Raw RSA

[Leichter, Jerry]

| > | It is known, that given such an oracle, the attacker can ask for
| > | "decryption"  of all primes less than B, and then he will be able to
| > | sign PKCS-1 encoded messages if the representative number is B-smooth,
| > | but is there any way to actually recover d itself?
| 
| > RSA is multiplicative, so, yes, this follows easily unless the encoding
| > used prevents it.
| 
| Could you describe this attack in more detail.  I do not see a scenario where
| it would be useful.
| 
| The attacker can encrypt a subset of numbers - those that encrypt to a B
| smooth number, but for this to be useful to him, he has to find a number in
| the subset set that corresponds to what he desires to encrypt, which  looks
| like a very long brute force search.
Let R(x) = x^k mod n - where k might be the public key - for encryption -
or the private key - for signing.  R(xy) = R(x)R(y) mod n.  For
encryption, this means little - since the encryption exponent is
public, anyone can compute R(xy) directly anyway.  But for signing,
it means that if I have my hands on signed copies of x and y, I can
forge a signature on xy.  Thus, if I am able to get signatures on a
good collection of primes, I can sign many messages easily.  Yet
another reason to sign hashes of messages, not raw messages - and
that the signer module should compute the hash itself, not rely on
the caller to do it.
							-- Jerry


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