Add missing accessors for X509 AuthorityKeyIdentifier

Complements commit b383aa208146, which added X509_get0_authority_key_id().

 const ASN1_OCTET_STRING *X509_get0_authority_key_id(X509 *x);
 const GENERAL_NAMES *X509_get0_authority_issuer(X509 *x);      [NEW]
 const ASN1_INTEGER *X509_get0_authority_serial(X509 *x);       [NEW]

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9493)

crypto/x509/v3_purp.c

diff --git a/crypto/x509/v3_purp.c b/crypto/x509/v3_purp.c
index d0ac7e6..40f976b 100644
--- a/crypto/x509/v3_purp.c
+++ b/crypto/x509/v3_purp.c
@@ -890,6 +890,20 @@
     return (x->akid != NULL ? x->akid->keyid : NULL);
 }
 
+const GENERAL_NAMES *X509_get0_authority_issuer(X509 *x)
+{
+    /* Call for side-effect of computing hash and caching extensions */
+    X509_check_purpose(x, -1, -1);
+    return (x->akid != NULL ? x->akid->issuer : NULL);
+}
+
+const ASN1_INTEGER *X509_get0_authority_serial(X509 *x)
+{
+    /* Call for side-effect of computing hash and caching extensions */
+    X509_check_purpose(x, -1, -1);
+    return (x->akid != NULL ? x->akid->serial : NULL);
+}
+
 long X509_get_pathlen(X509 *x)
 {
     /* Called for side effect of caching extensions */

doc/man3/X509_get_extension_flags.pod

diff --git a/doc/man3/X509_get_extension_flags.pod b/doc/man3/X509_get_extension_flags.pod
index 0c80796..34f811c 100644
--- a/doc/man3/X509_get_extension_flags.pod
+++ b/doc/man3/X509_get_extension_flags.pod
@@ -4,6 +4,8 @@
 
 X509_get0_subject_key_id,
 X509_get0_authority_key_id,
+X509_get0_authority_issuer,
+X509_get0_authority_serial,
 X509_get_pathlen,
 X509_get_extension_flags,
 X509_get_key_usage,
@@ -22,6 +24,8 @@
  uint32_t X509_get_extended_key_usage(X509 *x);
  const ASN1_OCTET_STRING *X509_get0_subject_key_id(X509 *x);
  const ASN1_OCTET_STRING *X509_get0_authority_key_id(X509 *x);
+ const GENERAL_NAMES *X509_get0_authority_issuer(X509 *x);
+ const ASN1_INTEGER *X509_get0_authority_serial(X509 *x);
  void X509_set_proxy_flag(X509 *x);
  void X509_set_proxy_pathlen(int l);
  long X509_get_proxy_pathlen(X509 *x);
@@ -115,6 +119,14 @@
 identifier of B<x> as an B<ASN1_OCTET_STRING> or B<NULL> if the extension
 is not present or cannot be parsed.
 
+X509_get0_authority_issuer() returns an internal pointer to the authority
+certificate issuer of B<x> as a stack of B<GENERAL_NAME> structures or
+B<NULL> if the extension is not present or cannot be parsed.
+
+X509_get0_authority_serial() returns an internal pointer to the authority
+certificate serial number of B<x> as an B<ASN1_INTEGER> or B<NULL> if the
+extension is not present or cannot be parsed.
+
 X509_set_proxy_flag() marks the certificate with the B<EXFLAG_PROXY> flag.
 This is for the users who need to mark non-RFC3820 proxy certificates as
 such, as OpenSSL only detects RFC3820 compliant ones.

include/openssl/x509v3.h

diff --git a/include/openssl/x509v3.h b/include/openssl/x509v3.h
index a743e59..bd60fcc 100644
--- a/include/openssl/x509v3.h
+++ b/include/openssl/x509v3.h
@@ -661,6 +661,8 @@
 uint32_t X509_get_extended_key_usage(X509 *x);
 const ASN1_OCTET_STRING *X509_get0_subject_key_id(X509 *x);
 const ASN1_OCTET_STRING *X509_get0_authority_key_id(X509 *x);
+const GENERAL_NAMES *X509_get0_authority_issuer(X509 *x);
+const ASN1_INTEGER *X509_get0_authority_serial(X509 *x);
 
 int X509_PURPOSE_get_count(void);
 X509_PURPOSE *X509_PURPOSE_get0(int idx);

util/libcrypto.num

diff --git a/util/libcrypto.num b/util/libcrypto.num
index 63cab32..e823aba 100644
--- a/util/libcrypto.num
+++ b/util/libcrypto.num
@@ -4700,3 +4700,5 @@
 ERR_set_debug                           4810	3_0_0	EXIST::FUNCTION:
 ERR_set_error                           4811	3_0_0	EXIST::FUNCTION:
 ERR_vset_error                          4812	3_0_0	EXIST::FUNCTION:
+X509_get0_authority_issuer              4813	3_0_0	EXIST::FUNCTION:
+X509_get0_authority_serial              4814	3_0_0	EXIST::FUNCTION: