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const-oid: fix large arc handling in encoder (#1522)
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BER encodings were being miscomputed for certain large arcs.

The previous method was a bit wacky (in addition to being buggy) and
attempted to encode each arc backwards within the BER output buffer.

This switches to a new method which splits the upper 7 bits from an arc
and encodes that as a byte, continuing until all bytes of the arc have
been encoded, which is much more straightforward.

The problematic cases which were reported have now been corrected.

Fixes #1520
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@tarcieri
tarcieri authored Sep 18, 2024
1 parent 895dbdf commit 7c4638b
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Showing 2 changed files with 87 additions and 59 deletions.
73 changes: 37 additions & 36 deletions const-oid/src/encoder.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -73,27 +73,9 @@ impl<const MAX_SIZE: usize> Encoder<MAX_SIZE> {
self.cursor = 1;
Ok(self)
}
// TODO(tarcieri): finer-grained overflow safety / checked arithmetic
#[allow(clippy::arithmetic_side_effects)]
State::Body => {
// Total number of bytes in encoded arc - 1
let nbytes = base128_len(arc);

// Shouldn't overflow on any 16-bit+ architectures
if self.cursor + nbytes + 1 > MAX_SIZE {
return Err(Error::Length);
}

let new_cursor = self.cursor + nbytes + 1;

// TODO(tarcieri): use `?` when stable in `const fn`
match self.encode_base128_byte(arc, nbytes, false) {
Ok(mut encoder) => {
encoder.cursor = new_cursor;
Ok(encoder)
}
Err(err) => Err(err),
}
self.encode_base128(arc, nbytes)
}
}
}
Expand All @@ -113,23 +95,19 @@ impl<const MAX_SIZE: usize> Encoder<MAX_SIZE> {
}

/// Encode a single byte of a Base 128 value.
const fn encode_base128_byte(mut self, mut n: u32, i: usize, continued: bool) -> Result<Self> {
let mask = if continued { 0b10000000 } else { 0 };

// Underflow checked by branch
#[allow(clippy::arithmetic_side_effects)]
if n > 0x80 {
self.bytes[checked_add!(self.cursor, i)] = (n & 0b1111111) as u8 | mask;
n >>= 7;

if i > 0 {
self.encode_base128_byte(n, i.saturating_sub(1), true)
} else {
Err(Error::Base128)
}
} else {
self.bytes[self.cursor] = n as u8 | mask;
Ok(self)
const fn encode_base128(mut self, n: u32, remaining_len: usize) -> Result<Self> {
if self.cursor >= MAX_SIZE {
return Err(Error::Length);
}

let mask = if remaining_len > 0 { 0b10000000 } else { 0 };
let (hi, lo) = split_high_bits(n);
self.bytes[self.cursor] = hi | mask;
self.cursor = checked_add!(self.cursor, 1);

match remaining_len.checked_sub(1) {
Some(len) => self.encode_base128(lo, len),
None => Ok(self),
}
}
}
Expand All @@ -145,6 +123,29 @@ const fn base128_len(arc: Arc) -> usize {
}
}

/// Split the highest 7-bits of an [`Arc`] from the rest of an arc.
///
/// Returns: `(hi, lo)`
// TODO(tarcieri): always use checked arithmetic
#[allow(clippy::arithmetic_side_effects)]
const fn split_high_bits(arc: Arc) -> (u8, Arc) {
if arc < 0x80 {
return (arc as u8, 0);
}

let hi_bit = 32 - arc.leading_zeros();
let hi_bit_mod7 = hi_bit % 7;
let upper_bit_pos = hi_bit
- if hi_bit > 0 && hi_bit_mod7 == 0 {
7
} else {
hi_bit_mod7
};
let upper_bits = arc >> upper_bit_pos;
let lower_bits = arc ^ (upper_bits << upper_bit_pos);
(upper_bits as u8, lower_bits)
}

