Why private_key 64 bits?

[iceriverdog]

Soory sir:

i have a confuse that why private key is 64 bits? the order of G is 2^255 +277....493, so privacy_key shouldnt be one number less order? So why 64 bits?

Thank you!
Wang

[Orson Peters]

First, private_key is 64 bytes, not 64 bits. The first 32 bytes store a curve point, and as you can see some bits are ignored/set to make it valid:

ed25519/src/keypair.c

Line 10 in 7fa6712

 private_key[0] &= 248; 

The second 32 bytes store a second portion of the hash of the seed as it's needed for Ed25519's generation of r.