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crypto/tls: reduce session ticket linkability

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Ever since session ticket key rotation was introduced in CL 9072, we've
been including a prefix in every ticket to identify what key it's
encrypted with. It's a small privacy gain, but the cost of trial
decryptions is also small, especially since the first key is probably
the most frequently used.

Also reissue tickets on every resumption so that the next connection
can't be linked to all the previous ones. Again the privacy gain is
small but the performance cost is small and it comes with a reduction in
complexity.

For #60105

Change-Id: I852f297162d2b79a3d9bf61f6171e8ce94b2537a
Reviewed-on: https://go-review.googlesource.com/c/go/+/496817
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
This commit is contained in:
Filippo Valsorda 2023-05-19 23:28:43 +02:00
parent b7691e8126
commit 65b9e15fc2
34 changed files with 2269 additions and 2303 deletions
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18
common.go
View file

@ -748,10 +748,6 @@ type Config struct {
}
const (
// ticketKeyNameLen is the number of bytes of identifier that is prepended to
// an encrypted session ticket in order to identify the key used to encrypt it.
ticketKeyNameLen = 16
// ticketKeyLifetime is how long a ticket key remains valid and can be used to
// resume a client connection.
ticketKeyLifetime = 7 * 24 * time.Hour // 7 days
@ -763,9 +759,6 @@ const (
// ticketKey is the internal representation of a session ticket key.
type ticketKey struct {
// keyName is an opaque byte string that serves to identify the session
// ticket key. It's exposed as plaintext in every session ticket.
keyName [ticketKeyNameLen]byte
aesKey [16]byte
hmacKey [16]byte
// created is the time at which this ticket key was created. See Config.ticketKeys.
@ -777,15 +770,18 @@ type ticketKey struct {
// bytes and this function expands that into sufficient name and key material.
func (c *Config) ticketKeyFromBytes(b [32]byte) (key ticketKey) {
hashed := sha512.Sum512(b[:])
copy(key.keyName[:], hashed[:ticketKeyNameLen])
copy(key.aesKey[:], hashed[ticketKeyNameLen:ticketKeyNameLen+16])
copy(key.hmacKey[:], hashed[ticketKeyNameLen+16:ticketKeyNameLen+32])
// The first 16 bytes of the hash used to be exposed on the wire as a ticket
// prefix. They MUST NOT be used as a secret. In the future, it would make
// sense to use a proper KDF here, like HKDF with a fixed salt.
const legacyTicketKeyNameLen = 16
copy(key.aesKey[:], hashed[legacyTicketKeyNameLen:])
copy(key.hmacKey[:], hashed[legacyTicketKeyNameLen+len(key.aesKey):])
key.created = c.time()
return key
}
// maxSessionTicketLifetime is the maximum allowed lifetime of a TLS 1.3 session
// ticket, and the lifetime we set for tickets we send.
// ticket, and the lifetime we set for all tickets we send.
const maxSessionTicketLifetime = 7 * 24 * time.Hour
// Clone returns a shallow clone of c or nil if c is nil. It is safe to clone a Config that is