#[cfg(test)]
#[allow(clippy::unwrap_used)]
mod tests {
Expand Down
73 changes: 50 additions & 23 deletions const-oid/tests/oid.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -23,10 +23,16 @@ const EXAMPLE_OID_2_BER: &[u8] = &hex!("60864801650304012A");
const EXAMPLE_OID_2: ObjectIdentifier = ObjectIdentifier::new_unwrap(EXAMPLE_OID_2_STR);

/// Example OID value with a large arc
const EXAMPLE_OID_LARGE_ARC_STR: &str = "0.9.2342.19200300.100.1.1";
const EXAMPLE_OID_LARGE_ARC_BER: &[u8] = &hex!("0992268993F22C640101");
const EXAMPLE_OID_LARGE_ARC: ObjectIdentifier =
ObjectIdentifier::new_unwrap("0.9.2342.19200300.100.1.1");
const EXAMPLE_OID_LARGE_ARC_0_STR: &str = "1.2.16384";
const EXAMPLE_OID_LARGE_ARC_0_BER: &[u8] = &hex!("2A818000");
const EXAMPLE_OID_LARGE_ARC_0: ObjectIdentifier =
ObjectIdentifier::new_unwrap(crate::EXAMPLE_OID_LARGE_ARC_0_STR);

/// Example OID value with a large arc
const EXAMPLE_OID_LARGE_ARC_1_STR: &str = "0.9.2342.19200300.100.1.1";
const EXAMPLE_OID_LARGE_ARC_1_BER: &[u8] = &hex!("0992268993F22C640101");
const EXAMPLE_OID_LARGE_ARC_1: ObjectIdentifier =
ObjectIdentifier::new_unwrap(EXAMPLE_OID_LARGE_ARC_1_STR);

/// Create an OID from a string.
pub fn oid(s: &str) -> ObjectIdentifier {
Expand All @@ -38,27 +44,37 @@ fn from_bytes() {
let oid0 = ObjectIdentifier::from_bytes(EXAMPLE_OID_0_BER).unwrap();
assert_eq!(oid0.arc(0).unwrap(), 0);
assert_eq!(oid0.arc(1).unwrap(), 9);
assert_eq!(oid0.arc(2).unwrap(), 2342);
assert_eq!(oid0, EXAMPLE_OID_0);

let oid1 = ObjectIdentifier::from_bytes(EXAMPLE_OID_1_BER).unwrap();
assert_eq!(oid1.arc(0).unwrap(), 1);
assert_eq!(oid1.arc(1).unwrap(), 2);
assert_eq!(oid1.arc(2).unwrap(), 840);
assert_eq!(oid1, EXAMPLE_OID_1);

let oid2 = ObjectIdentifier::from_bytes(EXAMPLE_OID_2_BER).unwrap();
assert_eq!(oid2.arc(0).unwrap(), 2);
assert_eq!(oid2.arc(1).unwrap(), 16);
assert_eq!(oid2.arc(2).unwrap(), 840);
assert_eq!(oid2, EXAMPLE_OID_2);

let oid3 = ObjectIdentifier::from_bytes(EXAMPLE_OID_LARGE_ARC_BER).unwrap();
assert_eq!(oid3.arc(0).unwrap(), 0);
assert_eq!(oid3.arc(1).unwrap(), 9);
assert_eq!(oid3.arc(2).unwrap(), 2342);
assert_eq!(oid3.arc(3).unwrap(), 19200300);
assert_eq!(oid3.arc(4).unwrap(), 100);
assert_eq!(oid3.arc(5).unwrap(), 1);
assert_eq!(oid3.arc(6).unwrap(), 1);
assert_eq!(oid3, EXAMPLE_OID_LARGE_ARC);
let oid_largearc0 = ObjectIdentifier::from_bytes(EXAMPLE_OID_LARGE_ARC_0_BER).unwrap();
assert_eq!(oid_largearc0.arc(0).unwrap(), 1);
assert_eq!(oid_largearc0.arc(1).unwrap(), 2);
assert_eq!(oid_largearc0.arc(2).unwrap(), 16384);
assert_eq!(oid_largearc0.arc(3), None);
assert_eq!(oid_largearc0, EXAMPLE_OID_LARGE_ARC_0);

