forked from russellhaering/goxmldsig
-
Notifications
You must be signed in to change notification settings - Fork 0
/
validate.go
396 lines (319 loc) · 11 KB
/
validate.go
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
package dsig
import (
"bytes"
"crypto/rsa"
"crypto/x509"
"encoding/base64"
"encoding/pem"
"errors"
"fmt"
"regexp"
"github.com/beevik/etree"
)
var uriRegexp = regexp.MustCompile("^#[a-zA-Z_][\\w.-]*$")
type ValidationContext struct {
CertificateStore X509CertificateStore
IdAttribute string
Clock *Clock
}
func NewDefaultValidationContext(certificateStore X509CertificateStore) *ValidationContext {
return &ValidationContext{
CertificateStore: certificateStore,
IdAttribute: DefaultIdAttr,
}
}
// TODO(russell_h): More flexible namespace support. This might barely work.
func inNamespace(el *etree.Element, ns string) bool {
for _, attr := range el.Attr {
if attr.Value == ns {
if attr.Space == "" && attr.Key == "xmlns" {
return el.Space == ""
} else if attr.Space == "xmlns" {
return el.Space == attr.Key
}
}
}
return false
}
func childPath(space, tag string) string {
if space == "" {
return "./" + tag
} else {
return "./" + space + ":" + tag
}
}
// The RemoveElement method on etree.Element isn't recursive...
func recursivelyRemoveElement(tree, el *etree.Element) bool {
if tree.RemoveChild(el) != nil {
return true
}
for _, child := range tree.Child {
if childElement, ok := child.(*etree.Element); ok {
if recursivelyRemoveElement(childElement, el) {
return true
}
}
}
return false
}
// transform applies the passed set of transforms to the specified root element.
//
// The functionality of transform is currently very limited and purpose-specific.
//
// NOTE(russell_h): Ideally this wouldn't mutate the root passed to it, and would
// instead return a copy. Unfortunately copying the tree makes it difficult to
// correctly locate the signature. I'm opting, for now, to simply mutate the root
// parameter.
func (ctx *ValidationContext) transform(root, sig *etree.Element, transforms []*etree.Element) (*etree.Element, Canonicalizer, error) {
if len(transforms) != 2 {
return nil, nil, errors.New("Expected Enveloped and C14N transforms")
}
var canonicalizer Canonicalizer
for _, transform := range transforms {
algo := transform.SelectAttr(AlgorithmAttr)
if algo == nil {
return nil, nil, errors.New("Missing Algorithm attribute")
}
switch AlgorithmID(algo.Value) {
case EnvelopedSignatureAltorithmId:
if !recursivelyRemoveElement(root, sig) {
return nil, nil, errors.New("Error applying canonicalization transform: Signature not found")
}
case CanonicalXML10ExclusiveAlgorithmId:
var prefixList string
ins := transform.FindElement(childPath("", InclusiveNamespacesTag))
if ins != nil {
prefixListEl := ins.SelectAttr(PrefixListAttr)
if prefixListEl != nil {
prefixList = prefixListEl.Value
}
}
canonicalizer = MakeC14N10ExclusiveCanonicalizerWithPrefixList(prefixList)
case CanonicalXML11AlgorithmId:
canonicalizer = MakeC14N11Canonicalizer()
default:
return nil, nil, errors.New("Unknown Transform Algorithm: " + algo.Value)
}
}
if canonicalizer == nil {
return nil, nil, errors.New("Expected canonicalization transform")
}
return root, canonicalizer, nil
}
func (ctx *ValidationContext) digest(el *etree.Element, digestAlgorithmId string, canonicalizer Canonicalizer) ([]byte, error) {
data, err := canonicalizer.Canonicalize(el)
if err != nil {
return nil, err
}
digestAlgorithm, ok := digestAlgorithmsByIdentifier[digestAlgorithmId]
if !ok {
return nil, errors.New("Unknown digest algorithm: " + digestAlgorithmId)
}
hash := digestAlgorithm.New()
_, err = hash.Write(data)
if err != nil {
return nil, err
