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crypto/tls: support TLS_FALLBACK_SCSV as a server.

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A new attack on CBC padding in SSLv3 was released yesterday[1]. Go only
supports SSLv3 as a server, not as a client. An easy fix is to change
the default minimum version to TLS 1.0 but that seems a little much
this late in the 1.4 process as it may break some things.

Thus this patch adds server support for TLS_FALLBACK_SCSV[2] -- a
mechanism for solving the fallback problem overall. Chrome has
implemented this since February and Google has urged others to do so in
light of yesterday's news.

With this change, clients can indicate that they are doing a fallback
connection and Go servers will be able to correctly reject them.

[1] http://googleonlinesecurity.blogspot.com/2014/10/this-poodle-bites-exploiting-ssl-30.html
[2] https://tools.ietf.org/html/draft-ietf-tls-downgrade-scsv-00

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/157090043
This commit is contained in:
Adam Langley 2014-10-15 17:54:04 -07:00
parent a59ca4a0e7
commit a367222d8d
5 changed files with 61 additions and 2 deletions
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5
cipher_suites.go
View file

@ -267,4 +267,9 @@ const (
TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA uint16 = 0xc014
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 uint16 = 0xc02f
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 uint16 = 0xc02b
// TLS_FALLBACK_SCSV isn't a standard cipher suite but an indicator
// that the client is doing version fallback. See
// https://tools.ietf.org/html/draft-ietf-tls-downgrade-scsv-00.
TLS_FALLBACK_SCSV uint16 = 0x5600
)