Comment by bastawhiz

AES would mean the encrypted parts of the id are ~28+ bytes. That's a long minimum identifier length.

What you're suggesting is perhaps true in the sense that the throughout is higher, but AES decryption carries a fairly high fixed overhead. If you're in a language like Ruby (as GitHub is) or Python/Node, you're probably calling out to openssl.

I did try to do my diligence and find data to support or refute your claim, but I wasn't able to find anything that does directly. That said, I'm not able to find any sources that support the idea that AES is faster at decryption than base64 in any context (for small plaintext values or in general). With SIMD, b64 often decodes in 0.2 CPU cycles or so per byte, while AES only manages 2.5-10.7 CPU cycles per byte. The numbers for AES get better as the plaintext size grow, though.

Do you happen to have data to support your claim?