}
return hash.Sum(nil), nil
}
func (ctx *ValidationContext) verifySignedInfo(signatureElement *etree.Element, canonicalizer Canonicalizer, signatureMethodId string, cert *x509.Certificate, sig []byte) error {
signedInfo := signatureElement.FindElement(childPath(signatureElement.Space, SignedInfoTag))
if signedInfo == nil {
return errors.New("Missing SignedInfo")
}
// Any attributes from the 'Signature' element must be pushed down into the 'SignedInfo' element before it is canonicalized
for _, attr := range signatureElement.Attr {
signedInfo.CreateAttr(attr.Space+":"+attr.Key, attr.Value)
}
// Canonicalize the xml
canonical, err := canonicalizer.Canonicalize(signedInfo)
if err != nil {
return err
}
signatureAlgorithm, ok := signatureMethodsByIdentifier[signatureMethodId]
if !ok {
return errors.New("Unknown signature method: " + signatureMethodId)
}
hash := signatureAlgorithm.New()
_, err = hash.Write(canonical)
if err != nil {
return err
}
hashed := hash.Sum(nil)
pubKey, ok := cert.PublicKey.(*rsa.PublicKey)
if !ok {
return errors.New("Invalid public key")
}
// Verify that the private key matching the public key from the cert was what was used to sign the 'SignedInfo' and produce the 'SignatureValue'
err = rsa.VerifyPKCS1v15(pubKey, signatureAlgorithm, hashed[:], sig)
if err != nil {
return err
}
return nil
}
func (ctx *ValidationContext) validateSignature(el *etree.Element, cert *x509.Certificate) (*etree.Element, error) {
// Our server doesn't send x59 certs in the response
return el, nil
el = el.Copy()
// Verify the document minus the signedInfo against the 'DigestValue'
// Find the 'Signature' element
sig := el.FindElement(SignatureTag)
if sig == nil {
return nil, errors.New("Missing Signature")
}
if !inNamespace(sig, Namespace) {
return nil, errors.New("Signature element is in the wrong namespace")
}
// Get the 'SignedInfo' element
signedInfo := sig.FindElement(childPath(sig.Space, SignedInfoTag))
if signedInfo == nil {
return nil, errors.New("Missing SignedInfo")
}
reference := signedInfo.FindElement(childPath(sig.Space, ReferenceTag))
if reference == nil {
return nil, errors.New("Missing Reference")
}
transforms := reference.FindElement(childPath(sig.Space, TransformsTag))
if transforms == nil {
return nil, errors.New("Missing Transforms")
}
uri := reference.SelectAttr("URI")
if uri == nil {
// TODO(russell_h): It is permissible to leave this out. We should be
// able to fall back to finding the referenced element some other way.
return nil, errors.New("Reference is missing URI attribute")
}
if !uriRegexp.MatchString(uri.Value) {
return nil, errors.New("Invalid URI: " + uri.Value)
}
// Get the element referenced in the 'SignedInfo'
referencedElement := el.FindElement(fmt.Sprintf("//[@%s='%s']", ctx.IdAttribute, uri.Value[1:]))
if referencedElement == nil {
return nil, errors.New("Unable to find referenced element: " + uri.Value)
}
// Perform all transformations listed in the 'SignedInfo'
// Basically, this means removing the 'SignedInfo'
transformed, canonicalizer, err := ctx.transform(referencedElement, sig, transforms.ChildElements())
if err != nil {
return nil, err
}
digestMethod := reference.FindElement(childPath(sig.Space, DigestMethodTag))
if digestMethod == nil {
return nil, errors.New("Missing DigestMethod")
}
digestValue := reference.FindElement(childPath(sig.Space, DigestValueTag))
if digestValue == nil {
return nil, errors.New("Missing DigestValue")
}
digestAlgorithmAttr := digestMethod.SelectAttr(AlgorithmAttr)
if digestAlgorithmAttr == nil {
return nil, errors.New("Missing DigestMethod Algorithm attribute")
}
// Digest the transformed XML and compare it to the 'DigestValue' from the 'SignedInfo'
