crypto/tls: randomly generate ticket_age_add

As required by RFC 8446, section 4.6.1, ticket_age_add now holds a
random 32-bit value. Before this change, this value was always set
to 0.

This change also documents the reasoning for always setting
ticket_nonce to 0. The value ticket_nonce must be unique per
connection, but we only ever send one ticket per connection.

Fixes #52814
Fixes CVE-2022-30629

Change-Id: I6c2fc6ca0376b7b968abd59d6d3d3854c1ab68bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/405994
Reviewed-by: Tatiana Bradley <tatiana@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Tatiana Bradley <tatiana@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

diff --git a/src/crypto/tls/handshake_server_tls13.go b/src/crypto/tls/handshake_server_tls13.go
index 09674764e2cf8c0ef2a0e6393ae704793985fa52..03a477f7bed54b658f069fec8f7eb15bf1b3de9f 100644 (file)
--- a/src/crypto/tls/handshake_server_tls13.go
+++ b/src/crypto/tls/handshake_server_tls13.go
@@ -10,6 +10,7 @@ import (
        "crypto"
        "crypto/hmac"
        "crypto/rsa"
+       "encoding/binary"
        "errors"
        "hash"
        "io"
@@ -745,6 +746,19 @@ func (hs *serverHandshakeStateTLS13) sendSessionTickets() error {
        }
        m.lifetime = uint32(maxSessionTicketLifetime / time.Second)
 
+       // ticket_age_add is a random 32-bit value. See RFC 8446, section 4.6.1
+       // The value is not stored anywhere; we never need to check the ticket age
+       // because 0-RTT is not supported.
+       ageAdd := make([]byte, 4)
+       _, err = hs.c.config.rand().Read(ageAdd)
+       if err != nil {
+               return err
+       }
+       m.ageAdd = binary.LittleEndian.Uint32(ageAdd)
+
+       // ticket_nonce, which must be unique per connection, is always left at
+       // zero because we only ever send one ticket per connection.
+
        if _, err := c.writeRecord(recordTypeHandshake, m.marshal()); err != nil {
                return err
        }