ASN1_ITEM_SIGN(3ossl)

📦 version 3.5.0, in opensuse-leap 16.0

okay bestie, gather round, because we're talking about ASN1_ITEM_SIGN(3ossl) from OpenSSL and honestly? it's giving cryptography main character energy 💅✨. Six functions, one very specific job: sign some raw ASN.1 and then verify it back. no cap, let's get into it.

🌸 NAME 🌸

ASN1_item_sign, ASN1_item_sign_ex, ASN1_item_sign_ctx, ASN1_item_verify, ASN1_item_verify_ex, ASN1_item_verify_ctx — the ASN1 sign and verify squad 🫶

✨ SYNOPSIS ✨

#include <openssl/x509.h>

int ASN1_item_sign_ex(const ASN1_ITEM *it, X509_ALGOR *algor1,
                      X509_ALGOR *algor2, ASN1_BIT_STRING *signature,
                      const void *data, const ASN1_OCTET_STRING *id,
                      EVP_PKEY *pkey, const EVP_MD *md, OSSL_LIB_CTX *libctx,
                      const char *propq);

int ASN1_item_sign(const ASN1_ITEM *it, X509_ALGOR *algor1, X509_ALGOR *algor2,
                   ASN1_BIT_STRING *signature, const void *data,
                   EVP_PKEY *pkey, const EVP_MD *md);

int ASN1_item_sign_ctx(const ASN1_ITEM *it, X509_ALGOR *algor1,
                       X509_ALGOR *algor2, ASN1_BIT_STRING *signature,
                       const void *data, EVP_MD_CTX *ctx);

int ASN1_item_verify_ex(const ASN1_ITEM *it, const X509_ALGOR *alg,
                        const ASN1_BIT_STRING *signature, const void *data,
                        const ASN1_OCTET_STRING *id, EVP_PKEY *pkey,
                        OSSL_LIB_CTX *libctx, const char *propq);

int ASN1_item_verify(const ASN1_ITEM *it, const X509_ALGOR *alg,
                     const ASN1_BIT_STRING *signature, const void *data,
                     EVP_PKEY *pkey);

int ASN1_item_verify_ctx(const ASN1_ITEM *it, const X509_ALGOR *alg,
                         const ASN1_BIT_STRING *signature, const void *data,
                         EVP_MD_CTX *ctx);

🎀 DESCRIPTION 🎀

Three for signing, three for verifying, all working on "arbitrary ASN1 data" — iykyk. Here's the tea on each one 🍓

💖 the signing besties 💖

ASN1_item_sign_ex() — this one signs arbitrary ASN1 data, using the data object data, the ASN.1 structure it, the private key pkey and the message digest md. The data that actually gets signed is formed by taking the data object in data and converting it to der format using the ASN.1 structure it 🔥
- the data that will be signed and a structure containing the signature may both have a copy of the X509_ALGOR. This function writes the correct X509_ALGOR into the structs based on the algorithms and parameters that have been set up ✨
- if one of algor1 or algor2 points to the X509_ALGOR of the data to be signed, then that X509_ALGOR gets written first, before the signature is generated 👑
- valid values for the ASN.1 structure it include ASN1_ITEM_rptr(X509_CINF), ASN1_ITEM_rptr(X509_REQ_INFO) and ASN1_ITEM_rptr(X509_CRL_INFO)
- the OSSL_LIB_CTX in libctx and the property query string in propq are used when searching for algorithms in providers 🔍
- the generated signature gets set into signature
- the optional parameter id can be NULL, but can be set for special key types — see EVP_PKEY_CTX_set1_id() for further info
- the output parameters algor1 and algor2 are ignored if they are NULL
ASN1_item_sign() — same vibe as ASN1_item_sign_ex(), but it uses default values of NULL for the id, libctx and propq 🦋
ASN1_item_sign_ctx() — same as ASN1_item_sign() but uses the parameters contained in the digest context ctx 🧚

💜 the verifying besties 💜

ASN1_item_verify_ex() — verifies the signature signature of internal data data using the public key pkey and the algorithm identifier alg. The data that gets verified is formed by taking the data object in data and converting it to der format using the ASN.1 structure it 🌈
- the OSSL_LIB_CTX in libctx and the property query string in propq are used when searching for algorithms in providers
- the optional parameter id can be NULL, but can be set for special key types — see EVP_PKEY_CTX_set1_id() for further info
ASN1_item_verify() — same as ASN1_item_verify_ex() but uses default values of NULL for the id, libctx and propq ⭐
ASN1_item_verify_ctx() — same as ASN1_item_verify() but uses the parameters contained in the digest context ctx 🎀

🔥 RETURN VALUES 🔥

okay this part matters, don't skip it 🙈

all sign functions return the size of the signature in bytes on success, and zero on failure 📏
all verify functions return 1 if the signature is valid, and 0 if the signature check fails. If the signature could not be checked at all — because it was ill-formed or some other error occurred — then -1 is returned 💀

bestie please actually check those return values. 0 vs -1 on verify is not a drill 😭

🌈 EXAMPLES 🌈

so in this example a MyObject object gets signed using the key contained in an EVP_MD_CTX. The signature is written to MyObject.signature, then the object is output in DER format, loaded back in, and verified. iconic behaviour, honestly 👑

