diff options
author | Justus Winter <justus@sequoia-pgp.org> | 2021-11-05 16:22:25 +0100 |
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committer | Justus Winter <justus@sequoia-pgp.org> | 2021-11-05 16:30:12 +0100 |
commit | aeacd64d736bbdbc5a0072b57605dcde7a2f0e44 (patch) | |
tree | d8bcccad7ede1a04e086976a40be12499aae5667 /openpgp | |
parent | d768ed170e5e43ddae2b53632a7b9a7e39d8fd49 (diff) |
openpgp: Implement CRC24 using a precomputed table.
- With this patch, Sequoia's armor encoding and decoding performance
roughly matches GnuPG's.
- Fixes #772.
Diffstat (limited to 'openpgp')
-rw-r--r-- | openpgp/src/armor/crc.rs | 41 |
1 files changed, 34 insertions, 7 deletions
diff --git a/openpgp/src/armor/crc.rs b/openpgp/src/armor/crc.rs index f9f695bd..45819692 100644 --- a/openpgp/src/armor/crc.rs +++ b/openpgp/src/armor/crc.rs @@ -18,16 +18,43 @@ impl CRC { CRC { n: CRC24_INIT } } + /// Updates the CRC sum using the given data. + /// + /// This implementation uses a lookup table. See: + /// + /// Sarwate, Dilip V. "Computation of cyclic redundancy checks via + /// table look-up." Communications of the ACM 31.8 (1988): + /// 1008-1013. pub fn update(&mut self, buf: &[u8]) -> &Self { - for octet in buf { - self.n ^= (*octet as u32) << 16; - for _ in 0..8 { - self.n <<= 1; - if self.n & 0x1000000 > 0 { - self.n ^= CRC24_POLY; + lazy_static::lazy_static! { + static ref TABLE: Vec<u32> = { + let mut t = vec![0u32; 256]; + + let mut crc = 0x80_0000; // 24 bit polynomial + let mut i = 1; + loop { + if crc & 0x80_0000 > 0 { + crc = (crc << 1) ^ CRC24_POLY; + } else { + crc <<= 1; + } + for j in 0..i { + t[i + j] = crc ^ t[j]; + } + i <<= 1; + if i == 256 { + break; + } } - } + t + }; } + + for octet in buf { + self.n = (self.n << 8) + ^ TABLE[(*octet ^ ((self.n >> 16) as u8)) as usize]; + } + self } |