ASN1_ITEM_SIGN(3)

NAME
ASN1_item_sign, ASN1_item_sign_ex, ASN1_item_sign_ctx, ASN1_item_verify, ASN1_item_verify_ex, ASN1_item_verify_ctx - ASN1 sign and verify

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
ASN1_item_sign_ex() is used to sign arbitrary ASN1 data using a data object data, the ASN.1 structure it, private key pkey, and message digest md. The data to be signed is formed by taking the data object in data and converting it to DER format using the ASN.1 type structure it. Examples of values that can be used for it are ASN1_ITEM_rptr(X509_CINF), ASN1_ITEM_rptr(X509_REQ_INFO), and ASN1_ITEM_rptr(X509_CRL_INFO).

signature, <algor1>, and algor2 are essentially output parameters. The generated signature is set into signature, which must have been allocated beforehand, e.g., using ASN1_BIT_STRING_new(). This may happen indirectly, for instance by having an ASN.1 structure like X509 that includes a non-optional signature field of type ASN1_BIT_STRING. The <algor1> and algor2 parameters are optional and support outputting up to two X509_ALGOR values representing the signature algorithm (typically including a digest algorithm, and/or possibly other parameters), which is useful for structures like X509 that must contain the same signature algorithm information in two places: sig_alg and cert_info.signature. Each of the <algor1> and algor2 pointers may be NULL, otherwise they must have been allocated beforehand (e.g., using X509_ALGOR_new(), or indirectly like for the X509fields sig_alg and cert_info.signature), and the ASN1_item_sign_ex() function sets into it the signature algorithm. This is done before generating the signature value and setting it into signature.

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.

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

ASN1_item_sign_ctx() is similar to ASN1_item_sign() but uses the parameters contained in digest context ctx.

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 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.

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 sign functions return the size of the signature in bytes for success and zero for 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.

EXAMPLES
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.

 #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.

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>.