- NAME
- SYNOPSIS
- DESCRIPTION
- NOTES
- SUPPORTED EXTENSIONS
  - PKIX Certificate Extensions
  - Netscape Certificate Extensions
  - Miscellaneous Certificate Extensions
  - PKIX CRL Extensions
  - OCSP Extensions
  - Certificate Transparency Extensions
- RETURN VALUES
- SEE ALSO
- HISTORY
- COPYRIGHT

# NAME

X509V3_get_d2i, X509V3_add1_i2d, X509V3_EXT_d2i, X509V3_EXT_i2d, X509_get_ext_d2i, X509_add1_ext_i2d, X509_ACERT_get_ext_d2i, X509_ACERT_add1_ext_i2d, X509_CRL_get_ext_d2i, X509_CRL_add1_ext_i2d, X509_REVOKED_get_ext_d2i, X509_REVOKED_add1_ext_i2d, X509_get0_extensions, X509_ACERT_get0_extensions, X509_CRL_get0_extensions, X509_REVOKED_get0_extensions - X509 extension decode and encode functions

# SYNOPSIS

    #include <openssl/x509v3.h>

    void *X509V3_get_d2i(const STACK_OF(X509_EXTENSION) *x, int nid, int *crit,
                         int *idx);
    int X509V3_add1_i2d(STACK_OF(X509_EXTENSION) **x, int nid, void *value,
                        int crit, unsigned long flags);

    void *X509V3_EXT_d2i(const X509_EXTENSION *ext);
    X509_EXTENSION *X509V3_EXT_i2d(int ext_nid, int crit, void *ext_struc);

    void *X509_get_ext_d2i(const X509 *x, int nid, int *crit, int *idx);
    int X509_add1_ext_i2d(X509 *x, int nid, void *value, int crit,
                          unsigned long flags);

    void *X509_ACERT_get_ext_d2i(const X509_ACERT *x, int nid, int *crit, int *idx);
    int X509_ACERT_add1_ext_i2d(X509_ACERT *x, int nid, void *value, int crit,
                                unsigned long flags);

    void *X509_CRL_get_ext_d2i(const X509_CRL *crl, int nid, int *crit, int *idx);
    int X509_CRL_add1_ext_i2d(X509_CRL *crl, int nid, void *value, int crit,
                              unsigned long flags);

    void *X509_REVOKED_get_ext_d2i(const X509_REVOKED *r, int nid, int *crit, int *idx);
    int X509_REVOKED_add1_ext_i2d(X509_REVOKED *r, int nid, void *value, int crit,
                                  unsigned long flags);

    const STACK_OF(X509_EXTENSION) *X509_get0_extensions(const X509 *x);
    const STACK_OF(X509_EXTENSION) *X509_ACERT_get0_extensions(const X509 *x);
    const STACK_OF(X509_EXTENSION) *X509_CRL_get0_extensions(const X509_CRL *crl);
    const STACK_OF(X509_EXTENSION) *X509_REVOKED_get0_extensions(const X509_REVOKED *r);

# DESCRIPTION

X509V3_get_d2i() looks for an extension with OID nid in the extensions x and, if found, decodes it. If idx is NULL then only one occurrence of an extension is permissible, otherwise the first extension after index *idx is returned and *idx updated to the location of the extension. If crit is not NULL then *crit is set to a status value: -2 if the extension occurs multiple times (this is only returned if idx is NULL), -1 if the extension could not be found, 0 if the extension is found and is not critical and 1 if critical. A pointer to an extension specific structure or NULL is returned.

X509V3_add1_i2d() adds extension value to STACK *x (allocating a new STACK if necessary) using OID nid and criticality crit according to flags.

X509V3_EXT_d2i() attempts to decode the ASN.1 data contained in extension ext and returns a pointer to an extension specific structure or NULL if the extension could not be decoded (invalid syntax or not supported).

X509V3_EXT_i2d() encodes the extension specific structure ext_struc with OID ext_nid and criticality crit.

X509_get_ext_d2i() and X509_add1_ext_i2d() operate on the extensions of certificate x. They are otherwise identical to X509V3_get_d2i() and X509V3_add1_i2d().

X509_ACERT_get_ext_d2i() and X509_ACERT_add1_ext_i2d() operate on the extensions of X509_ACERT structure x. They are otherwise identical to X509V3_get_d2i() and X509V3_add1_i2d().

X509_CRL_get_ext_d2i() and X509_CRL_add1_ext_i2d() operate on the extensions of CRL crl. They are otherwise identical to X509V3_get_d2i() and X509V3_add1_i2d().

X509_REVOKED_get_ext_d2i() and X509_REVOKED_add1_ext_i2d() operate on the extensions of X509_REVOKED structure r (i.e for CRL entry extensions). They are otherwise identical to X509V3_get_d2i() and X509V3_add1_i2d().

X509_get0_extensions(), X509_ACERT_get0_extensions(), X509_CRL_get0_extensions() and X509_REVOKED_get0_extensions() return a STACK of all the extensions of a certificate, an attribute certificate, a CRL or a CRL entry respectively.

# NOTES

In almost all cases an extension can occur at most once and multiple occurrences is an error. Therefore, the idx parameter is usually NULL.

The flags parameter may be one of the following values.

X509V3_ADD_DEFAULT appends a new extension only if the extension does not exist. An error is returned if the extension exists.

X509V3_ADD_APPEND appends a new extension, ignoring whether the extension exists.

X509V3_ADD_REPLACE replaces an existing extension. If the extension does not exist, appends a new extension.