let oid_largearc1 = ObjectIdentifier::from_bytes(EXAMPLE_OID_LARGE_ARC_1_BER).unwrap();
assert_eq!(oid_largearc1.arc(0).unwrap(), 0);
assert_eq!(oid_largearc1.arc(1).unwrap(), 9);
assert_eq!(oid_largearc1.arc(2).unwrap(), 2342);
assert_eq!(oid_largearc1.arc(3).unwrap(), 19200300);
assert_eq!(oid_largearc1.arc(4).unwrap(), 100);
assert_eq!(oid_largearc1.arc(5).unwrap(), 1);
assert_eq!(oid_largearc1.arc(6).unwrap(), 1);
assert_eq!(oid_largearc1, EXAMPLE_OID_LARGE_ARC_1);

// Empty
assert_eq!(ObjectIdentifier::from_bytes(&[]), Err(Error::Empty));
Expand All @@ -81,17 +97,25 @@ fn from_str() {
assert_eq!(oid2.arc(1).unwrap(), 16);
assert_eq!(oid2, EXAMPLE_OID_2);

let oid3 = EXAMPLE_OID_LARGE_ARC_STR
let oid_largearc0 = EXAMPLE_OID_LARGE_ARC_0_STR
.parse::<ObjectIdentifier>()
.unwrap();
assert_eq!(oid3.arc(0).unwrap(), 0);
assert_eq!(oid3.arc(1).unwrap(), 9);
assert_eq!(oid3.arc(2).unwrap(), 2342);
assert_eq!(oid3.arc(3).unwrap(), 19200300);
assert_eq!(oid3.arc(4).unwrap(), 100);
assert_eq!(oid3.arc(5).unwrap(), 1);
assert_eq!(oid3.arc(6).unwrap(), 1);
assert_eq!(oid3, EXAMPLE_OID_LARGE_ARC);
assert_eq!(oid_largearc0.arc(0).unwrap(), 1);
assert_eq!(oid_largearc0.arc(1).unwrap(), 2);
assert_eq!(oid_largearc0.arc(2).unwrap(), 16384);
assert_eq!(oid_largearc0, EXAMPLE_OID_LARGE_ARC_0);

let oid_largearc1 = EXAMPLE_OID_LARGE_ARC_1_STR
.parse::<ObjectIdentifier>()
.unwrap();
assert_eq!(oid_largearc1.arc(0).unwrap(), 0);
assert_eq!(oid_largearc1.arc(1).unwrap(), 9);
assert_eq!(oid_largearc1.arc(2).unwrap(), 2342);
assert_eq!(oid_largearc1.arc(3).unwrap(), 19200300);
assert_eq!(oid_largearc1.arc(4).unwrap(), 100);
assert_eq!(oid_largearc1.arc(5).unwrap(), 1);
assert_eq!(oid_largearc1.arc(6).unwrap(), 1);
assert_eq!(oid_largearc1, EXAMPLE_OID_LARGE_ARC_1);

// Truncated
assert_eq!(
Expand All @@ -117,7 +141,10 @@ fn display() {
assert_eq!(EXAMPLE_OID_0.to_string(), EXAMPLE_OID_0_STR);
assert_eq!(EXAMPLE_OID_1.to_string(), EXAMPLE_OID_1_STR);
assert_eq!(EXAMPLE_OID_2.to_string(), EXAMPLE_OID_2_STR);
assert_eq!(EXAMPLE_OID_LARGE_ARC.to_string(), EXAMPLE_OID_LARGE_ARC_STR);
assert_eq!(
EXAMPLE_OID_LARGE_ARC_1.to_string(),
EXAMPLE_OID_LARGE_ARC_1_STR
);
}

#[test]
Expand Down

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