Cipher_comparison.md

Cipher comparison

AES-GCM vs. ChaCha20-Poly1305

• If you have hardware acceleration (e.g. AES-NI), then AES-GCM provides better performance. On my benchmarks, it was faster by a factor of 1.28 on average.
  If you do not have hardware acceleration, AES-GCM is either slower than ChaCha20-Poly1305, or it leaks your encryption keys in cache timing.
• AES-GCM can target multiple security levels (128-bit, 192-bit, 256-bit), whereas ChaCha20-Poly1305 is only defined at the 256-bit security level.
• Nonce size:
  • AES-GCM: Varies, but the standard is 96-bit (12 bytes). If you supply a longer nonce, this gets hashed down to 16 bytes.
  • ChaCha20-Poly1305: The standardized version uses 96-bit nonce (12 bytes), but the original used 64-bit nonce (8 bytes).
• Wear-out of a single (key, nonce) pair:
  • AES-GCM: Messages must be less than 2^32 – 2 blocks (a.k.a. 2^36 – 32 bytes, a.k.a. 2^39 – 256-bit), that's roughly 64GB. This also makes the security analysis of AES-GCM with long nonces complicated since the hashed nonce doesn’t start with the lower 4 bytes set to 00 00 00 02.
  • ChaCha20-Poly1305: ChaCha has an internal counter (32-bit in the standardized IETF variant, 64-bit in the original design). Max message length is 2^39 - 256-bit, about 256GB
• Neither algorithm is nonce misuse-resistant.
• ChaChaPoly1305 is better at SIMD

Conclusion

Both are good options. AES-GCM can be faster with hardware support, but pure-software implementations of ChaCha20-Poly1305 are almost always fast and constant-time.