fix: support all base64 variants in DSSE decoding and normalize type filter

[Sandipmandal25]

Description

decodeBase64Any previously only used base64.StdEncoding. However, the DSSE spec (https://github.com/secure-systems-lab/dsse) encodes payloads using base64url without padding (RawURLEncoding). As a result, real witness run attestations failed to parse with an illegal base64 data error.

Additionally, ExtractAll stored raw type filter strings without normalization. This caused all files to be silently dropped when --types was passed as values like Command-Run or COMMAND-RUN, even though the parser layer already normalizes these values.

fixes #5

Changes Made

• Fixed parser.go:

  • decodeBase64Any now attempts all four base64 variants in order:
    RawURLEncoding → URLEncoding → StdEncoding → RawStdEncoding

• Fixed extractor.go:

  • Normalize type filter entries using:
    strings.ToLower(strings.TrimSpace(t))
  • Ensures consistency with extractAttestations

• Added tests:

  • parser_test.go to validate base64 decoding across variants
  • extractor_test.go to verify type normalization behavior

Testing

Before:

After: