HPPR-006 · Base64-Ascend (B64A)

B64A is an order-preserving Base64 variant.

Alphabet:

0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz~

Canonical form omits = padding and zero-fills trailing bits.

Alphabet Mapping

• 0..9 -> '0'..'9'
• 10..35 -> 'A'..'Z'
• 36 -> '_'
• 37..62 -> 'a'..'z'
• 63 -> '~'

~ keeps the alphabet ASCII-sorted and path-safe.

Encoding Rules

• Pack bits like RFC 4648 Base64 (MSB-first 6-bit groups).
• Do not emit =.
• Zero-fill the final partial 6-bit group.
• Output length is ceil(8*N/6) for N input bytes.

Decoding and Validation

• Reject bytes outside the B64A alphabet.
• Decode first, then validate tail bits:
  • len % 4 == 2: low 4 bits must be zero
  • len % 4 == 3: low 2 bits must be zero
• Implementations should validate by decoding, not by regex.

Ordering Guarantees

• Equal-length strings compare exactly like decoded bytes.
• If one canonical string is a strict prefix of another, the shorter string sorts first.

Conformance

• MUST emit and accept only the B64A alphabet above.
• MUST forbid = padding.
• MUST zero-fill trailing bits and reject non-zero filler bits.
• SHOULD validate by decoding plus explicit tail checks.

Test Vectors

Reject examples: 01, 001, ~m, ~l1, =, +, /.