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//! SKIP32 is a 80-bit key, 32-bit block cipher based on SKIPJACK.
//!
//! It has the uncommon properties of being fast, creating very dissimilar
//! encrypted values for consecutive input values, and producing output of the
//! same size as the input (32-bit). These make this cipher particularly useful
//! for obfuscating series of 32-bit integers (e.g.  auto-incremented database
//! ids). It is not appropriate for general cryptography.
//!
//! See [CPAN - Crypt::Skip32](http://search.cpan.org/%7Eesh/Crypt-Skip32-0.17/lib/Crypt/Skip32.pm)
//! for more information.
//!
//! # Example
//!
//! ```
//! let key: &[u8; 10] = &[1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
//! let input = 1000;
//! let encoded = skip32::encode(key, input);
//! let decoded = skip32::decode(key, encoded);
//! assert!(encoded == 2109307140);
//! assert!(decoded == input);
//! ```

static TABLE: &'static [u8; 256] = &[
    0xa3, 0xd7, 0x09, 0x83, 0xf8, 0x48, 0xf6, 0xf4, 0xb3, 0x21, 0x15, 0x78, 0x99, 0xb1, 0xaf, 0xf9,
    0xe7, 0x2d, 0x4d, 0x8a, 0xce, 0x4c, 0xca, 0x2e, 0x52, 0x95, 0xd9, 0x1e, 0x4e, 0x38, 0x44, 0x28,
    0x0a, 0xdf, 0x02, 0xa0, 0x17, 0xf1, 0x60, 0x68, 0x12, 0xb7, 0x7a, 0xc3, 0xe9, 0xfa, 0x3d, 0x53,
    0x96, 0x84, 0x6b, 0xba, 0xf2, 0x63, 0x9a, 0x19, 0x7c, 0xae, 0xe5, 0xf5, 0xf7, 0x16, 0x6a, 0xa2,
    0x39, 0xb6, 0x7b, 0x0f, 0xc1, 0x93, 0x81, 0x1b, 0xee, 0xb4, 0x1a, 0xea, 0xd0, 0x91, 0x2f, 0xb8,
    0x55, 0xb9, 0xda, 0x85, 0x3f, 0x41, 0xbf, 0xe0, 0x5a, 0x58, 0x80, 0x5f, 0x66, 0x0b, 0xd8, 0x90,
    0x35, 0xd5, 0xc0, 0xa7, 0x33, 0x06, 0x65, 0x69, 0x45, 0x00, 0x94, 0x56, 0x6d, 0x98, 0x9b, 0x76,
    0x97, 0xfc, 0xb2, 0xc2, 0xb0, 0xfe, 0xdb, 0x20, 0xe1, 0xeb, 0xd6, 0xe4, 0xdd, 0x47, 0x4a, 0x1d,
    0x42, 0xed, 0x9e, 0x6e, 0x49, 0x3c, 0xcd, 0x43, 0x27, 0xd2, 0x07, 0xd4, 0xde, 0xc7, 0x67, 0x18,
    0x89, 0xcb, 0x30, 0x1f, 0x8d, 0xc6, 0x8f, 0xaa, 0xc8, 0x74, 0xdc, 0xc9, 0x5d, 0x5c, 0x31, 0xa4,
    0x70, 0x88, 0x61, 0x2c, 0x9f, 0x0d, 0x2b, 0x87, 0x50, 0x82, 0x54, 0x64, 0x26, 0x7d, 0x03, 0x40,
    0x34, 0x4b, 0x1c, 0x73, 0xd1, 0xc4, 0xfd, 0x3b, 0xcc, 0xfb, 0x7f, 0xab, 0xe6, 0x3e, 0x5b, 0xa5,
    0xad, 0x04, 0x23, 0x9c, 0x14, 0x51, 0x22, 0xf0, 0x29, 0x79, 0x71, 0x7e, 0xff, 0x8c, 0x0e, 0xe2,
    0x0c, 0xef, 0xbc, 0x72, 0x75, 0x6f, 0x37, 0xa1, 0xec, 0xd3, 0x8e, 0x62, 0x8b, 0x86, 0x10, 0xe8,
    0x08, 0x77, 0x11, 0xbe, 0x92, 0x4f, 0x24, 0xc5, 0x32, 0x36, 0x9d, 0xcf, 0xf3, 0xa6, 0xbb, 0xac,
    0x5e, 0x6c, 0xa9, 0x13, 0x57, 0x25, 0xb5, 0xe3, 0xbd, 0xa8, 0x3a, 0x01, 0x05, 0x59, 0x2a, 0x46,
];

fn g(key: &[u8; 10], w: u16, k: usize) -> u16 {
    let (g1, g2) = ((w >> 8) as u8, w as u8);
    let g3 = TABLE[(g2 ^ key[(4 * k + 0) % 10]) as usize] ^ g1;
    let g4 = TABLE[(g3 ^ key[(4 * k + 1) % 10]) as usize] ^ g2;
    let g5 = TABLE[(g4 ^ key[(4 * k + 2) % 10]) as usize] ^ g3;
    let g6 = TABLE[(g5 ^ key[(4 * k + 3) % 10]) as usize] ^ g4;
    ((g5 as u16) << 8 | g6 as u16) ^ k as u16
}

fn skip32(key: &[u8; 10], x: u32, mut k: usize, kstep: usize) -> u32 {
    let (mut wl, mut wr) = ((x >> 16) as u16, x as u16);
    for _ in 0..12 {
        wr ^= g(key, wl, k);
        k = k.wrapping_add(kstep);
        wl ^= g(key, wr, k);
        k = k.wrapping_add(kstep);
    }
    (wr as u32) << 16 | wl as u32
}

/// Encode
pub fn encode(key: &[u8; 10], x: u32) -> u32 { skip32(key, x, 0, 1) }

/// Decode
pub fn decode(key: &[u8; 10], x: u32) -> u32 { skip32(key, x, 23, !0) }