ASN1_AUX_CB(3ossl)

NAME
ASN1_AUX, ASN1_PRINT_ARG, ASN1_STREAM_ARG, ASN1_aux_cb, ASN1_aux_const_cb - ASN.1 auxiliary data

SYNOPSIS
 #include <openssl/asn1t.h>

 struct ASN1_AUX_st {
     void *app_data;
     int flags;
     int ref_offset;             /* Offset of reference value */
     int ref_lock;               /* Offset to an CRYPTO_RWLOCK */
     ASN1_aux_cb *asn1_cb;
     int enc_offset;             /* Offset of ASN1_ENCODING structure */
     ASN1_aux_const_cb *asn1_const_cb; /* for ASN1_OP_I2D_ and ASN1_OP_PRINT_ */
 };
 typedef struct ASN1_AUX_st ASN1_AUX;

 struct ASN1_PRINT_ARG_st {
     BIO *out;
     int indent;
     const ASN1_PCTX *pctx;
 };
 typedef struct ASN1_PRINT_ARG_st ASN1_PRINT_ARG;

 struct ASN1_STREAM_ARG_st {
     BIO *out;
     BIO *ndef_bio;
     unsigned char **boundary;
 };
 typedef struct ASN1_STREAM_ARG_st ASN1_STREAM_ARG;

 typedef int ASN1_aux_cb(int operation, ASN1_VALUE **in, const ASN1_ITEM *it,
                         void *exarg);
 typedef int ASN1_aux_const_cb(int operation, const ASN1_VALUE **in,
                               const ASN1_ITEM *it, void *exarg);

DESCRIPTION
ASN.1 data structures can be associated with an ASN1_AUX object to supply additional information about the ASN.1 structure. An ASN1_AUX structure is associated with the structure during the definition of the ASN.1 template. For example an ASN1_AUX structure will be associated by using one of the various ASN.1 template definition macros that supply auxiliary information such as ASN1_SEQUENCE_enc(), ASN1_SEQUENCE_ref(), ASN1_SEQUENCE_cb_const_cb(), ASN1_SEQUENCE_const_cb(), ASN1_SEQUENCE_cb() or ASN1_NDEF_SEQUENCE_cb().

An ASN1_AUX structure contains the following information.

app_data
    Arbitrary application data

flags
    Flags which indicate the auxiliary functionality supported.

    The ASN1_AFLG_REFCOUNT flag indicates that objects support reference counting.

    The ASN1_AFLG_ENCODING flag indicates that the original encoding of the object will be saved.

    The ASN1_AFLG_BROKEN flag is a work around for broken encoders where the sequence length value may not be correct. This should generally not be used.

    The ASN1_AFLG_CONST_CB flag indicates that the "const" form of the ASN1_AUX callback should be used in preference to the non-const form.

ref_offset
    If the ASN1_AFLG_REFCOUNT flag is set then this value is assumed to be an offset into the ASN1_VALUE structure where a CRYPTO_REF_COUNT may be found for the purposes of reference counting.

ref_lock
    If the ASN1_AFLG_REFCOUNT flag is set then this value is assumed to be an offset into the ASN1_VALUE structure where a CRYPTO_RWLOCK may be found for the purposes of reference counting.

asn1_cb
    A callback that will be invoked at various points during the processing of the ASN1_VALUE. See below for further details.

enc_offset
    Offset into the ASN1_VALUE object where the original encoding of the object will be saved if the ASN1_AFLG_ENCODING flag has been set.

asn1_const_cb
    A callback that will be invoked at various points during the processing of the ASN1_VALUE. This is used in preference to the asn1_cb callback if the ASN1_AFLG_CONST_CB flag is set. See below for further details.

During the processing of an ASN1_VALUE object the callbacks set via asn1_cb or asn1_const_cb will be invoked as a result of various events indicated via the operation parameter. The value of *in will be the ASN1_VALUE object being processed based on the template in it. An additional operation specific parameter may be passed in exarg. The currently supported operations are as follows. Unless noted otherwise below, the callbacks should return a positive value on success and zero on error; some operations recognise additional return values, and a few do not consult the return value at all.

ASN1_OP_NEW_PRE
    Invoked when processing a CHOICE, SEQUENCE or NDEF_SEQUENCE structure prior to an ASN1_VALUE object being allocated. The callback may allocate the ASN1_VALUE itself and store it in *pval. If it does so it should return 2 from the callback. On error it should return 0.

ASN1_OP_NEW_POST
    Invoked when processing a CHOICE, SEQUENCE or NDEF_SEQUENCE structure after an ASN1_VALUE object has been allocated. The allocated object is in *pval.

ASN1_OP_FREE_PRE
    Invoked when processing a CHOICE, SEQUENCE or NDEF_SEQUENCE structure immediately before an ASN1_VALUE is freed. If the callback originally constructed the ASN1_VALUE via ASN1_OP_NEW_PRE then it should free it at this point and return 2; the caller will then skip its normal freeing. Any other return value (including zero) causes the caller to proceed with normal freeing; the hook cannot signal an error.

ASN1_OP_FREE_POST
    Invoked when processing a CHOICE, SEQUENCE or NDEF_SEQUENCE structure immediately after ASN1_VALUE sub-structures are freed. The caller does not consult the return value from this hook.