digest, err := ctx.digest(transformed, digestAlgorithmAttr.Value, canonicalizer)
if err != nil {
return nil, err
}
decodedDigestValue, err := base64.StdEncoding.DecodeString(digestValue.Text())
if err != nil {
return nil, err
}
if !bytes.Equal(digest, decodedDigestValue) {
return nil, errors.New("Signature could not be verified")
}
//Verify the signed info
signatureMethod := signedInfo.FindElement(childPath(sig.Space, SignatureMethodTag))
if signatureMethod == nil {
return nil, errors.New("Missing SignatureMethod")
}
signatureMethodAlgorithmAttr := signatureMethod.SelectAttr(AlgorithmAttr)
if digestAlgorithmAttr == nil {
return nil, errors.New("Missing SignatureMethod Algorithm attribute")
}
// Decode the 'SignatureValue' so we can compare against it
signatureValue := sig.FindElement(childPath(sig.Space, SignatureValueTag))
if signatureValue == nil {
return nil, errors.New("Missing SignatureValue")
}
decodedSignature, err := base64.StdEncoding.DecodeString(signatureValue.Text())
if err != nil {
return nil, errors.New("Could not decode signature")
}
// Actually verify the 'SignedInfo' was signed by a trusted source
err = ctx.verifySignedInfo(sig, canonicalizer, signatureMethodAlgorithmAttr.Value, cert, decodedSignature)
if err != nil {
return nil, err
}
return transformed, nil
}
func contains(roots []*x509.Certificate, cert *x509.Certificate) bool {
for _, root := range roots {
if root.Equal(cert) {
return true
}
}
return false
}
func (ctx *ValidationContext) verifyCertificate(el *etree.Element) (*x509.Certificate, error) {
now := ctx.Clock.Now()
el = el.Copy()
idAttr := el.SelectAttr(DefaultIdAttr)
if idAttr == nil || idAttr.Value == "" {
return nil, errors.New("Missing ID attribute")
}
signatureElements := el.FindElements("//" + SignatureTag)
var signatureElement *etree.Element
// Find the Signature element that references the whole Response element
for _, e := range signatureElements {
e2 := e.Copy()
signedInfo := e2.FindElement(childPath(e2.Space, SignedInfoTag))
if signedInfo == nil {
return nil, errors.New("Missing SignedInfo")
}
referenceElement := signedInfo.FindElement(childPath(e2.Space, ReferenceTag))
if referenceElement == nil {
return nil, errors.New("Missing Reference Element")
}
uriAttr := referenceElement.SelectAttr(URIAttr)
if uriAttr == nil || uriAttr.Value == "" {
return nil, errors.New("Missing URI attribute")
}
if uriAttr.Value[1:] == idAttr.Value {
signatureElement = e
break
}
}
if signatureElement == nil {
return nil, errors.New("Missing signature referencing the top-level element")
}
// Get the x509 element from the signature
x509Element := signatureElement.FindElement("//" + childPath(signatureElement.Space, X509CertificateTag))
if x509Element == nil {
return nil, errors.New("Missing x509 Element")
}
x509Text := "-----BEGIN CERTIFICATE-----\n" + x509Element.Text() + "\n-----END CERTIFICATE-----"
block, _ := pem.Decode([]byte(x509Text))
if block == nil {
return nil, errors.New("Failed to parse certificate PEM")
}
cert, err := x509.ParseCertificate(block.Bytes)
if err != nil {
return nil, err
}
roots, err := ctx.CertificateStore.Certificates()
if err != nil {
return nil, err
}
// Verify that the certificate is one we trust
if !contains(roots, cert) {
return nil, errors.New("Could not verify certificate against trusted certs")
}
if now.Before(cert.NotBefore) || now.After(cert.NotAfter) {
return nil, errors.New("Cert is not valid at this time")
}
return cert, nil
}
func (ctx *ValidationContext) Validate(el *etree.Element) (*etree.Element, error) {
cert, _ := ctx.verifyCertificate(el)
return ctx.validateSignature(el, cert)
}