uselesskey-core 0.8.0

//! DER corruption helpers for negative test fixtures.
//!
//! Provides deterministic truncation, byte-flipping, and combined corruption
//! strategies for DER-encoded blobs. Used by higher-level negative fixture
//! helpers (`uselesskey_core::srp::negative`) to generate invalid DER
//! artifacts that exercise parser error paths in tests.

use alloc::vec::Vec;

use crate::srp::hash::hash32;

/// Truncate `der` to at most `len` bytes, returning the original if already shorter.
pub fn truncate_der(der: &[u8], len: usize) -> Vec<u8> {
    if len >= der.len() {
        return der.to_vec();
    }
    der[..len].to_vec()
}

/// XOR the byte at `offset` with `0x01`, returning the original if `offset` is out of range.
pub fn flip_byte(der: &[u8], offset: usize) -> Vec<u8> {
    if offset >= der.len() {
        return der.to_vec();
    }

    let mut out = der.to_vec();
    out[offset] ^= 0x01;
    out
}

/// Choose a corruption strategy deterministically from `variant` and apply it to `der`.
///
/// The same `(der, variant)` pair always produces the same corrupted output.
pub fn corrupt_der_deterministic(der: &[u8], variant: &str) -> Vec<u8> {
    let digest = hash32(variant.as_bytes());
    let bytes = digest.as_bytes();

    match bytes[0] % 3 {
        0 => {
            let len = derived_truncate_len_bytes(der.len(), bytes);
            truncate_der(der, len)
        }
        1 => {
            let offset = derived_offset(der.len(), bytes[1]);
            flip_byte(der, offset)
        }
        _ => {
            let offset = derived_offset(der.len(), bytes[1]);
            let flipped = flip_byte(der, offset);
            let len = derived_truncate_len_bytes(flipped.len(), bytes);
            truncate_der(&flipped, len)
        }
    }
}

fn derived_offset(len: usize, selector: u8) -> usize {
    if len == 0 {
        return 0;
    }
    selector as usize % len
}

fn derived_truncate_len_bytes(len: usize, digest: &[u8; 32]) -> usize {
    if len <= 1 {
        return 0;
    }
    let span = len - 1;
    digest[2] as usize % span
}

#[cfg(all(test, feature = "std"))]
mod tests {
    use super::*;

    #[test]
    fn flip_byte_changes_only_target_offset() {
        let der = vec![0x30, 0x82, 0x01, 0x22];
        let flipped = flip_byte(&der, 0);

        assert_eq!(flipped[0], 0x31);
        assert_eq!(&flipped[1..], &der[1..]);
    }

    #[test]
    fn truncate_der_shortens_when_len_smaller() {
        let der = vec![0x30, 0x82, 0x01, 0x22];
        let truncated = truncate_der(&der, 2);
        assert_eq!(truncated, vec![0x30, 0x82]);
    }

    #[test]
    fn deterministic_der_corruption_is_stable_for_same_variant() {
        let der = vec![0x30, 0x82, 0x01, 0x22, 0x10, 0x20];
        let first = corrupt_der_deterministic(&der, "corrupt:variant-a");
        let second = corrupt_der_deterministic(&der, "corrupt:variant-a");
        assert_eq!(first, second);
        assert_ne!(first, der);
    }

    #[test]
    fn derived_truncate_len_bytes_exact_arithmetic() {
        let mut digest = [0u8; 32];
        digest[2] = 0x0B; // 11 % 4 = 3
        assert_eq!(derived_truncate_len_bytes(5, &digest), 3);
    }

    #[test]
    fn derived_truncate_len_bytes_single_returns_zero() {
        let digest = [0u8; 32];
        assert_eq!(derived_truncate_len_bytes(1, &digest), 0);
    }

    #[test]
    fn derived_offset_exact_arithmetic() {
        assert_eq!(derived_offset(5, 7), 2); // 7 % 5 = 2
    }

    #[test]
    fn derived_offset_zero_len_returns_zero() {
        assert_eq!(derived_offset(0, 7), 0);
    }

    #[test]
    fn flip_byte_xor_vs_or_on_set_bit() {
        // XOR: 0x01 ^ 0x01 = 0x00; OR mutation would give 0x01 | 0x01 = 0x01.
        let data = vec![0x01];
        let result = flip_byte(&data, 0);
        assert_eq!(result[0], 0x00);
    }

    #[test]
    fn deterministic_der_arm0_truncation() {
        let der = vec![0x30, 0x82, 0x01, 0x22, 0x10, 0x20];
        let variant = find_der_variant(0);
        let out = corrupt_der_deterministic(&der, &variant);
        assert!(out.len() < der.len());
        assert_eq!(&out[..], &der[..out.len()]);
    }

    #[test]
    fn deterministic_der_arm1_flip() {
        let der = vec![0x30, 0x82, 0x01, 0x22, 0x10, 0x20];
        let variant = find_der_variant(1);
        let out = corrupt_der_deterministic(&der, &variant);
        assert_eq!(out.len(), der.len());
        let diffs = out.iter().zip(der.iter()).filter(|(a, b)| a != b).count();
        assert_eq!(diffs, 1);
    }

    #[test]
    fn deterministic_der_arm2_flip_truncate() {
        let der = vec![0x30, 0x82, 0x01, 0x22, 0x10, 0x20];
        let variant = find_der_variant(2);
        let out = corrupt_der_deterministic(&der, &variant);
        assert!(out.len() < der.len());
    }

    #[test]
    fn deterministic_der_not_constant() {
        let der1 = vec![0x30, 0x82, 0x01, 0x22, 0x10, 0x20];
        let der2 = vec![0xFF, 0xFE, 0xFD, 0xFC, 0xFB, 0xFA];
        let out1 = corrupt_der_deterministic(&der1, "same-variant");
        let out2 = corrupt_der_deterministic(&der2, "same-variant");
        assert_ne!(out1, out2);
    }

    #[test]
    fn truncate_der_noop_when_len_exceeds_input() {
        let der = vec![0x30, 0x82];
        let out = truncate_der(&der, 100);
        assert_eq!(
            out, der,
            "truncate with len >= der.len() must return original"
        );
    }

    #[test]
    fn flip_byte_noop_when_offset_exceeds_input() {
        let der = vec![0x30, 0x82];
        let out = flip_byte(&der, 100);
        assert_eq!(
            out, der,
            "flip with offset >= der.len() must return original"
        );
    }

    #[test]
    fn derived_truncate_len_bytes_nonzero_for_large_input() {
        // Catches `return 0` and `return 1` mutations when tested
        // through corrupt_der_deterministic arm 0.
        let mut digest = [0u8; 32];
        digest[2] = 50;
        // len=10, span=9, 50%9=5
        assert_eq!(derived_truncate_len_bytes(10, &digest), 5);
    }

    #[test]
    fn derived_truncate_len_bytes_zero_len_returns_zero() {
        let digest = [0xFF; 32];
        assert_eq!(derived_truncate_len_bytes(0, &digest), 0);
    }

    fn find_der_variant(target: u8) -> String {
        use crate::srp::hash::hash32;

        for i in 0u64.. {
            let variant = format!("v{i}");
            if hash32(variant.as_bytes()).as_bytes()[0] % 3 == target {
                return variant;
            }
        }
        unreachable!()
    }
}