Preserve leading zeros in Diffie-Hellman secrets. #462
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Every other use of Diffie-Hellman preserves leading zeros. See PKCS tlswg#3 section 8.3, RFC 2631 section 2.1.2, and SP 800-56A section C.1. I assume X9.42 says something similar to RFC 2631, but I do not have a copy of it. This discrepancy has caused sporadic interoperability issues in TLS 1.2's DHE construction, so clearly it's confusing. Moreover, having variable-length secrets is generally dubious. It exposes an unnecessary side channel. Since TLS 1.2's server-fiat DHE and TLS 1.3's negotiated DHE are already very different animals (1.3 DHE is more like 1.2 ECDHE than anything else), change it to the more reasonable scheme going forward. This is not compatible with 1.2 and does risk a different set of sporadic interop issues if implementations do not realize this changed, but we already have those with 1.2 implementations today.
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Every other use of Diffie-Hellman preserves leading zeros. See PKCS #3 section
8.3, RFC 2631 section 2.1.2, and SP 800-56A section C.1. I assume X9.42 says
something similar to RFC 2631, but I do not have a copy of it. This discrepancy
has caused sporadic interoperability issues in TLS 1.2's DHE construction, so
clearly it's confusing.
Moreover, having variable-length secrets is generally dubious. It exposes an
unnecessary side channel.
Since TLS 1.2's server-fiat DHE and TLS 1.3's negotiated DHE are already very
different animals (1.3 DHE is more like 1.2 ECDHE than anything else), change
it to the more reasonable scheme going forward. This is not compatible with 1.2
and does risk a different set of sporadic interop issues if implementations do
not realize this changed, but we already have those with 1.2 implementations
today.