[openssl-users] How to parse DER encoded x509 attributes?

[Dave Coombs]

Hello,

You can do something like the following.

First make a type corresponding to a stack of x509 attributes:

	typedef STACK_OF(X509_ATTRIBUTE) SEQ_X509_ATTRIBUTE;
	DECLARE_ASN1_FUNCTIONS(SEQ_X509_ATTRIBUTE);

Then make an asn1 template that specifies how the stack should be encoded.  (You can use ASN1_TFLG_SET_OF instead of ..SEQUENCE_OF here, depending on the DER you're trying to interpret, I don't know.)

	ASN1_ITEM_TEMPLATE(SEQ_X509_ATTRIBUTE) =
		ASN1_EX_TEMPLATE_TYPE(ASN1_TFLG_SEQUENCE_OF, 0, x509attribute, X509_ATTRIBUTE)
	ASN1_ITEM_TEMPLATE_END(SEQ_X509_ATTRIBUTE)
	IMPLEMENT_ASN1_FUNCTIONS(SEQ_X509_ATTRIBUTE)

Now your type has its own d2i and i2d functions, and you can use them:

	STACK_OF(X509_ATTRIBUTE) *attrs = d2i_SEQ_X509_ATTRIBUTE(NULL, &data, length);

Cheers,
  -Dave


> On Nov 15, 2017, at 07:26, Libor Chocholaty <ossl at mts.cz> wrote:
> 
> Hello,
> 
> I would like to parse DER encoded x509 attributes and have no clue how to use d2i_ASN1_SET_OF_X509_ATTRIBUTE. There are params that cannot find what to pass like free_func.
> 
> I am trying to uderstand by collecting pieces of known code, looking into openssl sources but...
> 
> PKCS7_SIGNER_INFO *p7si = PKCS7_SIGNER_INFO_new();
> d2i_ASN1_SET_OF_X509_ATTRIBUTE(&p7si->auth_attr, &der_data, der_data_length, d2i_X509_ATTRIBUTE, free_func, V_ASN1_SET, V_ASN1_UNIVERSAL);
> 
> Could somebody help how to do it or give a link to some useful documentation?
> Search internet looks completely not useful in this topic... or do not know how to ask...
> 
> Regards,
> Libor
> 
>  
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