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crypto/tls: implement TLS 1.3 downgrade protection

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TLS_FALLBACK_SCSV is extremely fragile in the presence of sparse
supported_version, but gave it the best try I could.

Set the server random canaries but don't check them yet, waiting for the
browsers to clear the way of misbehaving middleboxes.

Updates #9671

Change-Id: Ie55efdec671d639cf1e716acef0c5f103e91a7ce
Reviewed-on: https://go-review.googlesource.com/c/147617
Reviewed-by: Adam Langley <agl@golang.org>
This commit is contained in:
Filippo Valsorda 2018-11-05 20:39:45 -05:00
parent b523d280e4
commit fc44e85605
5 changed files with 57 additions and 5 deletions
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18
handshake_server.go
View file

@ -210,7 +210,18 @@ Curves:
}
hs.hello.random = make([]byte, 32)
_, err := io.ReadFull(c.config.rand(), hs.hello.random)
serverRandom := hs.hello.random
// Downgrade protection canaries. See RFC 8446, Section 4.1.3.
maxVers := c.config.maxSupportedVersion(false)
if maxVers >= VersionTLS12 && c.vers < maxVers {
if c.vers == VersionTLS12 {
copy(serverRandom[24:], downgradeCanaryTLS12)
} else {
copy(serverRandom[24:], downgradeCanaryTLS11)
}
serverRandom = serverRandom[:24]
}
_, err := io.ReadFull(c.config.rand(), serverRandom)
if err != nil {
c.sendAlert(alertInternalError)
return err
@ -299,11 +310,10 @@ func (hs *serverHandshakeState) pickCipherSuite() error {
return errors.New("tls: no cipher suite supported by both client and server")
}
// See RFC 7507.
for _, id := range hs.clientHello.cipherSuites {
if id == TLS_FALLBACK_SCSV {
// The client is doing a fallback connection.
if hs.clientHello.vers < c.config.supportedVersions(false)[0] {
// The client is doing a fallback connection. See RFC 7507.
if hs.clientHello.vers < c.config.maxSupportedVersion(false) {
c.sendAlert(alertInappropriateFallback)
return errors.New("tls: client using inappropriate protocol fallback")
}