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;; This file is part of scheme-GNUnet, a partial Scheme port of GNUnet.
;; Copyright (C) 2005--2007, 2020 GNUnet e.V.
;;
;; GNUnet is free software: you can redistribute it and/or modify it
;; under the terms of the GNU Affero General Public License as published
;; by the Free Software Foundation, either version 3 of the License,
;; or (at your option) any later version.
;;
;; GNUnet is distributed in the hope that it will be useful, but
;; WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
;; Affero General Public License for more details.
;;
;; You should have received a copy of the GNU Affero General Public License
;; along with this program. If not, see <http://www.gnu.org/licenses/>.
;;
;; SPDX-License-Identifier: AGPL-3.0-or-later
;;
;; As a special exception to the GNU Affero General Public License,
;; the file may be relicensed under any license used for
;; most source code of GNUnet 0.13.1, or later versions, as published by
;; GNUnet e.V.
;; Extracted from src/util/strings.c by Nils Durner and Christian Grothoff.
;; Deviations from upstream:
;; * Alternative characters aren't recognised.
;; (The alternative encodings aren't generated by GNUnet anyway)
;; TODO: perhaps reject alternate encodings (e.g. "00" and "01" both
;; decode to 0 currently)
(library (gnu gnunet data-string (1 1))
(export data->string string->data)
(import (rnrs base)
(rnrs control)
(rnrs bytevectors)
(rnrs arithmetic bitwise))
(define charset "0123456789ABCDEFGHJKMNPQRSTVWXYZ")
(define data->string
(case-lambda
"Convert binary data to ASCII encoding using Crockford Base32 encoding.
@var{bv}: data to encode
@var{offset}: start position of data to encode
@var{size}: length of data to encode, in octets"
((bv) (data->string bv 0 (bytevector-length bv)))
((bv offset size)
(let loop ((vbit 0) (rpos offset) (bits 0) (accumulated '()))
(if (or (< (- rpos offset) size) (> vbit 0))
(begin
(when (and (< (- rpos offset) size) (< vbit 5))
(set! bits (bitwise-ior (bitwise-arithmetic-shift-left bits 8)
(bytevector-u8-ref bv rpos)))
(set! rpos (+ 1 rpos))
(set! vbit (+ vbit 8)))
(when (< vbit 5)
(set! bits (bitwise-arithmetic-shift-left bits (- 5 vbit)))
(assert (= vbit (mod (* 8 size) 5)))
(set! vbit 5))
(loop (- vbit 5) rpos bits
(cons (string-ref charset
(bitwise-and
(bitwise-arithmetic-shift-right
bits (- vbit 5)) 31))
accumulated)))
(begin (assert (= 0 vbit))
(apply string (reverse accumulated))))))))
(define (get-value ch)
"Get the decoded value corresponding to a character according to Crockford
Base32 encoding."
(cond ((and (char<=? #\0 ch) (char<=? ch #\9))
(- (char->integer ch) (char->integer #\0)))
((and (char<=? #\A ch) (char<=? ch #\H))
(- (char->integer ch) (char->integer #\A) -10))
((and (char<=? #\J ch) (char<=? ch #\K))
(- (char->integer ch) (char->integer #\J) -18))
((and (char<=? #\M ch) (char<=? ch #\N))
(- (char->integer ch) (char->integer #\M) -20))
((and (char<=? #\P ch) (char<=? ch #\T))
(- (char->integer ch) (char->integer #\P) -22))
((and (char<=? #\V ch) (char<=? ch #\Z))
(- (char->integer ch) (char->integer #\V) -27))
(else #f)))
(define string->data
(lambda (enc out-size)
"Convert Crockford Base32hex encoding back to data
Return the data as a bytevector on success, or return #f
if result has the wrong encoding.
@var{out-size} must exactly match the size of the data before it was encoded.
@var{enc} the encoding
@var{out-size} size of output buffer"
(let ((rpos (string-length enc))
(bits #f)
(vbit #f)
(ret #f)
(shift #f)
(encoded-len (* 8 out-size))
(uout (make-bytevector out-size)))
(if (= 0 (string-length enc))
(if (= 0 out-size)
#vu8()
#f)
(begin
(if (< 0 (mod encoded-len 5))
(begin ; padding!
(set! vbit (mod encoded-len 5))
(set! shift (- 5 vbit))
(set! rpos (- rpos 1))
(set! ret (get-value (string-ref enc rpos)))
(set! bits (bitwise-arithmetic-shift-right ret shift)))
(begin
(set! vbit 5)
(set! shift 0)
(set! rpos (- rpos 1))
(set! ret (get-value (string-ref enc rpos)))
(set! bits ret)))
(cond ((not (= (/ (+ encoded-len shift) 5)
(string-length enc)))
#f)
((not ret)
#f)
(else
(let loop ((wpos out-size))
(if (> wpos 0)
(begin
(assert (not (= 0 rpos)))
(set! rpos (- rpos 1))
(set! ret (get-value (string-ref enc rpos)))
(set! bits (bitwise-ior
(bitwise-arithmetic-shift-left
ret vbit)
bits))
(and ret
(begin
(set! vbit (+ vbit 5))
(when (>= vbit 8)
(set! wpos (- wpos 1))
(bytevector-u8-set! uout wpos
(bitwise-and bits
255))
(set! bits
(bitwise-arithmetic-shift-right bits 8))
(set! vbit (- vbit 8)))
(loop wpos))))
(if (and (= 0 rpos) (= 0 vbit))
uout
#f)))))))))))
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