base64ct/
alphabet.rs

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1//! Base64 alphabets.
2
3// TODO(tarcieri): explicitly checked/wrapped arithmetic
4#![allow(clippy::integer_arithmetic)]
5
6use core::{fmt::Debug, ops::RangeInclusive};
7
8pub mod bcrypt;
9pub mod crypt;
10pub mod shacrypt;
11pub mod standard;
12pub mod url;
13
14/// Core encoder/decoder functions for a particular Base64 alphabet.
15pub trait Alphabet: 'static + Copy + Debug + Eq + Send + Sized + Sync {
16    /// First character in this Base64 alphabet.
17    const BASE: u8;
18
19    /// Decoder passes
20    const DECODER: &'static [DecodeStep];
21
22    /// Encoder passes
23    const ENCODER: &'static [EncodeStep];
24
25    /// Is this encoding padded?
26    const PADDED: bool;
27
28    /// Unpadded equivalent of this alphabet.
29    ///
30    /// For alphabets that are unpadded to begin with, this should be `Self`.
31    type Unpadded: Alphabet;
32
33    /// Decode 3 bytes of a Base64 message.
34    #[inline(always)]
35    fn decode_3bytes(src: &[u8], dst: &mut [u8]) -> i16 {
36        debug_assert_eq!(src.len(), 4);
37        debug_assert!(dst.len() >= 3, "dst too short: {}", dst.len());
38
39        let c0 = Self::decode_6bits(src[0]);
40        let c1 = Self::decode_6bits(src[1]);
41        let c2 = Self::decode_6bits(src[2]);
42        let c3 = Self::decode_6bits(src[3]);
43
44        dst[0] = ((c0 << 2) | (c1 >> 4)) as u8;
45        dst[1] = ((c1 << 4) | (c2 >> 2)) as u8;
46        dst[2] = ((c2 << 6) | c3) as u8;
47
48        ((c0 | c1 | c2 | c3) >> 8) & 1
49    }
50
51    /// Decode 6-bits of a Base64 message.
52    fn decode_6bits(src: u8) -> i16 {
53        let mut ret: i16 = -1;
54
55        for step in Self::DECODER {
56            ret += match step {
57                DecodeStep::Range(range, offset) => {
58                    // Compute exclusive range from inclusive one
59                    let start = *range.start() as i16 - 1;
60                    let end = *range.end() as i16 + 1;
61                    (((start - src as i16) & (src as i16 - end)) >> 8) & (src as i16 + *offset)
62                }
63                DecodeStep::Eq(value, offset) => {
64                    let start = *value as i16 - 1;
65                    let end = *value as i16 + 1;
66                    (((start - src as i16) & (src as i16 - end)) >> 8) & *offset
67                }
68            };
69        }
70
71        ret
72    }
73
74    /// Encode 3-bytes of a Base64 message.
75    #[inline(always)]
76    fn encode_3bytes(src: &[u8], dst: &mut [u8]) {
77        debug_assert_eq!(src.len(), 3);
78        debug_assert!(dst.len() >= 4, "dst too short: {}", dst.len());
79
80        let b0 = src[0] as i16;
81        let b1 = src[1] as i16;
82        let b2 = src[2] as i16;
83
84        dst[0] = Self::encode_6bits(b0 >> 2);
85        dst[1] = Self::encode_6bits(((b0 << 4) | (b1 >> 4)) & 63);
86        dst[2] = Self::encode_6bits(((b1 << 2) | (b2 >> 6)) & 63);
87        dst[3] = Self::encode_6bits(b2 & 63);
88    }
89
90    /// Encode 6-bits of a Base64 message.
91    #[inline(always)]
92    fn encode_6bits(src: i16) -> u8 {
93        let mut diff = src + Self::BASE as i16;
94
95        for &step in Self::ENCODER {
96            diff += match step {
97                EncodeStep::Apply(threshold, offset) => ((threshold as i16 - diff) >> 8) & offset,
98                EncodeStep::Diff(threshold, offset) => ((threshold as i16 - src) >> 8) & offset,
99            };
100        }
101
102        diff as u8
103    }
104}
105
106/// Constant-time decoder step.
107#[derive(Debug)]
108pub enum DecodeStep {
109    /// Match the given range, offsetting the input on match.
110    Range(RangeInclusive<u8>, i16),
111
112    /// Match the given value, returning the associated offset on match.
113    Eq(u8, i16),
114}
115
116/// Constant-time encoder step.
117#[derive(Copy, Clone, Debug)]
118pub enum EncodeStep {
119    /// Apply the given offset to the cumulative result on match.
120    Apply(u8, i16),
121
122    /// Compute a difference using the given offset on match.
123    Diff(u8, i16),
124}
base64ct/alphabet.rs

base64ct/
alphabet.rs
