353 lines
15 KiB
Rust
353 lines
15 KiB
Rust
//! Provides the [GeneralPurpose] engine and associated config types.
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use crate::{
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alphabet,
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alphabet::Alphabet,
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engine::{Config, DecodeMetadata, DecodePaddingMode},
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DecodeSliceError,
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};
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use core::convert::TryInto;
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pub(crate) mod decode;
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pub(crate) mod decode_suffix;
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pub use decode::GeneralPurposeEstimate;
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pub(crate) const INVALID_VALUE: u8 = 255;
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/// A general-purpose base64 engine.
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///
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/// - It uses no vector CPU instructions, so it will work on any system.
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/// - It is reasonably fast (~2-3GiB/s).
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/// - It is not constant-time, though, so it is vulnerable to timing side-channel attacks. For loading cryptographic keys, etc, it is suggested to use the forthcoming constant-time implementation.
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#[derive(Debug, Clone)]
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pub struct GeneralPurpose {
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encode_table: [u8; 64],
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decode_table: [u8; 256],
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config: GeneralPurposeConfig,
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}
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impl GeneralPurpose {
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/// Create a `GeneralPurpose` engine from an [Alphabet].
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///
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/// While not very expensive to initialize, ideally these should be cached
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/// if the engine will be used repeatedly.
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pub const fn new(alphabet: &Alphabet, config: GeneralPurposeConfig) -> Self {
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Self {
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encode_table: encode_table(alphabet),
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decode_table: decode_table(alphabet),
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config,
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}
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}
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}
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impl super::Engine for GeneralPurpose {
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type Config = GeneralPurposeConfig;
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type DecodeEstimate = GeneralPurposeEstimate;
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fn internal_encode(&self, input: &[u8], output: &mut [u8]) -> usize {
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let mut input_index: usize = 0;
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const BLOCKS_PER_FAST_LOOP: usize = 4;
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const LOW_SIX_BITS: u64 = 0x3F;
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// we read 8 bytes at a time (u64) but only actually consume 6 of those bytes. Thus, we need
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// 2 trailing bytes to be available to read..
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let last_fast_index = input.len().saturating_sub(BLOCKS_PER_FAST_LOOP * 6 + 2);
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let mut output_index = 0;
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if last_fast_index > 0 {
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while input_index <= last_fast_index {
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// Major performance wins from letting the optimizer do the bounds check once, mostly
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// on the output side
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let input_chunk =
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&input[input_index..(input_index + (BLOCKS_PER_FAST_LOOP * 6 + 2))];
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let output_chunk =
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&mut output[output_index..(output_index + BLOCKS_PER_FAST_LOOP * 8)];
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// Hand-unrolling for 32 vs 16 or 8 bytes produces yields performance about equivalent
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// to unsafe pointer code on a Xeon E5-1650v3. 64 byte unrolling was slightly better for
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// large inputs but significantly worse for 50-byte input, unsurprisingly. I suspect
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// that it's a not uncommon use case to encode smallish chunks of data (e.g. a 64-byte
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// SHA-512 digest), so it would be nice if that fit in the unrolled loop at least once.
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// Plus, single-digit percentage performance differences might well be quite different
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// on different hardware.