#include <openssl/x509.h>
#include <openssl/asn1t.h>

/* An object used to store the ASN1 data fields that will be signed */
typedef struct MySignInfoObject_st
{
    ASN1_INTEGER *version;
    X509_ALGOR sig_alg;
} MySignInfoObject;

DECLARE_ASN1_FUNCTIONS(MySignInfoObject)
/*
 * A higher level object containing the ASN1 fields, signature alg and
 * output signature.
 */
typedef struct MyObject_st
{
    MySignInfoObject info;
    X509_ALGOR sig_alg;
    ASN1_BIT_STRING *signature;
} MyObject;

DECLARE_ASN1_FUNCTIONS(MyObject)

/* The ASN1 definition of MySignInfoObject */
ASN1_SEQUENCE_cb(MySignInfoObject, NULL) = {
    ASN1_SIMPLE(MySignInfoObject, version, ASN1_INTEGER)
    ASN1_EMBED(MySignInfoObject, sig_alg, X509_ALGOR),
} ASN1_SEQUENCE_END_cb(MySignInfoObject, MySignInfoObject)

/* new, free, d2i & i2d functions for MySignInfoObject */
IMPLEMENT_ASN1_FUNCTIONS(MySignInfoObject)

/* The ASN1 definition of MyObject */
ASN1_SEQUENCE_cb(MyObject, NULL) = {
    ASN1_EMBED(MyObject, info, MySignInfoObject),
    ASN1_EMBED(MyObject, sig_alg, X509_ALGOR),
    ASN1_SIMPLE(MyObject, signature, ASN1_BIT_STRING)
} ASN1_SEQUENCE_END_cb(MyObject, MyObject)

/* new, free, d2i & i2d functions for MyObject */
IMPLEMENT_ASN1_FUNCTIONS(MyObject)

int test_asn1_item_sign_verify(const char *mdname, EVP_PKEY *pkey, long version)
{
   int ret = 0;
   unsigned char *obj_der = NULL;
   const unsigned char *p = NULL;
   MyObject *obj = NULL, *loaded_obj = NULL;
   const ASN1_ITEM *it = ASN1_ITEM_rptr(MySignInfoObject);
   EVP_MD_CTX *sctx = NULL, *vctx = NULL;
   int len;

   /* Create MyObject and set its version */
   obj = MyObject_new();
   if (obj == NULL)
       goto err;
   if (!ASN1_INTEGER_set(obj->info.version, version))
       goto err;

   /* Set the key and digest used for signing */
   sctx = EVP_MD_CTX_new();
   if (sctx == NULL
       || !EVP_DigestSignInit_ex(sctx, NULL, mdname, NULL, NULL, pkey))
       goto err;

   /*
    * it contains the mapping between ASN.1 data and an object MySignInfoObject
    * obj->info is the 'MySignInfoObject' object that will be
    *   converted into DER data and then signed.
    * obj->signature will contain the output signature.
    * obj->sig_alg is filled with the private key's signing algorithm id.
    * obj->info.sig_alg is another copy of the signing algorithm id that sits
    * within MyObject.
    */
   len = ASN1_item_sign_ctx(it, &obj->sig_alg, &obj->info.sig_alg,
                            obj->signature, &obj->info, sctx);
   if (len <= 0
       || X509_ALGOR_cmp(&obj->sig_alg, &obj->info.sig_alg) != 0)
       goto err;

   /* Output MyObject in der form */
   len = i2d_MyObject(obj, &obj_der);
   if (len <= 0)
       goto err;

   /* Set the key and digest used for verifying */
   vctx = EVP_MD_CTX_new();
   if (vctx == NULL
       || !EVP_DigestVerifyInit_ex(vctx, NULL, mdname, NULL, NULL, pkey))
       goto err;

   /* Load the der data back into an object */
   p = obj_der;
   loaded_obj = d2i_MyObject(NULL, &p, len);
   if (loaded_obj == NULL)
       goto err;
   /* Verify the loaded object */
   ret = ASN1_item_verify_ctx(it, &loaded_obj->sig_alg, loaded_obj->signature,
                              &loaded_obj->info, vctx);
err:
   OPENSSL_free(obj_der);
   MyObject_free(loaded_obj);
   MyObject_free(obj);
   EVP_MD_CTX_free(sctx);
   EVP_MD_CTX_free(vctx);
   return ret;
}

👀 SEE ALSO 👀

X509_sign(3)
X509_verify(3)

🕰️ HISTORY 🕰️

ASN1_item_sign_ex() and ASN1_item_verify_ex() were added in OpenSSL 3.0 ✨ (yes the _ex girlies are the new kids, we love a glow-up)

📜 COPYRIGHT 📜

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>.

and that's the whole page bestie 🫶💖 mwah. this was version 3.5.0, dated 2026-09-25, ASN1_ITEM_SIGN(3ossl) — go sign something and verify it, slay 🌸

📦 where this page comes from

opensuse-leap 16.0: openssl-3-doc 3.5.0-160000.11.1 · Apache-2.0
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms