NAME

ASN1_put_object, ASN1_put_eoc, ASN1_object_size —

start and end the BER encoding of an arbitrary ASN.1 data element

SYNOPSIS

#include <openssl/asn1.h>

void
ASN1_put_object(unsigned char **ber_out, int constructed, int content_length, int tag, int class);

int
ASN1_put_eoc(unsigned char **ber_out);

int
ASN1_object_size(int constructed, int content_length, int tag);

DESCRIPTION

ASN1_put_object() begins writing the BER encoding of an arbitrary ASN.1 data element to the buffer *ber_out by writing the identifier and the length bytes. Making sure that there is sufficient space in the buffer is the responsibility of the caller. This function does not write any content bytes nor any end-of-content bytes.

The tag class can be V_ASN1_UNIVERSAL, V_ASN1_APPLICATION, V_ASN1_CONTEXT_SPECIFIC, or V_ASN1_PRIVATE and is written to the two most significant bits of the first byte written.

The constructed argument can have the following values:

0: Start a primitive value by setting the third most significant bit of the first byte written to 0. Always use the definite form.
1: Start a constructed value by setting the third most significant bit of the first byte written to 1, and use the definite form.
2: Start a constructed value and use the indefinite form,

If the tag is less than V_ASN1_PRIMITIVE_TAG (= 0x1f), it is written to the five least significant bits of the only identifier byte written. Otherwise, these five bits are all set to 1, and the tag is encoded in one or more following identifier bytes as needed.

After completing the identifier byte(s), when using the definite form, the given content_length is encoded in one or more bytes as needed, using the long form if and only if the content_length is greater than 127. When using the indefinite form, the special byte 0x80 is written instead and the content_length argument is ignored.

At the end, *ber_out is set to the byte following the last byte written. The calling code can then start writing content bytes.

If the indefinite form was selected, the calling code is also responsible for calling ASN1_put_eoc() which writes an end-of-content marker to *ber_out, consisting of two NUL bytes, and advances *ber_out by two bytes.

ASN1_object_size() calculates the total length in bytes of the BER encoding of an ASN.1 data element with the given tag and the number of content bytes given by content_length. The constructed argument has the same meaning as for ASN1_put_object(). The return value includes the identifier, length, and content bytes. If constructed is 2, it also includes the end-of-content bytes. For the definite form, only the short form is supported if the content_length is less than 128.

RETURN VALUES

ASN1_put_eoc() returns the number of bytes written, which is always 2.

ASN1_object_size() returns the total number of bytes in the encoding of the data element.

STANDARDS

ITU-T Recommendation X.690, also known as ISO/IEC 8825-1: Information technology - ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER), section 8.1: General rules for encoding

HISTORY

ASN1_put_object() and ASN1_object_size() first appeared in SSLeay 0.5.1 and have been available since OpenBSD 2.4.

ASN1_put_eoc() first appeared in OpenSSL 0.9.8 and has been available since OpenBSD 4.5.

CAVEATS

None of these functions do any sanity checking. When called in inconsistent ways, invalid content may result in *ber_out, for example

a tag number less than V_ASN1_PRIMITIVE_TAG with a class other than V_ASN1_UNIVERSAL
a tag number equal to V_ASN1_EOC (0x00) or V_ASN1_PRIMITIVE_TAG (0x1f)
a BOOLEAN, INTEGER, NULL etc. with the constructed bit set
a SEQUENCE or SET etc. without the constructed bit set
a content_length that makes no sense for the given tag
a content_length that disagrees with the following data
a BOOLEAN, INTEGER, NULL etc. in indefinite form
an end-of-content marker even though no indefinite form was started
...

If the calling code wants to find out how many bytes were written, it needs to save a copy of the pointer *ber_out before calling ASN1_put_object().