL_get_current_cipher(const SSL *s);
+__owur const SSL_CIPHER *SSL_get_pending_cipher(const SSL *s);
 __owur int SSL_CIPHER_get_bits(const SSL_CIPHER *c, int *alg_bits);
 __owur const char *SSL_CIPHER_get_version(const SSL_CIPHER *c);
 __owur const char *SSL_CIPHER_get_name(const SSL_CIPHER *c);
diff --git a/ssl/ssl_lib.c b/ssl/ssl_lib.c
index 3abb27140b..ed2113caa5 100644
--- a/ssl/ssl_lib.c
+++ b/ssl/ssl_lib.c
@@ -3613,6 +3613,11 @@ const SSL_CIPHER *SSL_get_current_cipher(const SSL *s)
     return (NULL);
 }
 
+const SSL_CIPHER *SSL_get_pending_cipher(const SSL *s)
+{
+    return s->s3->tmp.new_cipher;
+}
+
 const COMP_METHOD *SSL_get_current_compression(SSL *s)
 {
 #ifndef OPENSSL_NO_COMP
diff --git a/util/libssl.num b/util/libssl.num
index 78fb65a685..7d4c01e80f 100644
--- a/util/libssl.num
+++ b/util/libssl.num
@@ -463,3 +463,4 @@ OPENSSL_cipher_name                     463	1_1_1	EXIST::FUNCTION:
 SSL_alloc_buffers                       464	1_1_1	EXIST::FUNCTION:
 SSL_free_buffers                        465	1_1_1	EXIST::FUNCTION:
 SSL_SESSION_dup                         466	1_1_1	EXIST::FUNCTION:
+SSL_get_pending_cipher                  467	1_1_1	EXIST::FUNCTION: