On Wed, Sep 02, 2015, Sands, Daniel wrote: > > Well at first sight that looks like an IMPLICIT tag which replaces > > the normal > > tag with the context specific value. It is not possible to determine > > the > > underlying type from the encoding itself as it has been replaced. So > > you need > > the ASN.1 spec to see the appropriate type to use. > > > > In that particular example it looks like an SHA1 hash (20 bytes long) > > so we > > could use an implicitly tagged OCTET STRING like this: > > > > ASN1_IMP(struct_name, field_name, ASN1_OCTET_STRING, 0), > > And often, implicit tags are intended for ASN.1 Choice constructs so > that you will know which option of the choice to parse into. > Or OPTIONAL and occasionally one does see tagged types for no apparent reason. Also just because you see a universal tag doesn't mean that it isn't part of a CHOICE: which would mean the parser would choke when it encountered an unexpected tag. Steve. -- Dr Stephen N. Henson. OpenSSL project core developer. Commercial tech support now available see: http://www.openssl.org
