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

[Adam Langley <[e-mail]>]

Ponder, for a second, a world in which CFRG specified two schemes:
Ed25519(m) and Ed25519(H(m)).

Also consider a PKI system that issues certificates and CRLs.
Certificates are a type of message and are some serialisation of {0,
name, public-key of name}, where the zero is a single byte that serves
as a "type" indicator. CRLs are another type of message and are {1,
start-time, end-time, list of revoked certificate hashes}.

A signed message is {message, signature-algorithm, signature}.

Now, you might not think that this design is wise, but I would argue
that it's plausible as it's similar to X.509.

I, the attacker, can register a domain name and get a certificate for
it. The CA happens to sign with Ed25519(H(m)), even though the
messages aren't very large. So, offline, I iterate over many domain
names and pick one where the hash of the certificate structure for
that name and my public-key looks like a valid CRL message. I need to
fix the first byte to be one, probably fix a few other length bytes,
and obviously my fake CRL has to be exactly the length of a hash, but
the work-factor is pretty plausible.

Now I register that domain name and get a certificate for it. Then I
construct {message = fake CRL, signature-algorithm = Ed25519(m),
signature = signature from real certificate}. That should validate,
no? If I can construct a valid, signed CRL that covers a wide range of
time then I've broken revocation in this system.

The key here is that the attacker can specify the signature algorithm
and confuse the verifier. X.509 allows exactly this but, even if not,
I suspect that people will use the same key in several different
contexts like that.

So we would need to be careful with Ed25519(H(m)).


Cheers

AGL