1 files changed, 94 insertions, 0 deletions

diff --git a/crypto/tls/ocsp.go b/crypto/tls/ocsp.go
new file mode 100644
index 0000000..9ae9828
--- /dev/null
+++ b/crypto/tls/ocsp.go
@@ -0,0 +1,94 @@
+package tls
+
+import (
+        "bytes"
+        "crypto/tls"
+        "crypto/x509"
+        "fmt"
+        "io"
+        "net/http"
+
+        "golang.org/x/crypto/ocsp"
+)
+
+func GetOcspResponse(chain *tls.Certificate) ([]byte, *ocsp.Response, error) {
+        var certs []*x509.Certificate
+        for _, c := range chain.Certificate {
+                cert, err := x509.ParseCertificate(c)
+                if err != nil {
+                        return nil, nil, err
+                }
+                certs = append(certs, cert)
+        }
+        if len(certs) == 0 {
+                return nil, nil, fmt.Errorf("no certificates found in bundle")
+        }
+
+        // We expect the certificate slice to be ordered downwards the chain.
+        // SRV CRT -> CA. We need to pull the leaf and issuer certs out of it,
+        // which should always be the first two certificates. If there's no
+        // OCSP server listed in the leaf cert, there's nothing to do. And if
+        // we have only one certificate so far, we need to get the issuer cert.
+        leaf := certs[0]
+        if len(leaf.OCSPServer) == 0 {
+                return nil, nil, fmt.Errorf("no OCSP server specified in certificate")
+        }
+
+        if len(certs) == 1 {
+                if len(leaf.IssuingCertificateURL) == 0 {
+                        return nil, nil, fmt.Errorf("no URL to issuing certificate")
+                }
+
+                resp, err := http.Get(leaf.IssuingCertificateURL[0])
+                if err != nil {
+                        return nil, nil, fmt.Errorf("getting issuer certificate: %w", err)
+                }
+                defer resp.Body.Close()
+
+                issuerBytes, err := io.ReadAll(io.LimitReader(resp.Body, 1024*1024))
+                if err != nil {
+                        return nil, nil, fmt.Errorf("reading issuer certificate: %w", err)
+                }
+
+                issuer, err := x509.ParseCertificate(issuerBytes)
+                if err != nil {
+                        return nil, nil, fmt.Errorf("parsing issuer certificate: %w", err)
+                }
+
+                certs = append(certs, issuer)
+        }
+
+        issuer := certs[1]
+
+        req, err := ocsp.CreateRequest(leaf, issuer, nil)
+        if err != nil {
+                return nil, nil, fmt.Errorf("creating OCSP request: %w", err)
+        }
+
+        httpRes, err := http.Post(leaf.OCSPServer[0], "application/ocsp-request", bytes.NewReader(req))
+        if err != nil {
+                return nil, nil, fmt.Errorf("making OCSP request: %w", err)
+        }
+        defer httpRes.Body.Close()
+
+        rawRes, err := io.ReadAll(io.LimitReader(httpRes.Body, 1024*1024))
+        if err != nil {
+                return nil, nil, fmt.Errorf("reading OCSP response: %w", err)
+        }
+
+        res, err := ocsp.ParseResponse(rawRes, issuer)
+        if err != nil {
+                return nil, nil, fmt.Errorf("parsing OCSP response: %w", err)
+        }
+
+        if res.Status != ocsp.Good {
+                return nil, nil, fmt.Errorf("invalid: OCSP response was not of Good status")
+        }
+
+        // This is invalid, the response expires after the certificate
+        if res.NextUpdate.After(leaf.NotAfter) {
+                return nil, nil, fmt.Errorf("invalid: OCSP response valid after certificate expiration")
+        }
+
+        return rawRes, res, nil
+}