rsp6-decoder/src/main.rs

use crate::keys::IssuerKeyStore;
use crate::payload::Rsp6Ticket;
use anyhow::anyhow;
use num_bigint::BigUint;

mod keys;
mod payload;

fn base26_decode(input: &str) -> BigUint {
    let mut out = BigUint::new(Vec::new());
    for val in input.as_bytes().iter().rev() {
        out *= 26u32;
        out += *val - b'A';
    }
    BigUint::from_bytes_le(&out.to_bytes_be())
}

fn strip_padding(tkt: &[u8]) -> Option<&[u8]> {
    if tkt.is_empty() {
        return None;
    }
    match tkt[0] {
        1 => {
            // PKCS#1 v1
            let tkt = &tkt[1..];
            let mut iter = tkt.iter();
            loop {
                match iter.next()? {
                    0 => {
                        return Some(iter.as_slice());
                    }
                    255 => {}
                    _ => return None,
                }
            }
        }
        2 => {
            // PKCS#1 v2
            let tkt = &tkt[1..];
            let mut iter = tkt.iter();
            loop {
                match iter.next()? {
                    0 => {
                        return Some(iter.as_slice());
                    }
                    _ => {}
                }
            }
        }
        _ => None,
    }
}

fn main() -> anyhow::Result<()> {
    let iks = IssuerKeyStore::new();
    eprintln!("[+] Loaded {} public keys!", iks.keys.len());
    eprintln!("[+] mmm, give me a tasty ticket on stdin please");
    let ticket_str = std::io::stdin()
        .lines()
        .next()
        .ok_or_else(|| anyhow!("that was a bit rude, give me an actual ticket"))??;
    if ticket_str.len() < 16 {
        return Err(anyhow!("ticket too short"));
    }
    if &ticket_str[0..2] != "06" {
        return Err(anyhow!(
            "ticket isn't a RSP6 ticket: magic was {}",
            &ticket_str[0..2]
        ));
    }
    let issuer_id = &ticket_str[13..15];
    let ticket_reference = format!("{}{}", issuer_id, &ticket_str[2..11]);
    eprintln!("[+] RSP6 ticket, reference {}", ticket_reference);
    let ticket = base26_decode(&ticket_str[15..]);
    let keys = iks
        .keys
        .get(issuer_id)
        .ok_or_else(|| anyhow!("unknown issuer ID {}", issuer_id))?;
    for key in keys {
        let message = ticket.modpow(&key.public_exponent, &key.modulus);
        let message = message.to_bytes_be();
        if let Some(unpadded) = strip_padding(&message) {
            eprintln!("[+] decrypt done!");
            let ticket = Rsp6Ticket::decode(unpadded, issuer_id.into(), ticket_str[11..13].into())?;
            serde_json::to_writer_pretty(std::io::stdout(), &ticket)?;
            return Ok(());
        } else {
            eprintln!("[-] failed decrypt: {:?}", message);
        }
    }
    Err(anyhow!("no valid decryptions"))
}

#[cfg(test)]
mod test {
    #[test]
    fn test_base26() {
        assert_eq!(
            super::base26_decode("RANEBZCYPNQVMMYJBOJBONYSIYXTREYFSHTZFZEXWTVBNXJBFVOFBMXVQPZTFWVYSWYKINRXRVDCCUWUERKQZKYBPVIIAPJOOFJJXUBFGNVXGXTCFPBHXYVPEKWIURBEOYTYNZUXWVIXHAODACOQLZEQKRUNGWSJHIIWOYSNXJKVYWIGLWCIZKAHFKKAKRDUQSQBGEJMOFCSHSKXSFDDKYCFQI").to_bytes_be(),
            [53, 242, 184, 141, 14, 99, 169, 215, 200, 223, 85, 250, 45, 253, 184, 100, 225, 124, 82, 70, 138, 222, 246, 185, 192, 129, 247, 218, 24, 26, 249, 112, 74, 225, 71, 139, 27, 50, 218, 11, 93, 238, 232, 163, 151, 68, 159, 146, 80, 133, 11, 45, 57, 245, 163, 117, 218, 11, 187, 246, 18, 147, 88, 171, 133, 216, 166, 47, 232, 246, 198, 170, 99, 36, 120, 114, 73, 207, 19, 218, 202, 146, 158, 223, 107, 234, 171, 172, 20, 189, 133, 246, 192, 248, 57, 111, 65, 65, 135, 64, 241, 99, 87, 107, 75, 40, 224, 223, 100, 53, 180, 212, 53, 200, 172, 117, 127, 248, 193, 0, 147, 167, 222, 81, 135, 158, 135, 137]
        )
    }
}