The jq255e and jq255s are prime-order groups defined over some double-odd elliptic curves (i.e. elliptic curves over finite fields, whose order is twice an odd integer). This code implements cryptographic signatures and key exchange (ECDH).
The jq255.h file implements the API for these operations. The
code is meant to be portable, yet efficient; internally, it
automatically chooses between a 32-bit and a 64-bit backends, depending
on the local architecture. All operations (exception signature
verification) are constant-time. The code performs no heap memory
allocation; everything is contained on the stack. The jq255.c
file can be compiled for one or the other of the two groups, depending
on the value og hte JQ macro at the time of compilation; compile it
twice to use both curves (jq255e is the default and preferred group).
These groups offer 128-bit security level. All encodings are canonical and verified. A public key (group element) encodes over 32 bytes. A private key (scalar) encodes over 32 bytes. Signatures have length 48 bytes (i.e. they are shorter than the usual ECDSA or Ed25519 on other curves with comparable security, which use 64-byte signatures). Operations are fast (on an Intel "Coffee Lake" CPU in 64-bit mode, compiled with Clang 14.0.0, jq255e can sign in 56300 cycles, verify in 96500 cycles, and perform a key exchange in 95000 cycles; these figures include the cost of decoding the peer's public key from bytes).