Pfthb

I am working on a Golang implementation of DTLS, and I am not able to generate a valid value for VerifyData. I have a working example here this shows my code (and how I get something different then OpenSSL)

printf debugging OpenSSL it looks like the hash of handshake bodies is different then mine. It seems really unlikely, I would assume that I am collecting packets wrong. But putting printf statements before the

The cipher suite is TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and you can see the entire codebase at https://github.com/pions/dtls

VerifyData (in Finished) always uses the TLS-defined PRF, which for TLS1.2 (and DTLS1.2) is a doubled HMAC using a hash which can depend on the ciphersuite. For all pre-existing ciphersuites the D/TLS1.2 PRF uses SHA256, and the new-in-TLS1.2 suite you identified also uses SHA256. (In fact it uses SHA256 only for the PRF, because GCM suites don't use HMAC on data.) Some other new-in-TLS1.2 suites use SHA384. Also the length of VerifyData in 1.2 is formally dependent on the ciphersuite, but all pre-1.2 suites must use the pre-1.2 protocol size of 12, and all new-in-1.2 suites also do use 12, so in practice there is no difference.

However, going by your gist your problem is that you included the initial ClientHello and HelloVerifyRequest, which you shouldn't. See the antepenultimate (second to last, a word I don't get to use very often!) paragraph of section 4.2.1, near the top of page 18 in rfc6347 and section 4.2.6 which also notes, although this isn't an issue in your example, that if fragmentation is used for transmission you must still hash the unfragmented messages.