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use crate::{
engine::{general_purpose::INVALID_VALUE, DecodeMetadata, DecodePaddingMode},
DecodeError, PAD_BYTE,
};
/// Decode the last 1-8 bytes, checking for trailing set bits and padding per the provided
/// parameters.
///
/// Returns the decode metadata representing the total number of bytes decoded, including the ones
/// indicated as already written by `output_index`.
pub(crate) fn decode_suffix(
input: &[u8],
input_index: usize,
output: &mut [u8],
mut output_index: usize,
decode_table: &[u8; 256],
decode_allow_trailing_bits: bool,
padding_mode: DecodePaddingMode,
) -> Result<DecodeMetadata, DecodeError> {
// Decode any leftovers that aren't a complete input block of 8 bytes.
// Use a u64 as a stack-resident 8 byte buffer.
let mut leftover_bits: u64 = 0;
let mut morsels_in_leftover = 0;
let mut padding_bytes = 0;
let mut first_padding_index: usize = 0;
let mut last_symbol = 0_u8;
let start_of_leftovers = input_index;
for (i, &b) in input[start_of_leftovers..].iter().enumerate() {
// '=' padding
if b == PAD_BYTE {
// There can be bad padding bytes in a few ways:
// 1 - Padding with non-padding characters after it
// 2 - Padding after zero or one characters in the current quad (should only
// be after 2 or 3 chars)
// 3 - More than two characters of padding. If 3 or 4 padding chars
// are in the same quad, that implies it will be caught by #2.
// If it spreads from one quad to another, it will be an invalid byte
// in the first quad.
// 4 - Non-canonical padding -- 1 byte when it should be 2, etc.
// Per config, non-canonical but still functional non- or partially-padded base64
// may be treated as an error condition.
if i % 4 < 2 {
// Check for case #2.
let bad_padding_index = start_of_leftovers
+ if padding_bytes > 0 {
// If we've already seen padding, report the first padding index.
// This is to be consistent with the normal decode logic: it will report an
// error on the first padding character (since it doesn't expect to see
// anything but actual encoded data).
// This could only happen if the padding started in the previous quad since
// otherwise this case would have been hit at i % 4 == 0 if it was the same
// quad.
first_padding_index
} else {
// haven't seen padding before, just use where we are now
i
};
return Err(DecodeError::InvalidByte(bad_padding_index, b));
}
if padding_bytes == 0 {
first_padding_index = i;
}
padding_bytes += 1;
continue;
}
// Check for case #1.
// To make '=' handling consistent with the main loop, don't allow
// non-suffix '=' in trailing chunk either. Report error as first
// erroneous padding.
if padding_bytes > 0 {
return Err(DecodeError::InvalidByte(
start_of_leftovers + first_padding_index,
PAD_BYTE,
));
}
last_symbol = b;
// can use up to 8 * 6 = 48 bits of the u64, if last chunk has no padding.
// Pack the leftovers from left to right.
let shift = 64 - (morsels_in_leftover + 1) * 6;
let morsel = decode_table[b as usize];
if morsel == INVALID_VALUE {
return Err(DecodeError::InvalidByte(start_of_leftovers + i, b));
}
leftover_bits |= (morsel as u64) << shift;
morsels_in_leftover += 1;
}
match padding_mode {
DecodePaddingMode::Indifferent => { /* everything we care about was already checked */ }
DecodePaddingMode::RequireCanonical => {
if (padding_bytes + morsels_in_leftover) % 4 != 0 {
return Err(DecodeError::InvalidPadding);
}
}
DecodePaddingMode::RequireNone => {
if padding_bytes > 0 {
// check at the end to make sure we let the cases of padding that should be InvalidByte
// get hit
return Err(DecodeError::InvalidPadding);
}
}
}
// When encoding 1 trailing byte (e.g. 0xFF), 2 base64 bytes ("/w") are needed.
// / is the symbol for 63 (0x3F, bottom 6 bits all set) and w is 48 (0x30, top 2 bits
// of bottom 6 bits set).
// When decoding two symbols back to one trailing byte, any final symbol higher than
// w would still decode to the original byte because we only care about the top two
// bits in the bottom 6, but would be a non-canonical encoding. So, we calculate a
// mask based on how many bits are used for just the canonical encoding, and optionally
// error if any other bits are set. In the example of one encoded byte -> 2 symbols,
// 2 symbols can technically encode 12 bits, but the last 4 are non canonical, and
// useless since there are no more symbols to provide the necessary 4 additional bits
// to finish the second original byte.
let leftover_bits_ready_to_append = match morsels_in_leftover {
0 => 0,
2 => 8,
3 => 16,
4 => 24,
6 => 32,
7 => 40,
8 => 48,
// can also be detected as case #2 bad padding above
_ => unreachable!(
"Impossible: must only have 0 to 8 input bytes in last chunk, with no invalid lengths"
),
};
// if there are bits set outside the bits we care about, last symbol encodes trailing bits that
// will not be included in the output
let mask = !0 >> leftover_bits_ready_to_append;
if !decode_allow_trailing_bits && (leftover_bits & mask) != 0 {
// last morsel is at `morsels_in_leftover` - 1
return Err(DecodeError::InvalidLastSymbol(
start_of_leftovers + morsels_in_leftover - 1,
last_symbol,
));
}
// TODO benchmark simply converting to big endian bytes
let mut leftover_bits_appended_to_buf = 0;
while leftover_bits_appended_to_buf < leftover_bits_ready_to_append {
// `as` simply truncates the higher bits, which is what we want here
let selected_bits = (leftover_bits >> (56 - leftover_bits_appended_to_buf)) as u8;
output[output_index] = selected_bits;
output_index += 1;
leftover_bits_appended_to_buf += 8;
}
Ok(DecodeMetadata::new(
output_index,
if padding_bytes > 0 {
Some(input_index + first_padding_index)
} else {
None
},
))
}