goocsp/pkg/ocsp/responder.go

/*
Copyright 2022 CAcert Inc.
SPDX-License-Identifier: Apache-2.0

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

// Package ocsp implements an OCSP responder based on a generic storage backend.
// It provides a couple of sample implementations.
// Because OCSP responders handle high query volumes, we have to be careful
// about how much logging we do. Error-level logs are reserved for problems
// internal to the server, that can be fixed by an administrator. Any type of
// incorrect input from a user should be logged and Info or below. For things
// that are logged on every request, Debug is the appropriate level.
package ocsp

import (
	"crypto"
	"crypto/sha256"
	"encoding/base64"
	"encoding/json"
	"errors"
	"fmt"
	"io/ioutil"
	"net/http"
	"net/url"
	"regexp"
	"time"

	"github.com/jmhodges/clock"
	"github.com/sirupsen/logrus"
)

var (
	malformedRequestErrorResponse = []byte{0x30, 0x03, 0x0A, 0x01, 0x01}
	internalErrorErrorResponse    = []byte{0x30, 0x03, 0x0A, 0x01, 0x02}
	tryLaterErrorResponse         = []byte{0x30, 0x03, 0x0A, 0x01, 0x03}
	sigRequredErrorResponse       = []byte{0x30, 0x03, 0x0A, 0x01, 0x05}
	unauthorizedErrorResponse     = []byte{0x30, 0x03, 0x0A, 0x01, 0x06}

	// ErrNotFound indicates the request OCSP response was not found. It is used to
	// indicate that the responder should reply with unauthorizedErrorResponse.
	ErrNotFound = errors.New("Request OCSP Response not found")
)

// Source represents the logical source of OCSP responses, i.e.,
// the logic that actually chooses a response based on a request.  In
// order to create an actual responder, wrap one of these in a Responder
// object and pass it to http.Handle. By default the Responder will set
// the headers Cache-Control to "max-age=(response.NextUpdate-now), public, no-transform, must-revalidate",
// Last-Modified to response.ThisUpdate, Expires to response.NextUpdate,
// ETag to the SHA256 hash of the response, and Content-Type to
// application/ocsp-response. If you want to override these headers,
// or set extra headers, your source should return a http.Header
// with the headers you wish to set. If you don't want to set any
// extra headers you may return nil instead.
type Source interface {
	Response(*Request) ([]byte, http.Header, error)
}

// An InMemorySource is a map from serialNumber -> der(response)
type InMemorySource map[string][]byte

// Response looks up an OCSP response to provide for a given request.
// InMemorySource looks up a response purely based on serial number,
// without regard to what issuer the request is asking for.
func (src InMemorySource) Response(request *Request) ([]byte, http.Header, error) {
	response, present := src[request.SerialNumber.String()]
	if !present {
		return nil, nil, ErrNotFound
	}
	return response, nil, nil
}

// NewSourceFromFile reads the named file into an InMemorySource.
// The file read by this function must contain whitespace-separated OCSP
// responses. Each OCSP response must be in base64-encoded DER form (i.e.,
// PEM without headers or whitespace).  Invalid responses are ignored.
// This function pulls the entire file into an InMemorySource.
func NewSourceFromFile(responseFile string) (Source, error) {
	fileContents, err := ioutil.ReadFile(responseFile)
	if err != nil {
		return nil, err
	}

	responsesB64 := regexp.MustCompile("\\s").Split(string(fileContents), -1)
	src := InMemorySource{}
	for _, b64 := range responsesB64 {
		// if the line/space is empty just skip
		if b64 == "" {
			continue
		}
		der, tmpErr := base64.StdEncoding.DecodeString(b64)
		if tmpErr != nil {
			logrus.Errorf("Base64 decode error %s on: %s", tmpErr, b64)
			continue
		}

		response, tmpErr := ParseResponse(der, nil)
		if tmpErr != nil {
			logrus.Errorf("OCSP decode error %s on: %s", tmpErr, b64)
			continue
		}

		src[response.SerialNumber.String()] = der
	}

	logrus.Infof("Read %d OCSP responses", len(src))
	return src, nil
}

// Stats is a basic interface that allows users to record information
// about returned responses
type Stats interface {
	ResponseStatus(ResponseStatus)
}

// A Responder object provides the HTTP logic to expose a
// Source of OCSP responses.
type Responder struct {
	Source Source
	stats  Stats
	clk    clock.Clock
}