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let input_u64 = read_u64(&input_chunk[0..]);
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output_chunk[0] = self.encode_table[((input_u64 >> 58) & LOW_SIX_BITS) as usize];
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output_chunk[1] = self.encode_table[((input_u64 >> 52) & LOW_SIX_BITS) as usize];
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output_chunk[2] = self.encode_table[((input_u64 >> 46) & LOW_SIX_BITS) as usize];
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output_chunk[3] = self.encode_table[((input_u64 >> 40) & LOW_SIX_BITS) as usize];
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output_chunk[4] = self.encode_table[((input_u64 >> 34) & LOW_SIX_BITS) as usize];
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output_chunk[5] = self.encode_table[((input_u64 >> 28) & LOW_SIX_BITS) as usize];
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output_chunk[6] = self.encode_table[((input_u64 >> 22) & LOW_SIX_BITS) as usize];
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output_chunk[7] = self.encode_table[((input_u64 >> 16) & LOW_SIX_BITS) as usize];
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let input_u64 = read_u64(&input_chunk[6..]);
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output_chunk[8] = self.encode_table[((input_u64 >> 58) & LOW_SIX_BITS) as usize];
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output_chunk[9] = self.encode_table[((input_u64 >> 52) & LOW_SIX_BITS) as usize];
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output_chunk[10] = self.encode_table[((input_u64 >> 46) & LOW_SIX_BITS) as usize];
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output_chunk[11] = self.encode_table[((input_u64 >> 40) & LOW_SIX_BITS) as usize];
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output_chunk[12] = self.encode_table[((input_u64 >> 34) & LOW_SIX_BITS) as usize];
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output_chunk[13] = self.encode_table[((input_u64 >> 28) & LOW_SIX_BITS) as usize];
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output_chunk[14] = self.encode_table[((input_u64 >> 22) & LOW_SIX_BITS) as usize];
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output_chunk[15] = self.encode_table[((input_u64 >> 16) & LOW_SIX_BITS) as usize];
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let input_u64 = read_u64(&input_chunk[12..]);
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output_chunk[16] = self.encode_table[((input_u64 >> 58) & LOW_SIX_BITS) as usize];
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output_chunk[17] = self.encode_table[((input_u64 >> 52) & LOW_SIX_BITS) as usize];
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output_chunk[18] = self.encode_table[((input_u64 >> 46) & LOW_SIX_BITS) as usize];
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output_chunk[19] = self.encode_table[((input_u64 >> 40) & LOW_SIX_BITS) as usize];
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output_chunk[20] = self.encode_table[((input_u64 >> 34) & LOW_SIX_BITS) as usize];
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output_chunk[21] = self.encode_table[((input_u64 >> 28) & LOW_SIX_BITS) as usize];
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output_chunk[22] = self.encode_table[((input_u64 >> 22) & LOW_SIX_BITS) as usize];
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output_chunk[23] = self.encode_table[((input_u64 >> 16) & LOW_SIX_BITS) as usize];
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let input_u64 = read_u64(&input_chunk[18..]);
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output_chunk[24] = self.encode_table[((input_u64 >> 58) & LOW_SIX_BITS) as usize];
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output_chunk[25] = self.encode_table[((input_u64 >> 52) & LOW_SIX_BITS) as usize];
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output_chunk[26] = self.encode_table[((input_u64 >> 46) & LOW_SIX_BITS) as usize];
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output_chunk[27] = self.encode_table[((input_u64 >> 40) & LOW_SIX_BITS) as usize];
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output_chunk[28] = self.encode_table[((input_u64 >> 34) & LOW_SIX_BITS) as usize];
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output_chunk[29] = self.encode_table[((input_u64 >> 28) & LOW_SIX_BITS) as usize];
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output_chunk[30] = self.encode_table[((input_u64 >> 22) & LOW_SIX_BITS) as usize];
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output_chunk[31] = self.encode_table[((input_u64 >> 16) & LOW_SIX_BITS) as usize];
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output_index += BLOCKS_PER_FAST_LOOP * 8;
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input_index += BLOCKS_PER_FAST_LOOP * 6;
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}
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}
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// Encode what's left after the fast loop.
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const LOW_SIX_BITS_U8: u8 = 0x3F;
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let rem = input.len() % 3;
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let start_of_rem = input.len() - rem;
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// start at the first index not handled by fast loop, which may be 0.
