DER encoding - how to convert an implicit tag to an explicit tag

Question

I have an X.509 certificate that contains a dataset with the following IMPLICIT [0] tag:

A0 81 C6 (value)...

And I have this excerpt from the standards document:

The IMPLICIT [0] tag is not used for DER encoding, rather EXPLICIT The SET tag is used. That is, DER encoding of the EXPLICIT SET OF tag, not the IMPLICIT [0] tag, MUST be included along with the length and octets of the contents of the value.

I searched a lot, but I can’t understand what exactly the standard requires. I am looking for some clarification.

EDIT: Here is the standard I follow: http://tools.ietf.org/html/rfc3852

I am trying to verify the signature of X.509 and I need to calculate the message digest in order to do this. This certificate includes the optional SignedAttributes in the SignerInfo type. I havehed the signed content and verified that the message digest in SignedAttributes is correct. The standard states that if SignedAttributes are present, it must be hashed and encrypted to create a certificate signature. The standard also states that the SignedAttributes tag should be changed as described in the original question.

Here is the Asn.1 grammar for SignerInfo:

SignerInfo ::= SEQUENCE {
        version CMSVersion,
        sid SignerIdentifier,
        digestAlgorithm DigestAlgorithmIdentifier,
        signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL,
        signatureAlgorithm SignatureAlgorithmIdentifier,
        signature SignatureValue,
        unsignedAttrs [1] IMPLICIT UnsignedAttributes OPTIONAL }

SignerIdentifier ::= CHOICE {
        issuerAndSerialNumber IssuerAndSerialNumber,
        subjectKeyIdentifier [0] SubjectKeyIdentifier }

SignedAttributes ::= SET SIZE (1..MAX) OF Attribute

UnsignedAttributes ::= SET SIZE (1..MAX) OF Attribute

Attribute ::= SEQUENCE {
        attrType OBJECT IDENTIFIER,
        attrValues SET OF AttributeValue }

AttributeValue ::= ANY

SignatureValue ::= OCTET STRING

Answer 1

I am not sure how to interpret this comment. What standard are you reading? Do you have an ASN.1 grammar for structure?

. , SEQUENCE. SEQUENCE, 0x30. , SEQUENCE , EXPLICIT , . , .

, [0] EXPLICIT SET OF foo, ( ) (hex) A0 81 C9 31 81 C6 (value) ...

, , (A0), SET OF (31).

---

, , , , SET OF. , , "EXPLICIT", , . , 31 81 C6 (value) ... ( - 0xA0 SET OF 0x31).