# EPB2 compatibility specification

This remains the decoder for saved EPB2 records. New searches use EPB3. Your
complete private address, rather than its short ID, is needed for reconstruction.

The character alphabet is `abcdefghijklmnopqrstuvwxyz ,.` (digits0..28). A volume
has410 pages of3200 characters, in40 rows of80. EPB2 reading volumes reject
adjacent spaces. The public reference does not need your submitted passage.

The address is `epb2/H/wW/sS/vV` with optional `/pP`. H is canonical base29 using
`0123456789abcdefghijklmnopqrs`, without leading zeros except zero itself.
W ranges0..3, S0..4, V0..31; P is a one-based page1..410. Recover the scrambled
volume integer as H×640 + W×160 + S×32 + V; decode to exactly1,312,000 base29
digits with leading zeros retained. Overflow and noncanonical spellings fail.

Invert eight additive Feistel rounds on two656,000-digit halves. Forward round i
maps (L,R) to (R,(L+F_i(R)) mod29); inverse maps (L,R) to
((R-F_i(L)) mod29,L), processing rounds7 down to0. F_i uses domain
`EPB/v2/feistel/i`: BLAKE3(UTF8(domain)||zeroByte||halfDigits) supplies a seed.
For unsigned64 little-endian counters starting at0, BLAKE3 XOF of seed||counter
produces65,536 bytes per block. Reject bytes>=232, map remaining bytes modulo29,
and continue until656,000 digits. Concatenate the recovered halves and select the
requested3200-digit page. The reference provides exact executable byte framing.

`reconstruct.mjs`, fixed full-volume synthetic vectors and unchanged primitive
vectors accompany this document. The canonical address, volume, scrambledvolume
and page hashes are independently checked by that reference. Its BLAKE3 dependency
is pinned in package-lock.json. No private customer data or online API is needed.

Normalization/masks, purchase state and signed package authenticity are separate
from mathematical page reconstruction. Keep the downloaded original submission
as the authoritative source; a normalized mathematical page is not lossless.