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while input_index < start_of_rem {
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let input_chunk = &input[input_index..(input_index + 3)];
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let output_chunk = &mut output[output_index..(output_index + 4)];
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output_chunk[0] = self.encode_table[(input_chunk[0] >> 2) as usize];
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output_chunk[1] = self.encode_table
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[((input_chunk[0] << 4 | input_chunk[1] >> 4) & LOW_SIX_BITS_U8) as usize];
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output_chunk[2] = self.encode_table
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[((input_chunk[1] << 2 | input_chunk[2] >> 6) & LOW_SIX_BITS_U8) as usize];
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output_chunk[3] = self.encode_table[(input_chunk[2] & LOW_SIX_BITS_U8) as usize];
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input_index += 3;
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output_index += 4;
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}
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if rem == 2 {
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output[output_index] = self.encode_table[(input[start_of_rem] >> 2) as usize];
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output[output_index + 1] =
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self.encode_table[((input[start_of_rem] << 4 | input[start_of_rem + 1] >> 4)
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& LOW_SIX_BITS_U8) as usize];
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output[output_index + 2] =
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self.encode_table[((input[start_of_rem + 1] << 2) & LOW_SIX_BITS_U8) as usize];
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output_index += 3;
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} else if rem == 1 {
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output[output_index] = self.encode_table[(input[start_of_rem] >> 2) as usize];
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output[output_index + 1] =
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self.encode_table[((input[start_of_rem] << 4) & LOW_SIX_BITS_U8) as usize];
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output_index += 2;
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}
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output_index
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}
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fn internal_decoded_len_estimate(&self, input_len: usize) -> Self::DecodeEstimate {
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GeneralPurposeEstimate::new(input_len)
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}
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fn internal_decode(
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&self,
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input: &[u8],
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output: &mut [u8],
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estimate: Self::DecodeEstimate,
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) -> Result<DecodeMetadata, DecodeSliceError> {
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decode::decode_helper(
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input,
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estimate,
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output,
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&self.decode_table,
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self.config.decode_allow_trailing_bits,
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self.config.decode_padding_mode,
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)
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}
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fn config(&self) -> &Self::Config {
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&self.config
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}
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}
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/// Returns a table mapping a 6-bit index to the ASCII byte encoding of the index
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pub(crate) const fn encode_table(alphabet: &Alphabet) -> [u8; 64] {
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// the encode table is just the alphabet:
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// 6-bit index lookup -> printable byte
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let mut encode_table = [0_u8; 64];
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{
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let mut index = 0;
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while index < 64 {
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encode_table[index] = alphabet.symbols[index];
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index += 1;
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}
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}
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encode_table
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}
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/// Returns a table mapping base64 bytes as the lookup index to either:
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/// - [INVALID_VALUE] for bytes that aren't members of the alphabet
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/// - a byte whose lower 6 bits are the value that was encoded into the index byte
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pub(crate) const fn decode_table(alphabet: &Alphabet) -> [u8; 256] {
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let mut decode_table = [INVALID_VALUE; 256];
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// Since the table is full of `INVALID_VALUE` already, we only need to overwrite
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// the parts that are valid.
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let mut index = 0;
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while index < 64 {
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// The index in the alphabet is the 6-bit value we care about.
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// Since the index is in 0-63, it is safe to cast to u8.
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decode_table[alphabet.symbols[index] as usize] = index as u8;
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index += 1;
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}
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decode_table
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}
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#[inline]
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fn read_u64(s: &[u8]) -> u64 {
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u64::from_be_bytes(s[..8].try_into().unwrap())
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}
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/// Contains configuration parameters for base64 encoding and decoding.
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///
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/// ```
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/// # use base64::engine::GeneralPurposeConfig;
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/// let config = GeneralPurposeConfig::new()
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/// .with_encode_padding(false);
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/// // further customize using `.with_*` methods as needed
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/// ```
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///
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/// The constants [PAD] and [NO_PAD] cover most use cases.
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///
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/// To specify the characters used, see [Alphabet].
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#[derive(Clone, Copy, Debug)]
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pub struct GeneralPurposeConfig {
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encode_padding: bool,
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decode_allow_trailing_bits: bool,
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decode_padding_mode: DecodePaddingMode,
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}
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impl GeneralPurposeConfig {
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/// Create a new config with `padding` = `true`, `decode_allow_trailing_bits` = `false`, and
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/// `decode_padding_mode = DecodePaddingMode::RequireCanonicalPadding`.
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///
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/// This probably matches most people's expectations, but consider disabling padding to save
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/// a few bytes unless you specifically need it for compatibility with some legacy system.
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pub const fn new() -> Self {
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Self {
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// RFC states that padding must be applied by default
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encode_padding: true,
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decode_allow_trailing_bits: false,
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decode_padding_mode: DecodePaddingMode::RequireCanonical,
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}
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}
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/// Create a new config based on `self` with an updated `padding` setting.
