The [BSON corpus specification](https://github.com/mongodb/specifications/blob/master/source/bson-corpus/bson-corpus.md#validity-test-case-keys) says: > canonical_bson: an (uppercase) big-endian hex representation of a BSON byte string. Be sure to mangle the case as appropriate in any roundtrip tests. However, the [`double.json` example](https://github.com/mongodb/specifications/blob/f3549601e6bdfe4f18568985dfe706ca500dc679/source/bson-corpus/tests/double.json#L32) contains examples where the canonical BSON representation isn't in uppercase: - 1.2345678921232E+18 → `100000000164002a1bf5f41022b14300` - -1.2345678921232E+18 → `100000000164002a1bf5f41022b1c300`