Canonical Encoding
Compact values and the active Compact compiler/runtime define canonical field layout and persistentHash semantics. JSON is never a canonical signing input.
The TypeScript boundary frames Compact runtime Value chunks as:
- ASCII magic
MCV1; - a big-endian unsigned 32-bit chunk count; and
- for every chunk, a big-endian unsigned 32-bit length followed by its bytes.
The byte sequence is encoded as canonical unpadded base64url and identified by compact-value-v1.base64url. Decoders MUST reject an invalid magic value, truncation, non-canonical base64url, trailing bytes, and descriptor leftovers.
Typed decoding is also descriptor-canonical. After a descriptor consumes every chunk, the decoder MUST re-encode the decoded value with that descriptor and reject the payload unless the re-encoded payload is byte-for-byte identical. This rejects alternate chunk widths, including redundant zero padding, that a Compact runtime descriptor could otherwise normalize to the same typed value. It also fails closed when a descriptor's decode/encode round trip is lossy.
Untyped decoding cannot infer descriptor-relative widths and therefore remains width-preserving: decodeCompactValue returns the framed chunks exactly as supplied after transport-level validation. Callers that require typed canonicality MUST use decodeCompactPayload with the expected descriptor.
The bytes32-descriptor conformance vectors use CompactTypeBytes(32) from the Compact runtime version pinned by @midnight-ntwrk/credential-compact. For that descriptor, the canonical chunk ends at the final non-zero byte and an all-zero value uses an empty chunk. Zero-padded alternatives are invalid even when the descriptor can normalize them to the same 32-byte value.
Context tags, proof payload roots, and Schnorr challenges are defined in proof-semantics.md. Issuance, presentation, signer authorization, and verifier-request proofs MUST use different tags. The checked vectors in ../conformance/vectors/compact-generated.json are produced by generated Compact circuits and verified by TypeScript.
This draft does not define a portable external encoding for arbitrary generic VC/VP types. Credential families own any encoding beyond the Compact value framing defined here.