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

[Libor Chocholaty]

Hello, 

thanks a lot. Works nice. Just it is interesting that I get
X509_ATTRIBUTEs what should be a SEQUENCE but need to parse as SET. 

And to free the memory: 

X509_ATTRIBUTE *attr;
while ((attr = sk_X509_ATTRIBUTE_pop(attrs)) != NULL) {
    X509_ATTRIBUTE_free(attr);
}
sk_X509_ATTRIBUTE_free(attrs); 

Looks working. 

Regards,
Libor 

On 2017-11-15 15:31, Dave Coombs wrote:

> 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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