# Short Signatures

Since the Jacobi quartic $$(e,u)$$ coordinates come with an improved decoding process that changes the on-the-wire encoding of group elements, thereby breaking backward compatibility with the original do255e and do255s, we take the opportunity to define an altered signature scheme. That new scheme yields shorter signatures (48 bytes instead of 64), which is important for some applications (e.g. if the signature must fit in some payload in a cramped QR code). As a nice side-effect, the change also makes signature verification faster, while not changing the performance of signature generation.

The idea is, in fact, quite old, and should be well-known, since it was already proposed by Schnorr himself back in 1989, and also proposed again several times (at least by Naccache and Stern in 2001, and Neven, Smart and Warinschi in 2009). Weirdly enough, there is no standard that leverages it, even though the benefits are non-negligible. It is applicable to any Schnorr signature system, not only double-odd curves (but, of course, double-odd curves work nicely with it, with their very good performance and compact encoding).

A Schnorr signature scheme can be described as follows:

• The private key is a scalar $$d$$. The corresponding public key is $$Q = dG$$, with $$G$$ begin the conventional generator for a prime order group of order $$r$$.

• To sign a message $$m$$, a per-signature secret scalar $$k$$ is generated (uniformly among integers modulo $$r$$), then the point $$R = kG$$ is computed (this is the commitment). A challenge value is then computed with a hash function $$H$$ as $$c = H(R \parallel Q \parallel m)$$ (i.e. we hash the commitment, the public key and the message together to get the challenge). The challenge is interpreted as an integer, and the response to that challenge is $$s = k + cd$$. The signature value is nominally the triplet $$(R, c, s)$$.

• The verification of the signature entails checking that the challenge $$c$$ indeed matches $$H(R \parallel Q \parallel m)$$, and that the equation $$R = sG - cQ$$ is fulfilled.

Since verifying the challenge means recomputing it, that value does not need to be transmitted, leading to a signature containing exactly the pair $$(R,s)$$. This is exactly how such things are done in the well-known EdDSA scheme. In the case of Ed25519, $$R$$ and $$s$$ are encoded over 32 bytes each, for a total signature size of 64 bytes.

We can also choose to omit $$R$$ instead of $$c$$: given $$Q$$, $$s$$ and $$c$$, we can recompute $$R = sG - cQ$$. This leads to an alternate representation of the signature as the pair $$(c,s)$$. From this, we can get a gain in size by noticing that $$c$$ does not actually need to be as large as the group. It can be shorter.

The Neven-Smart-Warinschi paper includes some detailed security analysis, which we can summarize as follows:

• The security of Schnorr signatures does not depend upon the collision resistance of the hash function $$H$$.

• There are known proofs of security that reduce the security of Schnorr signatures to the discrete logarithm problem in the group with some conditions on the size of the group and the size of the output of $$H$$. They show that if you target "$$n$$ bits of security", then you will get them from a group of size $$2^{3n}$$ and a hash function output of $$2n$$ bits (assuming ideal group and hash function).

• The proofs are not tight; they don't show that attacks are possible up to these sizes. In fact, against the best known attacks, the $$n$$-bit security level is obtained with a $$2^{2n}$$ group (e.g. a 256-bit elliptic curve) and a hash output of only $$n$$ bits.

• We already disregard the proofs in practice. For instance, Ed25519 claims "128-bit security" with a 256-bit curve, while the proofs would call for a 384-bit curve.

From this analysis, we should get a practical security of 128 bits if we use a 256-bits-or-so elliptic curve (e.g. jq255e or jq255s) and a 128-bit challenge $$c$$. Combined with the $$(c,s)$$ representation of signatures, this yields a total signature size of 48 bytes.

In the fine details of the Neven-Smart-Warinschi analysis, we see that the hash function must behave quite closely to an ideal hash function, which rules out "narrow-pipe" designs; in plain words, we get our full 128-bit security only if the hash function $$H$$ is really a good hash function with a 256-bit output, that we then truncate to 128 bits. A standard 256-bit hash function should work; we choose BLAKE2s (which is secure but also quite fast and maps well to small 32-bit systems). SHA-256, or even SHA3-256, would also work.

The speed advantage comes from the fact that the computation of $$R = sG - cQ$$ can be split into: $\begin{equation*} R = (s \bmod 2^{128}) G + (\lfloor s/2^{128} \rfloor) (2^{128} G) + c (-Q) \end{equation*}$ i.e. a linear combination of three points (two of which being amenable to precomputations) with half-size (128 bits) coefficients. This yields better performance than the usual EdDSA signature verification, even if using the Antipa et al optimization with Lagrange's lattice basis reduction: the latter leads to a linear combination of four points with half-size coefficients (and adds the cost of Lagrange's algorithm, which we avoid here).

In total, we can get signature verification on jq255e and jq255s to run in less than 100k cycles on an Intel x86 CPU (Coffee Lake core). The main advantage of the short signatures is their reduced size, but the speed gain is nice as well.

It shall be noted that the shorter signature format is not compatible with batch verification. With EdDSA signatures, one can leverage the presence of $$R$$ in the signature to verify many signatures with a lower per-signature cost; this optimization cannot be applied to the shorter $$(c,s$$) signature format. Of course, the point of batch verification is to reduce the per-verification cost, and that is less needed if the verification of a single signature is already fast.