ASN1_ITEM_SIGN(3ossl)

💖 ASN1_ITEM_SIGN(3ossl) — bestie's guide to ASN1 sign & verify 💖

okay so, welcome to the ossl manual page for ASN1_item_sign, ASN1_item_sign_ex, ASN1_item_sign_ctx, ASN1_item_verify, ASN1_item_verify_ex, and ASN1_item_verify_ctx 🎀 — the whole "sign and verify ASN.1 data" friend group. They're all about ASN1 sign and verify. sit down, grab a boba, we're doing this. ✨

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🌸 NAME 🌸

ASN1_item_sign, ASN1_item_sign_ex, ASN1_item_sign_ctx, ASN1_item_verify, ASN1_item_verify_ex, ASN1_item_verify_ctx — it's giving ASN1 sign and verify. that's the whole vibe. 💅

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✨ SYNOPSIS ✨

first things first, bestie, you gotta include the header:

#include <openssl/x509.h>

then here's the whole lineup of function signatures, verbatim, no edits, i would never:

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);

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🦋 DESCRIPTION 🦋

okay so here's the tea, function by function. 🍓

ASN1_item_sign_ex() is the one used to sign arbitrary ASN1 data. it takes a data object data, the ASN.1 structure it, a private key pkey, and a message digest md. the way the data that gets signed is formed: you take the data object in data and convert 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. ASN1_item_sign_ex() will write the correct X509_ALGOR to 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 will first be written before the signature is generated. examples of valid values you can use for the ASN.1 structure it are ASN1_ITEM_rptr(X509_CINF), ASN1_ITEM_rptr(X509_REQ_INFO), and ASN1_ITEM_rptr(X509_CRL_INFO). the OSSL_LIB_CTX specified in libctx and the property query string specified in props are used when searching for algorithms in providers. the generated signature is 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. noted. 🫶

ASN1_item_sign() is similar to ASN1_item_sign_ex() but it uses default values of NULL for the id, libctx and propq. lowkey the chill version. 💜

ASN1_item_sign_ctx() is similar to ASN1_item_sign() but it uses the parameters contained in the digest context ctx. it's giving "i already have a ctx, don't make me build another one". 🌈

ASN1_item_verify_ex() is the one used to verify the signature signature of internal data data using the public key pkey and algorithm identifier alg. the data that is 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 specified in libctx and the property query string specified in props 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. slay. ✨

ASN1_item_verify() is similar to ASN1_item_verify_ex() but it uses default values of NULL for the id, libctx and propq.

ASN1_item_verify_ctx() is similar to ASN1_item_verify() but it uses the parameters contained in the digest context ctx. 🧚

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👑 RETURN VALUES 👑

all the sign functions return the size of the signature in bytes on success, and zero on failure. so like, if you get a zero, that's a no. 💀

all the verify functions return 1 if the signature is valid and 0 if the signature check fails. but wait — if the signature could not be checked at all because it was ill-formed or some other error occurred, then -1 is returned. three possible moods, bestie, choose wisely. 🔥

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🎀 EXAMPLES 🎀

okay this is the fun part, a whole real example. in the following example a 'MyObject' object is signed using the key contained in an EVP_MD_CTX. the signature is written to MyObject.signature. the object is then output in DER format and then loaded back in and verified. it's a whole round trip, main character energy fr fr. 🌟

#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;
}

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🔍 SEE ALSO 🔍

go read these too, i'm obsessed with cross-referencing:

• X509_sign(3)
• X509_verify(3)

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💜 HISTORY 💜

• ASN1_item_sign_ex() and ASN1_item_verify_ex() were added in OpenSSL 3.0. the ex's are the new kids, periodt. ⭐

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📜 COPYRIGHT 📜

Copyright 2020-2022 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>.

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and that's the whole page, bestie 🫶 this was the OpenSSL 3.5.9 version, dated 2026-09-29, from the ASN1_ITEM_SIGN(3ossl) manual. go forth and sign & verify responsibly, i believe in you, no cap 💖✨🎀