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c193fe9848
This fixes a vulnerability that compromises NIST P521 ECDSA keys when they are used with PuTTY's existing DSA nonce generation code. The vulnerability has been assigned the identifier CVE-2024-31497. PuTTY has been doing its DSA signing deterministically for literally as long as it's been doing it at all, because I didn't trust Windows's entropy generation. Deterministic nonce generation was introduced in commit d345ebc2a5a0b59, as part of the initial version of our DSA signing routine. At the time, there was no standard for how to do it, so we had to think up the details of our system ourselves, with some help from the Cambridge University computer security group. More than ten years later, RFC 6979 was published, recommending a similar system for general use, naturally with all the details different. We didn't switch over to doing it that way, because we had a scheme in place already, and as far as I could see, the differences were not security-critical - just the normal sort of variation you expect when any two people design a protocol component of this kind independently. As far as I know, the _structure_ of our scheme is still perfectly fine, in terms of what data gets hashed, how many times, and how the hash output is converted into a nonce. But the weak spot is the choice of hash function: inside our dsa_gen_k() function, we generate 512 bits of random data using SHA-512, and then reduce that to the output range by modular reduction, regardless of what signature algorithm we're generating a nonce for. In the original use case, this introduced a theoretical bias (the output size is an odd prime, which doesn't evenly divide the space of 2^512 possible inputs to the reduction), but the theory was that since integer DSA uses a modulus prime only 160 bits long (being based on SHA-1, at least in the form that SSH uses it), the bias would be too small to be detectable, let alone exploitable. Then we reused the same function for NIST-style ECDSA, when it arrived. This is fine for the P256 curve, and even P384. But in P521, the order of the base point is _greater_ than 2^512, so when we generate a 512-bit number and reduce it, the reduction never makes any difference, and our output nonces are all in the first 2^512 elements of the range of about 2^521. So this _does_ introduce a significant bias in the nonces, compared to the ideal of uniformly random distribution over the whole range. And it's been recently discovered that a bias of this kind is sufficient to expose private keys, given a manageably small number of signatures to work from. (Incidentally, none of this affects Ed25519. The spec for that system includes its own idea of how you should do deterministic nonce generation - completely different again, naturally - and we did it that way rather than our way, so that we could use the existing test vectors.) The simplest fix would be to patch our existing nonce generator to use a longer hash, or concatenate a couple of SHA-512 hashes, or something similar. But I think a more robust approach is to switch it out completely for what is now the standard system. The main reason why I prefer that is that the standard system comes with test vectors, which adds a lot of confidence that I haven't made some other mistake in following my own design. So here's a commit that adds an implementation of RFC 6979, and removes the old dsa_gen_k() function. Tests are added based on the RFC's appendix of test vectors (as many as are compatible with the more limited API of PuTTY's crypto code, e.g. we lack support for the NIST P192 curve, or for doing integer DSA with many different hash functions). One existing test changes its expected outputs, namely the one that has a sample key pair and signature for every key algorithm we support.