base64ct/
alphabet.rs

1//! Base64 alphabets.
2
3// TODO(tarcieri): explicitly checked/wrapped arithmetic
4#![allow(clippy::arithmetic_side_effects)]
5
6use core::{fmt::Debug, ops::RangeInclusive};
7
8pub mod bcrypt;
9pub mod crypt;
10pub mod pbkdf2;
11pub mod shacrypt;
12pub mod standard;
13pub mod url;
14
15/// Core encoder/decoder functions for a particular Base64 alphabet.
16pub trait Alphabet: 'static + Copy + Debug + Eq + Send + Sized + Sync {
17    /// First character in this Base64 alphabet.
18    const BASE: u8;
19
20    /// Decoder passes
21    const DECODER: &'static [DecodeStep];
22
23    /// Encoder passes
24    const ENCODER: &'static [EncodeStep];
25
26    /// Is this encoding padded?
27    const PADDED: bool;
28
29    /// Unpadded equivalent of this alphabet.
30    ///
31    /// For alphabets that are unpadded to begin with, this should be `Self`.
32    type Unpadded: Alphabet;
33
34    /// Decode 3 bytes of a Base64 message.
35    #[inline(always)]
36    fn decode_3bytes(src: &[u8], dst: &mut [u8]) -> i16 {
37        debug_assert_eq!(src.len(), 4);
38        debug_assert!(dst.len() >= 3, "dst too short: {}", dst.len());
39
40        let c0 = Self::decode_6bits(src[0]);
41        let c1 = Self::decode_6bits(src[1]);
42        let c2 = Self::decode_6bits(src[2]);
43        let c3 = Self::decode_6bits(src[3]);
44
45        dst[0] = ((c0 << 2) | (c1 >> 4)) as u8;
46        dst[1] = ((c1 << 4) | (c2 >> 2)) as u8;
47        dst[2] = ((c2 << 6) | c3) as u8;
48
49        ((c0 | c1 | c2 | c3) >> 8) & 1
50    }
51
52    /// Decode 6-bits of a Base64 message.
53    fn decode_6bits(src: u8) -> i16 {
54        let mut ret: i16 = -1;
55
56        for step in Self::DECODER {
57            ret += match step {
58                DecodeStep::Range(range, offset) => {
59                    // Compute exclusive range from inclusive one
60                    let start = *range.start() as i16 - 1;
61                    let end = *range.end() as i16 + 1;
62                    (((start - src as i16) & (src as i16 - end)) >> 8) & (src as i16 + *offset)
63                }
64                DecodeStep::Eq(value, offset) => {
65                    let start = *value as i16 - 1;
66                    let end = *value as i16 + 1;
67                    (((start - src as i16) & (src as i16 - end)) >> 8) & *offset
68                }
69            };
70        }
71
72        ret
73    }
74
75    /// Encode 3-bytes of a Base64 message.
76    #[inline(always)]
77    fn encode_3bytes(src: &[u8], dst: &mut [u8]) {
78        debug_assert_eq!(src.len(), 3);
79        debug_assert!(dst.len() >= 4, "dst too short: {}", dst.len());
80
81        let b0 = src[0] as i16;
82        let b1 = src[1] as i16;
83        let b2 = src[2] as i16;
84
85        dst[0] = Self::encode_6bits(b0 >> 2);
86        dst[1] = Self::encode_6bits(((b0 << 4) | (b1 >> 4)) & 63);
87        dst[2] = Self::encode_6bits(((b1 << 2) | (b2 >> 6)) & 63);
88        dst[3] = Self::encode_6bits(b2 & 63);
89    }
90
91    /// Encode 6-bits of a Base64 message.
92    #[inline(always)]
93    fn encode_6bits(src: i16) -> u8 {
94        let mut diff = src + Self::BASE as i16;
95
96        for &step in Self::ENCODER {
97            diff += match step {
98                EncodeStep::Apply(threshold, offset) => ((threshold as i16 - diff) >> 8) & offset,
99                EncodeStep::Diff(threshold, offset) => ((threshold as i16 - src) >> 8) & offset,
100            };
101        }
102
103        diff as u8
104    }
105}
106
107/// Constant-time decoder step.
108#[derive(Debug)]
109pub enum DecodeStep {
110    /// Match the given range, offsetting the input on match.
111    Range(RangeInclusive<u8>, i16),
112
113    /// Match the given value, returning the associated offset on match.
114    Eq(u8, i16),
115}
116
117/// Constant-time encoder step.
118#[derive(Copy, Clone, Debug)]
119pub enum EncodeStep {
120    /// Apply the given offset to the cumulative result on match.
121    Apply(u8, i16),
122
123    /// Compute a difference using the given offset on match.
124    Diff(u8, i16),
125}
base64ct/alphabet.rs

base64ct/
alphabet.rs
