Cryptography and SSL/TLS Toolkit



d2i_X509, i2d_X509, d2i_X509_bio, d2i_X509_fp, i2d_X509_bio, i2d_X509_fp - X509 encode and decode functions


 #include <openssl/x509.h>

 X509 *d2i_X509(X509 **px, const unsigned char **in, int len);
 int i2d_X509(X509 *x, unsigned char **out);

 X509 *d2i_X509_bio(BIO *bp, X509 **x);
 X509 *d2i_X509_fp(FILE *fp, X509 **x);

 int i2d_X509_bio(BIO *bp, X509 *x);
 int i2d_X509_fp(FILE *fp, X509 *x);

 int i2d_re_X509_tbs(X509 *x, unsigned char **out);


The X509 encode and decode routines encode and parse an X509 structure, which represents an X509 certificate.

d2i_X509() attempts to decode len bytes at *in. If successful a pointer to the X509 structure is returned. If an error occurred then NULL is returned. If px is not NULL then the returned structure is written to *px. If *px is not NULL then it is assumed that *px contains a valid X509 structure and an attempt is made to reuse it. This "reuse" capability is present for historical compatibility but its use is strongly discouraged (see BUGS below, and the discussion in the RETURN VALUES section).

If the call is successful *in is incremented to the byte following the parsed data.

i2d_X509() encodes the structure pointed to by x into DER format. If out is not NULL is writes the DER encoded data to the buffer at *out, and increments it to point after the data just written. If the return value is negative an error occurred, otherwise it returns the length of the encoded data.

If *out is NULL memory will be allocated for a buffer and the encoded data written to it. In this case *out is not incremented and it points to the start of the data just written.

d2i_X509_bio() is similar to d2i_X509() except it attempts to parse data from BIO bp.

d2i_X509_fp() is similar to d2i_X509() except it attempts to parse data from FILE pointer fp.

i2d_X509_bio() is similar to i2d_X509() except it writes the encoding of the structure x to BIO bp and it returns 1 for success and 0 for failure.

i2d_X509_fp() is similar to i2d_X509() except it writes the encoding of the structure x to BIO bp and it returns 1 for success and 0 for failure.

i2d_re_X509_tbs() is similar to i2d_X509() except it encodes only the TBSCertificate portion of the certificate.


The letters i and d in for example i2d_X509 stand for "internal" (that is an internal C structure) and "DER". So i2d_X509 converts from internal to DER. The "re" in i2d_re_X509_tbs stands for "re-encode", and ensures that a fresh encoding is generated in case the object has been modified after creation (see the BUGS section).

The functions can also understand BER forms.

The actual X509 structure passed to i2d_X509() must be a valid populated X509 structure it can not simply be fed with an empty structure such as that returned by X509_new().

The encoded data is in binary form and may contain embedded zeroes. Therefore any FILE pointers or BIOs should be opened in binary mode. Functions such as strlen() will not return the correct length of the encoded structure.

The ways that *in and *out are incremented after the operation can trap the unwary. See the WARNINGS section for some common errors.

The reason for the auto increment behaviour is to reflect a typical usage of ASN1 functions: after one structure is encoded or decoded another will processed after it.


Allocate and encode the DER encoding of an X509 structure:

 int len;
 unsigned char *buf;

 buf = NULL;
 len = i2d_X509(x, &buf);
 if (len < 0)
        /* error */

Attempt to decode a buffer:

 X509 *x;
 unsigned char *buf, *p;
 int len;

 /* Something to setup buf and len */
 p = buf;
 x = d2i_X509(NULL, &p, len);

 if (x == NULL)
    /* Some error */

Alternative technique:

 X509 *x;
 unsigned char *buf, *p;
 int len;

 /* Something to setup buf and len */
 p = buf;
 x = NULL;

 if (!d2i_X509(&x, &p, len))
    /* Some error */


The use of temporary variable is mandatory. A common mistake is to attempt to use a buffer directly as follows:

 int len;
 unsigned char *buf;

 len = i2d_X509(x, NULL);
 buf = OPENSSL_malloc(len);
 if (buf == NULL)
        /* error */

 i2d_X509(x, &buf);
 /* Other stuff ... */

This code will result in buf apparently containing garbage because it was incremented after the call to point after the data just written. Also buf will no longer contain the pointer allocated by OPENSSL_malloc() and the subsequent call to OPENSSL_free() may well crash.

Another trap to avoid is misuse of the xp argument to d2i_X509():

 X509 *x;

 if (!d2i_X509(&x, &p, len))
        /* Some error */

This will probably crash somewhere in d2i_X509(). The reason for this is that the variable x is uninitialized and an attempt will be made to interpret its (invalid) value as an X509 structure, typically causing a segmentation violation. If x is set to NULL first then this will not happen.


In some versions of OpenSSL the "reuse" behaviour of d2i_X509() when *px is valid is broken and some parts of the reused structure may persist if they are not present in the new one. As a result the use of this "reuse" behaviour is strongly discouraged.

i2d_X509() will not return an error in many versions of OpenSSL, if mandatory fields are not initialized due to a programming error then the encoded structure may contain invalid data or omit the fields entirely and will not be parsed by d2i_X509(). This may be fixed in future so code should not assume that i2d_X509() will always succeed.

The encoding of the TBSCertificate portion of a certificate is cached in the X509 structure internally to improve encoding performance and to ensure certificate signatures are verified correctly in some certificates with broken (non-DER) encodings.

Any function which encodes an X509 structure such as i2d_X509(), i2d_X509_fp() or i2d_X509_bio() may return a stale encoding if the X509 structure has been modified after deserialization or previous serialization.

If, after modification, the X509 object is re-signed with X509_sign(), the encoding is automatically renewed. Otherwise, the encoding of the TBSCertificate portion of the X509 can be manually renewed by calling i2d_re_X509_tbs().


d2i_X509(), d2i_X509_bio() and d2i_X509_fp() return a valid X509 structure or NULL if an error occurs. The error code that can be obtained by ERR_get_error. If the "reuse" capability has been used with a valid X509 structure being passed in via px then the object is not freed in the event of error but may be in a potentially invalid or inconsistent state.

i2d_X509() returns the number of bytes successfully encoded or a negative value if an error occurs. The error code can be obtained by ERR_get_error.

i2d_X509_bio() and i2d_X509_fp() return 1 for success and 0 if an error occurs The error code can be obtained by ERR_get_error.


ERR_get_error X509_CRL_get0_by_serial, X509_get0_signature, X509_get_ext_d2i, X509_get_extension_flags, X509_get_pubkey, X509_get_subject_name, X509_get_version, X509_NAME_add_entry_by_txt, X509_NAME_ENTRY_get_object, X509_NAME_get_index_by_NID, X509_NAME_print_ex, X509_new, X509_sign, X509V3_get_d2i, X509_verify_cert