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///
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/// If `padding` is `true`, encoding will append either 1 or 2 `=` padding characters as needed
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/// to produce an output whose length is a multiple of 4.
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///
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/// Padding is not needed for correct decoding and only serves to waste bytes, but it's in the
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/// [spec](https://datatracker.ietf.org/doc/html/rfc4648#section-3.2).
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///
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/// For new applications, consider not using padding if the decoders you're using don't require
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/// padding to be present.
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pub const fn with_encode_padding(self, padding: bool) -> Self {
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Self {
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encode_padding: padding,
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..self
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}
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}
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/// Create a new config based on `self` with an updated `decode_allow_trailing_bits` setting.
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///
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/// Most users will not need to configure this. It's useful if you need to decode base64
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/// produced by a buggy encoder that has bits set in the unused space on the last base64
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/// character as per [forgiving-base64 decode](https://infra.spec.whatwg.org/#forgiving-base64-decode).
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/// If invalid trailing bits are present and this is `true`, those bits will
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/// be silently ignored, else `DecodeError::InvalidLastSymbol` will be emitted.
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pub const fn with_decode_allow_trailing_bits(self, allow: bool) -> Self {
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Self {
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decode_allow_trailing_bits: allow,
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..self
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}
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}
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/// Create a new config based on `self` with an updated `decode_padding_mode` setting.
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///
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/// Padding is not useful in terms of representing encoded data -- it makes no difference to
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/// the decoder if padding is present or not, so if you have some un-padded input to decode, it
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/// is perfectly fine to use `DecodePaddingMode::Indifferent` to prevent errors from being
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/// emitted.
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///
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/// However, since in practice
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/// [people who learned nothing from BER vs DER seem to expect base64 to have one canonical encoding](https://eprint.iacr.org/2022/361),
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/// the default setting is the stricter `DecodePaddingMode::RequireCanonicalPadding`.
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///
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/// Or, if "canonical" in your circumstance means _no_ padding rather than padding to the
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/// next multiple of four, there's `DecodePaddingMode::RequireNoPadding`.
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pub const fn with_decode_padding_mode(self, mode: DecodePaddingMode) -> Self {
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Self {
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decode_padding_mode: mode,
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..self
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}
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}
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}
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impl Default for GeneralPurposeConfig {
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/// Delegates to [GeneralPurposeConfig::new].
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fn default() -> Self {
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Self::new()
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}
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}
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impl Config for GeneralPurposeConfig {
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fn encode_padding(&self) -> bool {
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self.encode_padding
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}
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}
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/// A [GeneralPurpose] engine using the [alphabet::STANDARD] base64 alphabet and [PAD] config.
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pub const STANDARD: GeneralPurpose = GeneralPurpose::new(&alphabet::STANDARD, PAD);
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/// A [GeneralPurpose] engine using the [alphabet::STANDARD] base64 alphabet and [NO_PAD] config.
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pub const STANDARD_NO_PAD: GeneralPurpose = GeneralPurpose::new(&alphabet::STANDARD, NO_PAD);
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/// A [GeneralPurpose] engine using the [alphabet::URL_SAFE] base64 alphabet and [PAD] config.
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pub const URL_SAFE: GeneralPurpose = GeneralPurpose::new(&alphabet::URL_SAFE, PAD);
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/// A [GeneralPurpose] engine using the [alphabet::URL_SAFE] base64 alphabet and [NO_PAD] config.
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pub const URL_SAFE_NO_PAD: GeneralPurpose = GeneralPurpose::new(&alphabet::URL_SAFE, NO_PAD);
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/// Include padding bytes when encoding, and require that they be present when decoding.
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///
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/// This is the standard per the base64 RFC, but consider using [NO_PAD] instead as padding serves
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/// little purpose in practice.
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pub const PAD: GeneralPurposeConfig = GeneralPurposeConfig::new();
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/// Don't add padding when encoding, and require no padding when decoding.
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pub const NO_PAD: GeneralPurposeConfig = GeneralPurposeConfig::new()
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.with_encode_padding(false)
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.with_decode_padding_mode(DecodePaddingMode::RequireNone);
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