fn nonnegative_integer()

in aws-lc-rs/src/io/der.rs [123:179]

• 49 lines of code
• 8 McCabe index (conditional complexity)

fn nonnegative_integer<'a>(
    input: &mut untrusted::Reader<'a>,
    min_value: u8,
) -> Result<untrusted::Input<'a>, error::Unspecified> {
    // Verify that |input|, which has had any leading zero stripped off, is the
    // encoding of a value of at least |min_value|.
    fn check_minimum(input: untrusted::Input, min_value: u8) -> Result<(), error::Unspecified> {
        input.read_all(error::Unspecified, |input| {
            let first_byte = input.read_byte()?;
            if input.at_end() && first_byte < min_value {
                return Err(error::Unspecified);
            }
            let _: untrusted::Input = input.read_bytes_to_end();
            Ok(())
        })
    }

    let value = expect_tag_and_get_value(input, Tag::Integer)?;

    value.read_all(error::Unspecified, |input| {
        // Empty encodings are not allowed.
        let first_byte = input.read_byte()?;

        if first_byte == 0 {
            if input.at_end() {
                // |value| is the legal encoding of zero.
                if min_value > 0 {
                    return Err(error::Unspecified);
                }
                return Ok(value);
            }

            let r = input.read_bytes_to_end();
            r.read_all(error::Unspecified, |input| {
                let second_byte = input.read_byte()?;
                if (second_byte & 0x80) == 0 {
                    // A leading zero is only allowed when the value's high bit
                    // is set.
                    return Err(error::Unspecified);
                }
                let _: untrusted::Input = input.read_bytes_to_end();
                Ok(())
            })?;
            check_minimum(r, min_value)?;
            return Ok(r);
        }

        // Negative values are not allowed.
        if (first_byte & 0x80) != 0 {
            return Err(error::Unspecified);
        }

        let _: untrusted::Input = input.read_bytes_to_end();
        check_minimum(value, min_value)?;
        Ok(value)
    })
}