ASN1_OP_D2I_PRE
    Invoked when processing a CHOICE, SEQUENCE or NDEF_SEQUENCE structure immediately before a "d2i" operation for the ASN1_VALUE.

ASN1_OP_D2I_POST
    Invoked when processing a CHOICE, SEQUENCE or NDEF_SEQUENCE structure immediately after a "d2i" operation for the ASN1_VALUE.

ASN1_OP_I2D_PRE
    Invoked when processing a CHOICE, SEQUENCE or NDEF_SEQUENCE structure immediately before a "i2d" operation for the ASN1_VALUE.

ASN1_OP_I2D_POST
    Invoked when processing a CHOICE, SEQUENCE or NDEF_SEQUENCE structure immediately after a "i2d" operation for the ASN1_VALUE.

ASN1_OP_PRINT_PRE
    Invoked when processing a SEQUENCE or NDEF_SEQUENCE structure immediately before printing the ASN1_VALUE. The exarg argument will be a pointer to an ASN1_PRINT_ARG structure (see below). If the callback has fully printed the value itself it should return 2; the caller will then skip the per-field printing loop and the matching ASN1_OP_PRINT_POST callback. Return zero on error or any other positive value to continue with normal printing.

ASN1_OP_PRINT_POST
    Invoked when processing a SEQUENCE or NDEF_SEQUENCE structure immediately after printing the ASN1_VALUE. The exarg argument will be a pointer to an ASN1_PRINT_ARG structure (see below).

ASN1_OP_STREAM_PRE
    Invoked immediately prior to streaming the ASN1_VALUE data using indefinite length encoding. The exarg argument will be a pointer to a ASN1_STREAM_ARG structure (see below).

ASN1_OP_STREAM_POST
    Invoked immediately after streaming the ASN1_VALUE data using indefinite length encoding. The exarg argument will be a pointer to a ASN1_STREAM_ARG structure (see below).

ASN1_OP_DETACHED_PRE
    Invoked immediately prior to processing the ASN1_VALUE data as a "detached" value (as used in CMS and PKCS7). The exarg argument will be a pointer to a ASN1_STREAM_ARG structure (see below).

ASN1_OP_DETACHED_POST
    Invoked immediately after processing the ASN1_VALUE data as a "detached" value (as used in CMS and PKCS7). The exarg argument will be a pointer to a ASN1_STREAM_ARG structure (see below).

ASN1_OP_DUP_PRE
    Invoked immediate prior to an ASN1_VALUE being duplicated via a call to ASN1_item_dup().

ASN1_OP_DUP_POST
    Invoked immediate after to an ASN1_VALUE has been duplicated via a call to ASN1_item_dup().

ASN1_OP_GET0_LIBCTX
    Invoked in order to obtain the OSSL_LIB_CTX associated with an ASN1_VALUE if any. A pointer to an OSSL_LIB_CTX should be stored in *exarg if such a value exists.

ASN1_OP_GET0_PROPQ
    Invoked in order to obtain the property query string associated with an ASN1_VALUE if any. A pointer to the property query string should be stored in *exarg if such a value exists.

An ASN1_PRINT_ARG object is used during processing of ASN1_OP_PRINT_PRE and ASN1_OP_PRINT_POST callback operations. It contains the following information.

out The BIO being used to print the data out.

ndef_bio
    The current number of indent spaces that should be used for printing this data.

pctx
    The context for the ASN1_PCTX operation.

An ASN1_STREAM_ARG object is used during processing of ASN1_OP_STREAM_PRE, ASN1_OP_STREAM_POST, ASN1_OP_DETACHED_PRE and ASN1_OP_DETACHED_POST callback operations. It contains the following information.

out The BIO to stream through

ndef_bio
    The BIO with filters appended

boundary
    The streaming I/O boundary.

RETURN VALUES
In general the callbacks return zero on error and a positive value on success. Several operations have additional or different return-value semantics, summarised here:

•   ASN1_OP_NEW_PRE recognises a return of 2, meaning that the callback has allocated the ASN1_VALUE itself and normal allocation should be skipped.

•   ASN1_OP_FREE_PRE recognises a return of 2, meaning that the callback has freed the ASN1_VALUE itself and normal freeing should be skipped. Other return values (including zero) cause normal freeing to proceed; the hook cannot signal an error.

•   ASN1_OP_FREE_POST's return value is not consulted by the caller.

•   ASN1_OP_PRINT_PRE recognises a return of 2, meaning that the callback has printed the value itself; the caller will skip the per-field printing loop and the matching ASN1_OP_PRINT_POST invocation.

•   ASN1_OP_STREAM_PRE, ASN1_OP_STREAM_POST, ASN1_OP_DETACHED_PRE, and ASN1_OP_DETACHED_POST treat any non-positive return value (zero or negative) as an error.

SEE ALSO
ASN1_item_new_ex(3)

HISTORY
The ASN1_aux_const_cb() callback and the ASN1_OP_GET0_LIBCTX and ASN1_OP_GET0_PROPQ operation types were added in OpenSSL 3.0.

COPYRIGHT
Copyright 2021-2026 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>.