taler-mailbox

helper.go (3769B)

---

 // This file is part of taldir, the Taler Directory implementation.
 // Copyright (C) 2025 Martin Schanzenbach
 //
 // Taldir 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.
 //
 // Taldir 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: AGPL3.0-or-later
 
 package util
 
 import (
   "errors"
   "strings"
 )
 
 //------------------------------------------------------------------------
 // Base32 conversion between binary data and string representation
 //------------------------------------------------------------------------
 //
 // A binary array of size m is viewed as a consecutive stream of bits
 // from left to right. Bytes are ordered with ascending address, while
 // bits (in a byte) are ordered MSB to LSB.
 
 // For encoding the stream is partitioned into 5-bit chunks; the last chunk
 // is right-padded with 0's if 8*m is not divisible by 5. Each chunk (value
 // between 0 and 31) is encoded into a character; the mapping for encoding
 // is the same as in [https://www.crockford.com/wrmg/base32.html].
 //
 // For decoding each character is converted to a 5-bit chunk based on the
 // encoder mapping (with one addition: the character 'U' maps to the value
 // 27). The chunks are concatenated to produce the bit stream to be stored
 // in the output array.
 
 // character set used for encoding/decoding
 const xlate = "0123456789ABCDEFGHJKMNPQRSTVWXYZ"
 
 var (
 	// ErrInvalidEncoding signals an invalid encoding
 	ErrInvalidEncoding = errors.New("invalid encoding")
 	// ErrBufferTooSmall signalsa too small buffer for decoding
 	ErrBufferTooSmall = errors.New("buffer to small")
 )
 
 // EncodeBinaryToString encodes a byte array into a string.
 func Base32CrockfordEncode(data []byte) string {
 	size, pos, bits, n := len(data), 0, 0, 0
 	out := ""
 	for {
 		if n < 5 {
 			if pos < size {
 				bits = (bits << 8) | (int(data[pos]) & 0xFF)
 				pos++
 				n += 8
 			} else if n > 0 {
 				bits <<= uint(5 - n)
 				n = 5
 			} else {
 				break
 			}
 		}
 		out += string(xlate[(bits>>uint(n-5))&0x1F])
 		n -= 5
 	}
 	return out
 }
 
 // Base32CrockfordDecode decodes a string into a byte array.
 // The function expects the size of the output buffer to be specified as an
 // argument ('num'); the function returns an error if the buffer is overrun
 // or if an invalid character is found in the encoded string. If the decoded
 // bit stream is smaller than the output buffer, it is padded with 0's.
 func Base32CrockfordDecode(s string, num int) ([]byte, error) {
 	size := len(s)
 	out := make([]byte, num)
 	rpos, wpos, n, bits := 0, 0, 0, 0
 	for {
 		if n < 8 {
 			if rpos < size {
 				c := rune(s[rpos])
 				rpos++
 				v := strings.IndexRune(xlate, c)
 				if v == -1 {
 					switch c {
 					case 'O':
 						v = 0
 					case 'I', 'L':
 						v = 1
 					case 'U':
 						v = 27
 					default:
 						return nil, ErrInvalidEncoding
 					}
 				}
 				bits = (bits << 5) | (v & 0x1F)
 				n += 5
 			} else {
 				if wpos < num {
 					out[wpos] = byte(bits & ((1 << uint(n+1)) - 1))
 					wpos++
 					for i := wpos; i < num; i++ {
 						out[i] = 0
 					}
 				}
 				break
 			}
 		} else {
 			if wpos < num {
 				out[wpos] = byte((bits >> uint(n-8)) & 0xFF)
 				wpos++
 				n -= 8
 			} else {
 				return nil, ErrBufferTooSmall
 			}
 		}
 	}
 	return out, nil
 }