// NewResponder instantiates a Responder with the give Source.
func NewResponder(source Source, stats Stats) *Responder {
	return &Responder{
		Source: source,
		stats:  stats,
		clk:    clock.New(),
	}
}

func overrideHeaders(response http.ResponseWriter, headers http.Header) {
	for k, v := range headers {
		if len(v) == 1 {
			response.Header().Set(k, v[0])
		} else if len(v) > 1 {
			response.Header().Del(k)
			for _, e := range v {
				response.Header().Add(k, e)
			}
		}
	}
}

type logEvent struct {
	IP       string        `json:"ip,omitempty"`
	UA       string        `json:"ua,omitempty"`
	Method   string        `json:"method,omitempty"`
	Path     string        `json:"path,omitempty"`
	Body     string        `json:"body,omitempty"`
	Received time.Time     `json:"received,omitempty"`
	Took     time.Duration `json:"took,omitempty"`
	Headers  http.Header   `json:"headers,omitempty"`

	Serial         string `json:"serial,omitempty"`
	IssuerKeyHash  string `json:"issuerKeyHash,omitempty"`
	IssuerNameHash string `json:"issuerNameHash,omitempty"`
	HashAlg        string `json:"hashAlg,omitempty"`
}

// hashToString contains mappings for the only hash functions
// x/crypto/ocsp supports
var hashToString = map[crypto.Hash]string{
	crypto.SHA1:   "SHA1",
	crypto.SHA256: "SHA256",
	crypto.SHA384: "SHA384",
	crypto.SHA512: "SHA512",
}

// A Responder can process both GET and POST requests.  The mapping
// from an OCSP request to an OCSP response is done by the Source;
// the Responder simply decodes the request, and passes back whatever
// response is provided by the source.
// Note: The caller must use http.StripPrefix to strip any path components
// (including '/') on GET requests.
// Do not use this responder in conjunction with http.NewServeMux, because the
// default handler will try to canonicalize path components by changing any
// strings of repeated '/' into a single '/', which will break the base64
// encoding.
func (rs Responder) ServeHTTP(response http.ResponseWriter, request *http.Request) {
	le := logEvent{
		IP:       request.RemoteAddr,
		UA:       request.UserAgent(),
		Method:   request.Method,
		Path:     request.URL.Path,
		Received: time.Now(),
	}
	defer func() {
		le.Headers = response.Header()
		le.Took = time.Since(le.Received)
		jb, err := json.Marshal(le)
		if err != nil {
			// we log this error at the debug level as if we aren't at that level anyway
			// we shouldn't really care about marshalling the log event object
			logrus.Debugf("failed to marshal log event object: %s", err)
			return
		}
		logrus.Debugf("Received request: %s", string(jb))
	}()
	// By default we set a 'max-age=0, no-cache' Cache-Control header, this
	// is only returned to the client if a valid authorized OCSP response
	// is not found or an error is returned. If a response if found the header
	// will be altered to contain the proper max-age and modifiers.
	response.Header().Add("Cache-Control", "max-age=0, no-cache")
	// Read response from request
	var requestBody []byte
	var err error
	switch request.Method {
	case "GET":
		base64Request, err := url.QueryUnescape(request.URL.Path)
		if err != nil {
			logrus.Debugf("Error decoding URL: %s", request.URL.Path)
			response.WriteHeader(http.StatusBadRequest)
			return
		}
		// url.QueryUnescape not only unescapes %2B escaping, but it additionally
		// turns the resulting '+' into a space, which makes base64 decoding fail.
		// So we go back afterwards and turn ' ' back into '+'. This means we
		// accept some malformed input that includes ' ' or %20, but that's fine.
		base64RequestBytes := []byte(base64Request)
		for i := range base64RequestBytes {
			if base64RequestBytes[i] == ' ' {
				base64RequestBytes[i] = '+'
			}
		}
		// In certain situations a UA may construct a request that has a double
		// slash between the host name and the base64 request body due to naively
		// constructing the request URL. In that case strip the leading slash
		// so that we can still decode the request.
		if len(base64RequestBytes) > 0 && base64RequestBytes[0] == '/' {
			base64RequestBytes = base64RequestBytes[1:]
		}
		requestBody, err = base64.StdEncoding.DecodeString(string(base64RequestBytes))
		if err != nil {
			logrus.Debugf("Error decoding base64 from URL: %s", string(base64RequestBytes))
			response.WriteHeader(http.StatusBadRequest)
			return
		}
	case "POST":
		requestBody, err = ioutil.ReadAll(request.Body)
		if err != nil {
			logrus.Errorf("Problem reading body of POST: %s", err)
			response.WriteHeader(http.StatusBadRequest)
			return
		}
	default:
		response.WriteHeader(http.StatusMethodNotAllowed)
		return
	}
	b64Body := base64.StdEncoding.EncodeToString(requestBody)
	logrus.Debugf("Received OCSP request: %s", b64Body)
	if request.Method == http.MethodPost {
		le.Body = b64Body
	}

