I meant to say that I don't know of any specific reason other than not changing the underlying type. I imagine that not changing the underlying type can be important/helpful in some situations. An example being an encoded certificate as a member of some other structure. In order to "hand" the certificate portion of the DER to an X.509 decoder an implicit tag would have to changed to the universal tag expected of the certificate. For verifying the signature it would also be necessary to "correct" an implicit tag. Using an explicit tag instead means that the underlying object is still a standalone certificate.
-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Steven Reddie Sent: Thursday, 8 September 2005 2:17 PM To: openssl-users@openssl.org Subject: RE: OCSP, Nonce and the requestExtensions When working with encodings of an existing data model then the use of implicit vs explicit comes down to what the designers specified. ie. for interoperability you can't work against the specification. When designing a data model with ASN.1 I don't know of any specific reason for using one over the other. Explicit tags wrapper the underlying object and as such add a little bloat but leave the underlying encoded object unchanged. Implicit tags replace the underlying tag in the encoding, avoiding the little bloat, but altering the encoded representation of the underlying object. For example, the INTEGER zero is encoded in DER as 02 01 00. Applying a context-specific tag of 2 results in: Implicit: 82 01 00 Explicit: 82 03 02 01 00 Regards, Steven -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of sravan Sent: Thursday, 8 September 2005 1:55 PM To: openssl-users@openssl.org Subject: Re: OCSP, Nonce and the requestExtensions Hi Steven and others, i have a doubt regd these tags in ASN1: when do we use implicit tags & when do we use explicit tags? i have read the 'layman's guide to a subset of ASN.1, BER & DER' but it seems i didn't get the exact difference b/n the two types of tags - in the sense of exact context in which each of these types of tags are used. bye & thnx - sravan Steven Reddie wrote: >I should clarify that tags aren't blindly used to identify members of >structured types, only when there would otherwise be ambiguity such as >with optional members in a SEQUENCE, or in a CHOICE. > >-----Original Message----- >From: [EMAIL PROTECTED] >[mailto:[EMAIL PROTECTED] On Behalf Of Steven Reddie >Sent: Thursday, 8 September 2005 11:07 AM >To: openssl-users@openssl.org >Subject: RE: OCSP, Nonce and the requestExtensions > >Do a search for a document titled "A Layman's Guide to a Subset of >ASN.1, BER, and DER". The tags in this case, and generally, are used >to identify the components in the structure. Since the last two >members are optional it is necessary to encode the structure so that it >is possible to tell which of the optional components is present. I >don't know why the version has an explicit tag -- it seems redundant. > > > ______________________________________________________________________ OpenSSL Project http://www.openssl.org User Support Mailing List openssl-users@openssl.org Automated List Manager [EMAIL PROTECTED] ______________________________________________________________________ OpenSSL Project http://www.openssl.org User Support Mailing List openssl-users@openssl.org Automated List Manager [EMAIL PROTECTED] ______________________________________________________________________ OpenSSL Project http://www.openssl.org User Support Mailing List openssl-users@openssl.org Automated List Manager [EMAIL PROTECTED]
