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

base64ct/
alphabet.rs
