about summary refs log tree commit diff
path: root/tvix/nix-compat/src/nixbase32.rs
blob: c4c2f2b96761674fa2ebbc37c00a22461372f3ec (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
//! Implements the slightly odd "base32" encoding that's used in Nix.
//!
//! Nix uses a custom alphabet. Contrary to other implementations (RFC4648),
//! encoding to "nix base32" doesn't use any padding, and reads in characters
//! in reverse order.
//!
//! This is also the main reason why we can't use `data_encoding::Encoding` -
//! it gets things wrong if there normally would be a need for padding.

use std::fmt::Write;

use data_encoding::{DecodeError, DecodeKind};

const ALPHABET: &[u8; 32] = b"0123456789abcdfghijklmnpqrsvwxyz";

/// Returns encoded input
pub fn encode(input: &[u8]) -> String {
    let output_len = encode_len(input.len());
    let mut output = String::with_capacity(output_len);

    for n in (0..output_len).rev() {
        let b = n * 5; // bit offset within the entire input
        let i = b / 8; // input byte index
        let j = b % 8; // bit offset within that input byte

        // 5-bit words aren't aligned to bytes
        // we can only read byte-aligned units
        // read 16 bits then shift and mask to 5
        let c = {
            let mut word = input[i] as u16;
            if let Some(&msb) = input.get(i + 1) {
                word |= (msb as u16) << 8;
            }
            (word >> j) & 0x1f
        };

        output.write_char(ALPHABET[c as usize] as char).unwrap();
    }

    output
}

/// This maps a nixbase32-encoded character to its binary representation, which
/// is also the index of the character in the alphabet. Invalid characters are
/// mapped to 0xFF, which is itself an invalid value.
const BASE32_ORD: [u8; 256] = {
    let mut ord = [0xFF; 256];
    let mut alphabet = ALPHABET.as_slice();
    let mut i = 0;

    while let &[c, ref tail @ ..] = alphabet {
        ord[c as usize] = i;
        alphabet = tail;
        i += 1;
    }

    ord
};

/// Returns decoded input
pub fn decode(input: impl AsRef<[u8]>) -> Result<Vec<u8>, DecodeError> {
    let input = input.as_ref();

    let output_len = decode_len(input.len());
    let mut output: Vec<u8> = vec![0x00; output_len];

    decode_inner(input, &mut output)?;
    Ok(output)
}

pub fn decode_fixed<const K: usize>(input: impl AsRef<[u8]>) -> Result<[u8; K], DecodeError> {
    let input = input.as_ref();

    if input.len() != encode_len(K) {
        return Err(DecodeError {
            position: input.len().min(encode_len(K)),
            kind: DecodeKind::Length,
        });
    }

    let mut output = [0; K];
    decode_inner(input, &mut output)?;
    Ok(output)
}

fn decode_inner(input: &[u8], output: &mut [u8]) -> Result<(), DecodeError> {
    // loop over all characters in reverse, and keep the iteration count in n.
    let mut carry = 0;
    let mut mask = 0;
    for (n, &c) in input.iter().rev().enumerate() {
        let b = n * 5;
        let i = b / 8;
        let j = b % 8;

        let digit = BASE32_ORD[c as usize];
        let value = (digit as u16) << j;
        output[i] |= value as u8 | carry;
        carry = (value >> 8) as u8;

        mask |= digit;
    }

    if mask == 0xFF {
        return Err(DecodeError {
            position: find_invalid(input),
            kind: DecodeKind::Symbol,
        });
    }

    // if we're at the end, but have a nonzero carry, the encoding is invalid.
    if carry != 0 {
        return Err(DecodeError {
            position: 0,
            kind: DecodeKind::Trailing,
        });
    }

    Ok(())
}

fn find_invalid(input: &[u8]) -> usize {
    for (i, &c) in input.iter().enumerate() {
        if !ALPHABET.contains(&c) {
            return i;
        }
    }

    unreachable!()
}

/// Returns the decoded length of an input of length len.
pub const fn decode_len(len: usize) -> usize {
    (len * 5) / 8
}

/// Returns the encoded length of an input of length len
pub const fn encode_len(len: usize) -> usize {
    (len * 8 + 4) / 5
}

#[cfg(test)]
mod tests {
    use hex_literal::hex;
    use test_case::test_case;

    #[test_case("", &[]; "empty bytes")]
    #[test_case("0z", &hex!("1f"); "one byte")]
    #[test_case("00bgd045z0d4icpbc2yyz4gx48ak44la", &hex!("8a12321522fd91efbd60ebb2481af88580f61600"); "store path")]
    #[test_case("0c5b8vw40dy178xlpddw65q9gf1h2186jcc3p4swinwggbllv8mk", &hex!("b3a24de97a8fdbc835b9833169501030b8977031bcb54b3b3ac13740f846ab30"); "sha256")]
    fn encode(enc: &str, dec: &[u8]) {
        assert_eq!(enc, super::encode(dec));
    }

    #[test_case("", Some(&[]) ; "empty bytes")]
    #[test_case("0z", Some(&hex!("1f")); "one byte")]
    #[test_case("00bgd045z0d4icpbc2yyz4gx48ak44la", Some(&hex!("8a12321522fd91efbd60ebb2481af88580f61600")); "store path")]
    #[test_case("0c5b8vw40dy178xlpddw65q9gf1h2186jcc3p4swinwggbllv8mk", Some(&hex!("b3a24de97a8fdbc835b9833169501030b8977031bcb54b3b3ac13740f846ab30")); "sha256")]
    // this is invalid encoding, because it encodes 10 1-bits, so the carry
    // would be 2 1-bits
    #[test_case("zz", None; "invalid encoding-1")]
    // this is an even more specific example - it'd decode as 00000000 11
    #[test_case("c0", None; "invalid encoding-2")]

    fn decode(enc: &str, dec: Option<&[u8]>) {
        match dec {
            Some(dec) => {
                // The decode needs to match what's passed in dec
                assert_eq!(dec, super::decode(enc).unwrap());
            }
            None => {
                // the decode needs to be an error
                assert!(super::decode(enc).is_err());
            }
        }
    }

    #[test]
    fn decode_fixed() {
        assert_eq!(
            super::decode_fixed("00bgd045z0d4icpbc2yyz4gx48ak44la").unwrap(),
            hex!("8a12321522fd91efbd60ebb2481af88580f61600")
        );
        assert_eq!(
            super::decode_fixed::<32>("00").unwrap_err(),
            super::DecodeError {
                position: 2,
                kind: super::DecodeKind::Length
            }
        );
    }

    #[test]
    fn encode_len() {
        assert_eq!(super::encode_len(0), 0);
        assert_eq!(super::encode_len(20), 32);
    }

    #[test]
    fn decode_len() {
        assert_eq!(super::decode_len(0), 0);
        assert_eq!(super::decode_len(32), 20);
    }
}