Some docs relating to X509 ASN1 functions.

=pod

=head1 NAME

X509_new, X509_free, - X509 certificate ASN1 allocation functions

=head1 SYNOPSIS

 X509 *X509_new(void);

 void X509_free(X509 *a);

=head1 DESCRIPTION

The X509 ASN1 allocation routines, allocate and free an

X509 structure, which represents an X509 certificate.

X509_new() allocates and initializes a X509 structure.

X509_free() frees up the B<X509> structure B<a>.

=head1 RETURN VALUES

If the allocation fails, X509_new() returns B<NULL> and sets an error

code that can be obtained by L<ERR_get_error(3)|ERR_get_error(3)>.

Otherwise it returns a pointer to the newly allocated structure.

X509_free() returns no value.

=head1 SEE ALSO

L<ERR_get_error(3)|ERR_get_error(3)>, L<d2i_X509(3)|d2i_X509(3)>

=head1 HISTORY

X509_new() and X509_free() are available in all versions of SSLeay and OpenSSL.

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=pod

=head1 NAME

d2i_X509, i2d_X509, d2i_X509_bio, d2i_X509_fp, i2d_X509_bio,

i2d_X509_fp - X509 encode and decode functions

=head1 SYNOPSIS

 X509 *d2i_X509(X509 **px, 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(X509 *x, BIO *bp);

 int i2d_X509_fp(X509 *x, FILE *fp);

=head1 DESCRIPTION

The X509 encode and decode routines encode and parse an

B<X509> structure, which represents an X509 certificate.

d2i_X509() attempts to decode B<len> bytes at B<*out>. If 

successful a pointer to the B<X509> structure is returned. If an error

occurred then B<NULL> is returned. If B<px> is not B<NULL> then the

returned structure is written to B<*px>. If B<*px> is not B<NULL>

then it is assumed that B<*px> contains a valid B<X509>

structure and an attempt is made to reuse it. If the call is

successful B<*out> is incremented to the byte following the

parsed data.

i2d_X509() encodes the structure pointed to by B<x> into DER format.

If B<out> is not B<NULL> is writes the DER encoded data to the buffer

at B<*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. 

For OpenSSL 0.9.7 and later if B<*out> is B<NULL> memory will be

allocated for a buffer and the encoded data written to it. In this

case B<*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 B<bp>.

d2i_X509_fp() is similar to d2i_X509() except it attempts

to parse data from FILE pointer B<fp>.

i2d_X509_bio() is similar to i2d_X509() except it writes

the encoding of the structure B<x> to BIO B<bp>.

i2d_X509_fp() is similar to i2d_X509() except it writes

the encoding of the structure B<x> to BIO B<bp>.

=head1 NOTES

The letters B<i> and B<d> in for example B<i2d_X509> stand for

"internal" (that is an internal C structure) and "DER". So that

B<i2d_X509> converts from internal to DER.

The functions can also understand B<BER> forms.

The actual X509 structure passed to i2d_X509() must be a valid

populated B<X509> structure it can B<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 B<strlen()> will B<not> return the correct length

of the encoded structure.

The ways that B<*in> and B<*out> are incremented after the operation

can trap the unwary. See the B<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.

=head1 EXAMPLES

Allocate and encode the DER encoding of an X509 structure:

 int len;

 unsigned char *buf, *p;

 len = i2d_X509(x, NULL);

 buf = OPENSSL_malloc(len);

 if (buf == NULL)

	/* error */

 p = buf;

 i2d_X509(x, &p);

If you are using OpenSSL 0.9.7 or later then this can be

simplified to:

 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 */

=head1 WARNINGS

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 ... */

 OPENSSL_free(buf);

This code will result in B<buf> apparently containing garbage because

it was incremented after the call to point after the data just written.

Also B<buf> will no longer contain the pointer allocated by B<OPENSSL_malloc()>

and the subsequent call to B<OPENSSL_free()> may well crash.

The auto allocation feature (setting buf to NULL) only works on OpenSSL

0.9.7 and later. Attempts to use it on earlier versions will typically

cause a segmentation violation.

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

 X509 *x;

 if (!d2i_X509(&x, &p, len))

	/* Some error */

This will probably crash somewhere in B<d2i_X509()>. The reason for this

is that the variable B<x> is uninitialized and an attempt will be made to

interpret its (invalid) value as an B<X509> structure, typically causing

a segmentation violation. If B<x> is set to NULL first then this will not

happen.

=head1 BUGS

In some versions of OpenSSL the "reuse" behaviour of d2i_X509() when 

B<*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.

=head1 RETURN VALUES

d2i_X509(), d2i_X509_bio() and d2i_X509_fp() return a valid B<X509> structure

or B<NULL> if an error occurs. The error code that can be obtained by

L<ERR_get_error(3)|ERR_get_error(3)>. 

i2d_X509(), i2d_X509_bio() and i2d_X509_fp() return a the number of bytes

successfully encoded or a negative value if an error occurs. The error code

can be obtained by L<ERR_get_error(3)|ERR_get_error(3)>. 

=head1 SEE ALSO

L<ERR_get_error(3)|ERR_get_error(3)>

=head1 HISTORY

d2i_X509, i2d_X509, d2i_X509_bio, d2i_X509_fp, i2d_X509_bio and i2d_X509_fp

are available in all versions of SSLeay and OpenSSL.

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