	// All responses after this point will be OCSP.
	// We could check for the content type of the request, but that
	// seems unnecessariliy restrictive.
	response.Header().Add("Content-Type", "application/ocsp-response")

	// Parse response as an OCSP request
	// XXX: This fails if the request contains the nonce extension.
	//      We don't intend to support nonces anyway, but maybe we
	//      should return unauthorizedRequest instead of malformed.
	ocspRequest, err := ParseRequest(requestBody)
	if err != nil {
		logrus.Debugf("Error decoding request body: %s", b64Body)
		response.WriteHeader(http.StatusBadRequest)
		response.Write(malformedRequestErrorResponse)
		if rs.stats != nil {
			rs.stats.ResponseStatus(Malformed)
		}
		return
	}
	le.Serial = fmt.Sprintf("%x", ocspRequest.SerialNumber.Bytes())
	le.IssuerKeyHash = fmt.Sprintf("%x", ocspRequest.IssuerKeyHash)
	le.IssuerNameHash = fmt.Sprintf("%x", ocspRequest.IssuerNameHash)
	le.HashAlg = hashToString[ocspRequest.HashAlgorithm]

	// Look up OCSP response from source
	ocspResponse, headers, err := rs.Source.Response(ocspRequest)
	if err != nil {
		if err == ErrNotFound {
			logrus.Infof("No response found for request: serial %x, request body %s",
				ocspRequest.SerialNumber, b64Body)
			response.Write(unauthorizedErrorResponse)
			if rs.stats != nil {
				rs.stats.ResponseStatus(Unauthorized)
			}
			return
		}
		logrus.Infof("Error retrieving response for request: serial %x, request body %s, error: %s",
			ocspRequest.SerialNumber, b64Body, err)
		response.WriteHeader(http.StatusInternalServerError)
		response.Write(internalErrorErrorResponse)
		if rs.stats != nil {
			rs.stats.ResponseStatus(InternalError)
		}
		return
	}

	parsedResponse, err := ParseResponse(ocspResponse, nil)
	if err != nil {
		logrus.Errorf("Error parsing response for serial %x: %s",
			ocspRequest.SerialNumber, err)
		response.Write(internalErrorErrorResponse)
		if rs.stats != nil {
			rs.stats.ResponseStatus(InternalError)
		}
		return
	}

	// Write OCSP response to response
	response.Header().Add("Last-Modified", parsedResponse.ThisUpdate.Format(time.RFC1123))
	response.Header().Add("Expires", parsedResponse.NextUpdate.Format(time.RFC1123))
	now := rs.clk.Now()
	maxAge := 0
	if now.Before(parsedResponse.NextUpdate) {
		maxAge = int(parsedResponse.NextUpdate.Sub(now) / time.Second)
	} else {
		// TODO(#530): we want max-age=0 but this is technically an authorized OCSP response
		//             (despite being stale) and 5019 forbids attaching no-cache
		maxAge = 0
	}
	response.Header().Set(
		"Cache-Control",
		fmt.Sprintf(
			"max-age=%d, public, no-transform, must-revalidate",
			maxAge,
		),
	)
	responseHash := sha256.Sum256(ocspResponse)
	response.Header().Add("ETag", fmt.Sprintf("\"%X\"", responseHash))

	if headers != nil {
		overrideHeaders(response, headers)
	}

	// RFC 7232 says that a 304 response must contain the above
	// headers if they would also be sent for a 200 for the same
	// request, so we have to wait until here to do this
	if etag := request.Header.Get("If-None-Match"); etag != "" {
		if etag == fmt.Sprintf("\"%X\"", responseHash) {
			response.WriteHeader(http.StatusNotModified)
			return
		}
	}
	response.WriteHeader(http.StatusOK)
	response.Write(ocspResponse)
	if rs.stats != nil {
		rs.stats.ResponseStatus(Success)
	}
}