X509V3_GET_D2I(3SSL)

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version 4.0.3 · 6 versions
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✨ X509V3_GET_D2I ✨

okay so bestie, this is the x509v3_get_d2i page from the 3SSL manual — yeah, it lives in the OpenSSL section, section 3, we love her 💅. It's all about decoding and encoding X509 extensions, which honestly is giving "the spicy little add-ons that live on certificates, CRLs and attribute certs" energy. Grab a boba, we're going in 🧋

🌸 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 — literally the X509 extension decode and encode functions. That's the whole girlboss squad right there 👑

✨ 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() goes hunting for an extension with OID nid inside the extensions x, and if it finds one, it decodes it, slay. Now here's the tea about idx: if idx is NULL, then only one occurrence of an extension is allowed — no multiples, we don't do that here. But if idx isn't NULL, then the first extension after index *idx gets returned and *idx gets updated to the location of that extension 📍

And if crit isn't NULL, then *crit gets set to a status value, and these numbers matter so pay attention bestie:

  • -2 — the extension occurs multiple times (this is only returned if idx is NULL)
  • -1 — the extension could not be found 😭
  • 0 — the extension is found and it's not critical
  • 1 — it's critical

Then you get back either a pointer to an extension specific structure, or NULL. That's it, that's the deal ✨

X509V3_add1_i2d() adds extension value to STACK *x (and yes, it allocates a new STACK if it needs to, she's resourceful 💅) using OID nid and criticality crit, behaving according to flags.

X509V3_EXT_d2i() tries to decode the ASN.1 data sitting inside extension ext and returns a pointer to an extension specific structure — or NULL if the extension couldn't be decoded (invalid syntax, or just not supported, rip 🙈).

X509V3_EXT_i2d() is the other direction: it encodes the extension specific structure ext_struc with OID ext_nid and criticality crit. Encoding era 💫

  • X509_get_ext_d2i() and X509_add1_ext_i2d() operate on the extensions of certificate x. Otherwise they are 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. Otherwise they are 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. Otherwise they are 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, okurrr). Otherwise they are 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. The whole collection, no filter 💅

🎀 NOTES 🎀

In almost all cases an extension can occur at most once, and multiple occurrences is an error. So yeah, the idx parameter is usually NULL. Most of us are not doing the multi-occurrence thing, periodt.

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. Yolo energy, she does not check.
  • X509V3_ADD_REPLACE — replaces an existing extension. If the extension does not exist, it appends a new extension.
  • X509V3_ADD_REPLACE_EXISTING — replaces an existing extension. If the extension does not exist, it 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. Chill vibes.
  • X509V3_ADD_DELETE — deletes and frees an existing extension. If the extension does not exist, it returns an error. No new extension is added. Bye bye 👋
  • If X509V3_ADD_SILENT is bitwise ORed with flags: any error returned will not be added to the error queue. Secretive queen 🤫

X509V3_get_d2i() and its variants will return NULL if the extension is not found, occurs multiple times, or cannot be decoded. And you can figure out the precise reason by checking the value of *crit — that's literally what it's for bestie. Oh, and the returned pointer must be explicitly freed. Like, you own it now. It's your responsibility. Don't ghost it 🫶

X509V3_add1_i2d() and its variants allocate X509_EXTENSION objects on STACK *x depending on flags. Those X509_EXTENSION objects must be explicitly freed using X509_EXTENSION_free(). Do not forget. I'm watching 👀

🌈 SUPPORTED EXTENSIONS 🌈

The following sections contain a list of all supported extensions including their name and NID. The whole menu, let's go 🍓

🦋 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. Vintage, but make it deprecated 💀

  • 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. Three possible vibes. Know the difference 😌

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 — empty stack, empty life, it happens 🥲

👑 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. The ACERT girlies are new to the group chat 💅

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>.

okay that's the whole page bestie, you're officially certified in extension decoding 🎓✨ OpenSSL 4.0.3, dated 2026-09-29 — go forth and d2i responsibly, mwah 🫶💖

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