//! Domain-separated SHA-256 identity of the exact file-backed bytes installed
//! in a narrow ELF user program. This is an identity witness, not a signature.
use sha2::{Digest, Sha256};

pub const DOMAIN: &[u8] = b"TurkOS/ELF-PROGRAM/v1\0";

pub fn digest(entry: u64, code: &[u8], data: &[u8]) -> [u8; 32] {
    let mut sha = Sha256::new();
    sha.update(DOMAIN);
    sha.update(entry.to_le_bytes());
    sha.update((code.len() as u64).to_le_bytes());
    sha.update(code);
    sha.update((data.len() as u64).to_le_bytes());
    sha.update(data);
    sha.finalize().into()
}

pub fn hex_lower(bytes: &[u8; 32]) -> [u8; 64] {
    let mut output = [0u8; 64];
    const HEX: &[u8; 16] = b"0123456789abcdef";
    for (index, byte) in bytes.iter().enumerate() {
        output[2 * index] = HEX[(byte >> 4) as usize];
        output[2 * index + 1] = HEX[(byte & 15) as usize];
    }
    output
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn sha256_library_matches_published_hello_world_vector() {
        let digest = Sha256::digest(b"hello world");
        let expected = b"b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9";
        assert_eq!(hex_lower(&digest.into()), *expected);
    }

    #[test]
    fn program_identity_binds_entry_lengths_and_file_backed_bytes() {
        let baseline = digest(0x8000_0000, b"CODE", b"HELLO");
        assert_ne!(baseline, digest(0x8000_0004, b"CODE", b"HELLO"));
        assert_ne!(baseline, digest(0x8000_0000, b"CODF", b"HELLO"));
        assert_ne!(baseline, digest(0x8000_0000, b"CODE", b"WORLD"));
        assert_ne!(baseline, digest(0x8000_0000, b"CODE\0", b"HELLO"));
        assert_eq!(hex_lower(&baseline).len(), 64);
    }
}