ASN1_ITEM_SIGN(3SSL)
💖 ASN1_ITEM_SIGN 💖
okay bestie, this is the ASN1_ITEM_SIGN(3SSL) page — we're in the OpenSSL manuals, section 3SSL, and today's topic is ASN1 sign and verify. yes, signing. verifying. the whole cryptographic glow-up. let's go 🫶
✨ NAME ✨
ASN1_item_sign, ASN1_item_sign_ex, ASN1_item_sign_ctx, ASN1_item_verify, ASN1_item_verify_ex, ASN1_item_verify_ctx — literally just ASN1 sign and verify, no cap 💅
🌸 SYNOPSIS 🌸
okay so first you gotta bring the header in, then it's function time:
#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 🦋
signing girlies ✍️
ASN1_item_sign_ex() is the one that signs arbitrary ASN1 data — you hand it a data object data, the ASN.1 structure it, a private key pkey, and a message digest md. the way it works is: it takes the data object in data and converts it to DER format using the ASN.1 type structure it. so data + it = the actual bytes getting signed, periodt.
things you can drop into it, for example: ASN1_ITEM_rptr(X509_CINF), ASN1_ITEM_rptr(X509_REQ_INFO), and ASN1_ITEM_rptr(X509_CRL_INFO). iconic lineup honestly.
signature, algor1 and algor2 are basically output parameters. the generated signature gets set into signature, and that thing must have been allocated beforehand, e.g. by using ASN1_BIT_STRING_new(). it can also happen indirectly, like if you have an ASN.1 structure such as X509 that already includes a non-optional signature field of type ASN1_BIT_STRING. no pre-allocation, no vibes, it just won't work 💀
the algor1 and algor2 parameters are optional, and they let you output up to two X509_ALGOR values representing the signature algorithm (typically including a digest algorithm, and/or possibly other parameters). why two? because structures like X509 have to contain the exact same signature algorithm info in two places: sig_alg and cert_info.signature. that's it, that's the reason.
each of the algor1 and algor2 pointers may be NULL. if they're not NULL, they must have been allocated beforehand — e.g. using X509_ALGOR_new(), or indirectly like for the X509 fields sig_alg and cert_info.signature — and then ASN1_item_sign_ex() sets the signature algorithm into it. important timing detail: this happens before generating the signature value and setting it into signature. order matters, bestie. 🎀
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 it can be set for special key types. see EVP_PKEY_CTX_set1_id() for further info if you want the deep lore.
ASN1_item_sign() is similar to ASN1_item_sign_ex() but uses default values of NULL for the id, libctx and propq. the chill, no-config version, we love her.
ASN1_item_sign_ctx() is similar to ASN1_item_sign() but uses the parameters contained in digest context ctx. everything already packed in a context? she's ready. 💜
verifying girlies ✅
ASN1_item_verify_ex() is used to verify the signature signature of internal data data using the public key pkey and 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. same deal as signing, other direction.
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 it can be set for special key types. see EVP_PKEY_CTX_set1_id() for further info. yes, same as above, she's consistent 💅
ASN1_item_verify() is similar to ASN1_item_verify_ex() but uses default values of NULL for the id, libctx and propq.
ASN1_item_verify_ctx() is similar to ASN1_item_verify() but uses the parameters contained in digest context ctx.
🎀 RETURN VALUES 🎀
all the sign functions return the size of the signature in bytes on success, and zero for failure. bytes, bestie, count them 📏
all the verify functions return 1 if the signature is valid, and 0 if the signature check fails. and if the signature could not be checked at all because it was ill-formed or some other error occurred, then -1 is returned. so like: 1 = slay, 0 = nope, -1 = we literally couldn't even look at it 😭
🌈 EXAMPLES 🌈
okay so here's the whole scenario: a 'MyObject' object gets signed using the key contained in an EVP_MD_CTX. the signature gets written to MyObject.signature. then the object is output in DER format, loaded back in, and verified. full circle moment 🔥
#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;
}not me putting the entire lifecycle in one function and it actually working 🙈
👑 SEE ALSO 👑
X509_sign(3), X509_verify(3) — go read those too, they're the cousins 🫶
⭐ HISTORY ⭐
ASN1_item_sign_ex() and ASN1_item_verify_ex() were added in OpenSSL 3.0. the glow-up era ✨
🧚 COPYRIGHT 🧚
Copyright 2020-2025 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>.
and that's the whole page bestie 💖 this was ASN1_ITEM_SIGN(3SSL), version 4.0.3, dated 2026-09-29, from the OpenSSL crew — mwah, take care of your keys, hydrate, and always check your return values 😭💅🍓
📦 where this page comes from
- ubuntu stonking: libssl-doc 4.0.3-1ubuntu1 · Apache-2.0
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on : a language model's rewrite of the original, under its terms