add bbs work item

work_items/bbs_signatures.md

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+# BBS Signatures
+
+BBS Signatures are a form of short group digital signature that supports multi-message signing with selective reveal on proof derivation. These properties are advantageous within a variety of applications however most notably within attribute based credential technologies such as Verifiable Credentials.
+
+This work items focus is to formally document the cryptographic representation of and process of producing a BBS signature along side associated structures such as derived proofs of knowledge of a BBS signature.
+
+As a proposed starting point for the work item, the following document is offered (https://github.com/mattrglobal/bbs-signatures-spec).
+
+## Work Item Owners
+- Tobias Looker (@tplooker, tobias.looker@mattr.global)
+- Mike Lodder (@mikelodder7, mike@cryptid.tech)
+- Andrew Whitehead (@andrewwhitehead, cywolf@gmail.com)
+
+## Outcome
+
+The overall intent from this work item is to produce a document that can be proposed as an input to the IETF CFRG for formal standardization of BBS Signatures.
+
+## Deliverables
+- A document that formally defines the cryptographic representation of and process of producing a BBS signature along side associated structures such as derived proofs of knowledge of a BBS signature.
+- A set of test fixtures to support the development of interoperable implementations.
+
+## Meetings
+- A weekly work item call with a primary focus on issue review and general evolution of the BBS signature document.