go-sct: Verifying Signed Certificate Timestamps in Go.

Warning:

This is a prototype, no guarantees are provided regarding correctness.

Use:

Perform Signed Certificate Timestamp verification for TLS connections.

To install:

go get github.com/mberhault/go-sct

Using it to verify a simple https Get:

package main

import (
  "log"
  "net/http"

  "github.com/mberhault/go-sct"
)

func main() {
  resp, err := http.Get("https://www.certificate-transparency.org")
  if err != nil {
    log.Fatalf("get failed for %s: %v", url, err)
  }

  err = sct.CheckConnectionState(resp.TLS)
  if err != nil {
    log.Fatalf("failed SCT check: %v", err)
  }

  log.Printf("OK")
}

See the examples directory for various methods of verifying the tls.ConnectionState:

• examples/check_get_response to verify a http.Response
• examples/dial_tls to verify a tls.Conn
• examples/tls_config_verify to use the VerifyConnection callback of a tls.Config

Signed Certificate Timestamp acceptance:

Two types of SCTs (Signed Certificate Timestamps) are examined:

• embedded in a x509 certificate
• included in the TLS handshake as a TLS extension

SCTs are verified using the following:

• extract SCTs from x509 certificate or TLS extension
• lookup corresponding log in the Chrome CT log list, specifically https://www.gstatic.com/ct/log_list/v2/log_list.json
• verify SCT signature using the log's public key
• check the log for inclusion

sct.CheckConnectionState returns success when the first valid SCT is encountered, skipping all others.

Caveats:

There are a few noteworthy caveats:

• this is a prototype
• SCTs included in the OCSP response are not examined
• the log list is not refreshed after initialization
• if the issuer certificate is missing, embedded SCTs cannot be verified and will fail
• if the SCT is not included in the tree but its timestamp is before Maximum Merge Delay, the check passes
• no configuration is currently possible
• the set of dependencies is massive, pulling a large portion of certificate-transparency-go and its dependencies.

Performance:

This is a prototype for validation only, many aspects remain unoptimized. Expect severely increased latency.