Some XML schemas need opaque binary identifiers that remain valid as NMTOKEN or Name values. Instead of + and /, they swap the last two Base64 alphabet characters:
| Variant | Chars 62 / 63 | Padding | Typical use |
|---|---|---|---|
| XML name token | . - |
none | NMTOKEN / NMTOKENS attributes |
| XML identifier | _ : |
none | XML Name / ID-like values |
Both omit padding. They are rare compared with RFC 4648 and Base64URL, but appear in niche enterprise XML and legacy interoperability specs.
How to work with them here
- Paste a suspicious value into the Base64 standard detector.
- Convert to standard Base64 with normalization.
- Or decode directly on Base64 decode after selecting the matching XML standard.
If your value contains - and _ only, it is more often Base64URL (JWT, OAuth) than an XML variant.
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