X509V3_ADD_REPLACE_EXISTING replaces an existing extension. If the extension does not exist, returns an error.

X509V3_ADD_KEEP_EXISTING appends a new extension only if the extension does not exist. An error is not returned if the extension exists.

X509V3_ADD_DELETE deletes and frees an existing extension. If the extension does not exist, returns an error. No new extension is added.

If X509V3_ADD_SILENT is bitwise ORed with flags: any error returned will not be added to the error queue.

The function X509V3_get_d2i() and its variants will return NULL if the extension is not found, occurs multiple times or cannot be decoded. It is possible to determine the precise reason by checking the value of *crit. The returned pointer must be explicitly freed.

The function X509V3_add1_i2d() and its variants allocate X509_EXTENSION objects on STACK *x depending on flags. The X509_EXTENSION objects must be explicitly freed using X509_EXTENSION_free().

# SUPPORTED EXTENSIONS

The following sections contain a list of all supported extensions including their name and NID.

## PKIX Certificate Extensions

The following certificate extensions are defined in PKIX standards such as RFC5280.

    Basic Constraints                  NID_basic_constraints
    Key Usage                          NID_key_usage
    Extended Key Usage                 NID_ext_key_usage

    Subject Key Identifier             NID_subject_key_identifier
    Authority Key Identifier           NID_authority_key_identifier

    Private Key Usage Period           NID_private_key_usage_period

    Subject Alternative Name           NID_subject_alt_name
    Issuer Alternative Name            NID_issuer_alt_name

    Authority Information Access       NID_info_access
    Subject Information Access         NID_sinfo_access

    Name Constraints                   NID_name_constraints

    Certificate Policies               NID_certificate_policies
    Policy Mappings                    NID_policy_mappings
    Policy Constraints                 NID_policy_constraints
    Inhibit Any Policy                 NID_inhibit_any_policy

    TLS Feature                        NID_tlsfeature

## Netscape Certificate Extensions

The following are (largely obsolete) Netscape certificate extensions.

    Netscape Cert Type                 NID_netscape_cert_type
    Netscape Base Url                  NID_netscape_base_url
    Netscape Revocation Url            NID_netscape_revocation_url
    Netscape CA Revocation Url         NID_netscape_ca_revocation_url
    Netscape Renewal Url               NID_netscape_renewal_url
    Netscape CA Policy Url             NID_netscape_ca_policy_url
    Netscape SSL Server Name           NID_netscape_ssl_server_name
    Netscape Comment                   NID_netscape_comment

## Miscellaneous Certificate Extensions

    Strong Extranet ID                 NID_sxnet
    Proxy Certificate Information      NID_proxyCertInfo

## PKIX CRL Extensions

The following are CRL extensions from PKIX standards such as RFC5280.

    CRL Number                         NID_crl_number
    CRL Distribution Points            NID_crl_distribution_points
    Delta CRL Indicator                NID_delta_crl
    Freshest CRL                       NID_freshest_crl
    Invalidity Date                    NID_invalidity_date
    Issuing Distribution Point         NID_issuing_distribution_point

The following are CRL entry extensions from PKIX standards such as RFC5280.

    CRL Reason Code                    NID_crl_reason
    Certificate Issuer                 NID_certificate_issuer

## OCSP Extensions

    OCSP Nonce                         NID_id_pkix_OCSP_Nonce
    OCSP CRL ID                        NID_id_pkix_OCSP_CrlID
    Acceptable OCSP Responses          NID_id_pkix_OCSP_acceptableResponses
    OCSP No Check                      NID_id_pkix_OCSP_noCheck
    OCSP Archive Cutoff                NID_id_pkix_OCSP_archiveCutoff
    OCSP Service Locator               NID_id_pkix_OCSP_serviceLocator
    Hold Instruction Code              NID_hold_instruction_code

## Certificate Transparency Extensions

The following extensions are used by certificate transparency, RFC6962

    CT Precertificate SCTs             NID_ct_precert_scts
    CT Certificate SCTs                NID_ct_cert_scts

# RETURN VALUES

X509V3_get_d2i(), its variants, and X509V3_EXT_d2i() return a pointer to an extension specific structure or NULL if an error occurs.

X509V3_add1_i2d() and its variants return 1 if the operation is successful and 0 if it fails due to a non-fatal error (extension not found, already exists, cannot be encoded) or -1 due to a fatal error such as a memory allocation failure.

X509V3_EXT_i2d() returns a pointer to an X509_EXTENSION structure or NULL if an error occurs.

X509_get0_extensions(), X509_CRL_get0_extensions() and X509_REVOKED_get0_extensions() return a stack of extensions. They return NULL if no extensions are present.

# SEE ALSO

d2i_X509(3), ERR_get_error(3), X509_CRL_get0_by_serial(3), X509_get0_signature(3), X509_get_ext_d2i(3), X509_get_extension_flags(3), X509_get_pubkey(3), X509_get_subject_name(3), X509_get_version(3), X509_NAME_add_entry_by_txt(3), X509_NAME_ENTRY_get_object(3), X509_NAME_get_index_by_NID(3), X509_NAME_print_ex(3), X509_new(3), X509_sign(3), X509_verify_cert(3)

# HISTORY

X509_ACERT_get_ext_d2i(), X509_ACERT_add1_ext_i2d(), X509_ACERT_get0_extensions() were added in OpenSSL 3.4.

# COPYRIGHT

Copyright 2015-2026 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at https://www.openssl.org/source/license.html.