[Cfrg] Summary of the poll: Elliptic Curves - signature scheme: friendliness to low memory implementations (ends on June 3rd)

[Alexey Melnikov <[e-mail]>]

On 20/05/2015 22:16, Alexey Melnikov wrote:
> Signature schemes have, traditionally, hashed the message to be signed to
> a representative value and then actually signed that value. This has led
> to APIs that follow the Init, Update*, Final (IUF) pattern that allows for
> fragmented or very large messages to be signed with constant memory usage.
>
> Alternatively, some signature schemes hash the message to be signed twice,
> where a prefix of the second hash depends on the result of the first.
> Ed25519 is an example of a scheme of this type. This typically allows the
> signature scheme to require fewer assumptions about the strength of the
> hash function that is employed. However, this approach implies that the signing
> algorithm would have to buffer the entire message. That could lead to
> unacceptable memory usage for applications that sign very large messages.
>
> Penalising large messages might be seen as a positive by some because
> signing arbitrarily large messages invites implementations to process
> unauthenticated data because of its potential size. Also, designs that
> really wish to do this can always define that the message to be signed is
> actually a hash of the data. In doing so they would reintroduce the
> requirement that the hash function be collision resistant, but can
> implement an IUF API in constant space again.
>
> On the other hand, if we wish a signature primitive to be easy to
> substitute into existing protocols then supporting a constant-space IUF
> API might be seen as a requirement.
>
> Chairs wish to poll the group on the following topic (please pick one):
>
> #1: The signature scheme should follow the traditional model of hashing
> the message to be signed, thus trivially supporting IUF APIs in
> constant-space, at the cost of requiring collision resistant hash
> functions.
>
> #2: The signature scheme should not depend on collision resistance, at the
> cost of the signing algorithm having to buffer messages to be signed.
>
> #3: option #2, but with an additional "large message" mode defined for
> protocols that feel that they need it, where the message to be signed is
> actually a hash of the true message.

After discussing answers to this poll chairs concluded that despite
security limitations of Initiliase-Update-Finalise (IUF) APIs on verifier's
end, there is rough consensus that any recommended CFRG signature scheme
should support such APIs.

The next step for CFRG is to solicit proposals for EC signature schemes that
satisfy the previously agreed requirements, including support for the IUF APIs.