How to encode [0] IMPLICIT SET OF?

[stevemit]

Prerequisites

• Checked the developer manual
• Checked that your issue isn't already filed: https://github.com/issues?utf8=✓&q=repo%3Alibtom%2Flibtomcrypt
• Checked that your issue isn't due to the fact that you're using asymmetric cryptography and you forgot linking in and/or setting an MPI provider (usually this causes either random crashes or runtime errors like LTC_ARGCHK 'ltc_mp.name != NULL' failure ...). c.f. Ch. "Math Descriptors" of the developer manual.
• Checked that your issue isn't related to TomsFastMath's limitation that PK operations can by default only be done with max. 2048bit keys

Description

How can I construct, using libtomcrypt release 1.18.2, a struct of type ltc_asn1_list that, when nested inside a SEQUENCE and encoded, will produce an encoded DER SET OF with an IMPLICIT CONTEXT SENSITIVE [0] tag (as in CMS SignedData certificates, for example)?

If I set list.type = LTC_ASN1_SETOF, I get a properly encoded SET but the tag is 0x31 (UNIVERSAL SET). I can manually change the tag after encoding to 0xA0, except that the SET is nested inside layers of ASN.1 data. If I use list.type = LTC_ASN1_CUSTOM_TYPE, I can get the tag I want, but the function der_encode_custom_type() encodes the items like a SEQUENCE, not in sorted order like a DER SET. If I use the macro LTC_SET_ASN1_CUSTOM_CONSTRUCTED, I can nest a SET inside a custom typed SEQUENCE, creating an EXPLICIT tag.

Am I missing something? Is there a solution in the latest developer version? It seems that I must either change the tag manually after encoding, or else pre-sort the items manually within the array list.data. A fix would be to look during der_encode_custom_type() for list.used=LTC_ASN1_SETOF.

Steps to Reproduce

Version

Release 1.18.2

Additional Information

[sjaeckel]

It's possible that this currently can't be done with any ltc version.

Can you provide an example "CMS SignedData certificate" in encoded format (i.e. the expected output)?

[stevemit]

The CMS spec RFC 5652 uses a CONTEXT SENSITIVE [0] IMPLICIT SET OF for the certificates member of a SignedData structure. A second example in CMS is the signedAttrs member of a SignerInfo structure.

But, maybe a toy example will be more useful here. In the example below, the integers are sorted by their DER encoding. This is implemented in der_encode_setof() but it isn't called from der_encode_custom_type().

Toy ::= SEQUENCE {
  hello        UTF8String,
  numbers [0]  IMPLICIT SET OF Number  }

Number ::= INTEGER { zero(0), one(1) }

30 0F 
  0C 02 68 69 
  A0 09 
    02 01 00
    02 01 00
    02 01 01

When the data type LTC_ASN1_CUSTOM_TYPE represents a primitive ASN.1 data type, the member used indicates the structure of the data member. Maybe that API could work also for a constructed ASN.1 data type, to distinguish SEQUENCE, SET and SET OF.

[stevemit]

The function der_encode_set() overwrites the member ltc_asn1_list.used, then passes the list to the function der_encode_sequence_ex(). Maybe der_encode_set() should use the member ltc_asn1_list.optional instead for set ordering.

It wouldn't be a bad idea to modify ltc_asn1_list to use an anonymous union to rename the members used and optional (for decoding) to custom_type and set_order (for encoding).