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|
//! 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 thiserror::Error;
const ALPHABET: &[u8; 32] = b"0123456789abcdfghijklmnpqrsvwxyz";
/// Errors that can occur while decoding nixbase32-encoded data.
#[derive(Debug, Eq, PartialEq, Error)]
pub enum Nixbase32DecodeError {
#[error("character {0:x} not in alphabet")]
CharacterNotInAlphabet(u8),
#[error("nonzero carry")]
NonzeroCarry(),
}
/// Returns encoded input
pub fn encode(input: &[u8]) -> String {
let output_len = encode_len(input.len());
let mut output = String::with_capacity(output_len);
if output_len > 0 {
for n in (0..=output_len - 1).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
let mut c = input[i] >> j;
if i + 1 < input.len() {
// we want to right shift, and discard shifted out bits (unchecked)
// To do this without panicing, we need to do the shifting in u16
// and convert back to u8 afterwards.
c |= ((input[i + 1] as u16) << (8 - j as u16)) as u8
}
output
.write_char(ALPHABET[(c & 0x1f) 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.
fn decode_char(encoded_char: u8) -> Option<u8> {
Some(match encoded_char {
b'0'..=b'9' => encoded_char - b'0',
b'a'..=b'd' => encoded_char - b'a' + 10_u8,
b'f'..=b'n' => encoded_char - b'f' + 14_u8,
b'p'..=b's' => encoded_char - b'p' + 23_u8,
b'v'..=b'z' => encoded_char - b'v' + 27_u8,
_ => return None,
})
}
/// Returns decoded input
pub fn decode(input: &[u8]) -> Result<Vec<u8>, Nixbase32DecodeError> {
let output_len = decode_len(input.len());
let mut output: Vec<u8> = vec![0x00; output_len];
// loop over all characters in reverse, and keep the iteration count in n.
let mut carry = 0;
for (n, &c) in input.iter().rev().enumerate() {
if let Some(digit) = decode_char(c) {
let b = n * 5;
let i = b / 8;
let j = b % 8;
let value = (digit as u16) << j;
output[i] |= value as u8 | carry;
carry = (value >> 8) as u8;
} else {
return Err(Nixbase32DecodeError::CharacterNotInAlphabet(c));
}
}
// if we're at the end, but have a nonzero carry, the encoding is invalid.
if carry != 0 {
return Err(Nixbase32DecodeError::NonzeroCarry());
}
Ok(output)
}
/// Returns the decoded length of an input of length len.
pub fn decode_len(len: usize) -> usize {
(len * 5) / 8
}
/// Returns the encoded length of an input of length len
pub fn encode_len(len: usize) -> usize {
(len * 8 + 4) / 5
}
#[cfg(test)]
mod tests {
use test_case::test_case;
#[test_case("", vec![] ; "empty bytes")]
#[test_case("0z", vec![0x1f]; "one byte")]
#[test_case("00bgd045z0d4icpbc2yyz4gx48ak44la", vec![
0x8a, 0x12, 0x32, 0x15, 0x22, 0xfd, 0x91, 0xef, 0xbd, 0x60, 0xeb, 0xb2, 0x48, 0x1a,
0xf8, 0x85, 0x80, 0xf6, 0x16, 0x00]; "store path")]
#[test_case("0c5b8vw40dy178xlpddw65q9gf1h2186jcc3p4swinwggbllv8mk", vec![
0xb3, 0xa2, 0x4d, 0xe9, 0x7a, 0x8f, 0xdb, 0xc8, 0x35, 0xb9, 0x83, 0x31, 0x69, 0x50, 0x10, 0x30,
0xb8, 0x97, 0x70, 0x31, 0xbc, 0xb5, 0x4b, 0x3b, 0x3a, 0xc1, 0x37, 0x40, 0xf8, 0x46, 0xab, 0x30,
]; "sha256")]
fn encode(enc: &str, dec: Vec<u8>) {
assert_eq!(enc, super::encode(&dec));
}
#[test_case("", Some(vec![]) ; "empty bytes")]
#[test_case("0z", Some(vec![0x1f]); "one byte")]
#[test_case("00bgd045z0d4icpbc2yyz4gx48ak44la", Some(vec![
0x8a, 0x12, 0x32, 0x15, 0x22, 0xfd, 0x91, 0xef, 0xbd, 0x60, 0xeb, 0xb2, 0x48, 0x1a,
0xf8, 0x85, 0x80, 0xf6, 0x16, 0x00]); "store path")]
#[test_case("0c5b8vw40dy178xlpddw65q9gf1h2186jcc3p4swinwggbllv8mk", Some(vec![
0xb3, 0xa2, 0x4d, 0xe9, 0x7a, 0x8f, 0xdb, 0xc8, 0x35, 0xb9, 0x83, 0x31, 0x69, 0x50, 0x10, 0x30,
0xb8, 0x97, 0x70, 0x31, 0xbc, 0xb5, 0x4b, 0x3b, 0x3a, 0xc1, 0x37, 0x40, 0xf8, 0x46, 0xab, 0x30,
]); "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<Vec<u8>>) {
match dec {
Some(dec) => {
// The decode needs to match what's passed in dec
assert_eq!(dec, super::decode(enc.as_bytes()).unwrap());
}
None => {
// the decode needs to be an error
assert!(super::decode(enc.as_bytes()).is_err());
}
}
}
#[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);
}
}
|
