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