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package dsig
import (
"bytes"
"crypto/rsa"
"crypto/x509"
"encoding/base64"
"errors"
"fmt"
"regexp"
"github.com/beevik/etree"
"github.com/russellhaering/goxmldsig/etreeutils"
"github.com/russellhaering/goxmldsig/types"
)
var uriRegexp = regexp.MustCompile("^#[a-zA-Z_][\\w.-]*$")
var whiteSpace = regexp.MustCompile("\\s+")
var (
// ErrMissingSignature indicates that no enveloped signature was found referencing
// the top level element passed for signature verification.
ErrMissingSignature = errors.New("Missing signature referencing the top-level element")
ErrInvalidSignature = errors.New( "Invalid Signature")
)
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
}
}
func mapPathToElement(tree, el *etree.Element) []int {
for i, child := range tree.Child {
if child == el {
return []int{i}
}
}
for i, child := range tree.Child {
if childElement, ok := child.(*etree.Element); ok {
childPath := mapPathToElement(childElement, el)
if childElement != nil {
return append([]int{i}, childPath...)
}
}
}
return nil
}
func removeElementAtPath(el *etree.Element, path []int) bool {
if len(path) == 0 {
return false
}
if len(el.Child) <= path[0] {
return false
}
childElement, ok := el.Child[path[0]].(*etree.Element)
if !ok {
return false
}
if len(path) == 1 {
el.RemoveChild(childElement)
return true
}
return removeElementAtPath(childElement, path[1:])
}
// Transform returns a new element equivalent to the passed root el, but with
// the set of transformations described by the ref applied.
//
// The functionality of transform is currently very limited and purpose-specific.
func (ctx *ValidationContext) transform(
el *etree.Element,
sig *types.Signature,
ref *types.Reference) (*etree.Element, Canonicalizer, error) {
transforms := ref.Transforms.Transforms
if len(transforms) != 2 {
return nil, nil, errors.New("Expected Enveloped and C14N transforms")
}
// map the path to the passed signature relative to the passed root, in
// order to enable removal of the signature by an enveloped signature
// transform
signaturePath := mapPathToElement(el, sig.UnderlyingElement())
// make a copy of the passed root
el = el.Copy()
var canonicalizer Canonicalizer
for _, transform := range transforms {
algo := transform.Algorithm
switch AlgorithmID(algo) {
case EnvelopedSignatureAltorithmId:
if !removeElementAtPath(el, signaturePath) {
return nil, nil, errors.New("Error applying canonicalization transform: Signature not found")
}
case CanonicalXML10ExclusiveAlgorithmId:
var prefixList string
if transform.InclusiveNamespaces != nil {
prefixList = transform.InclusiveNamespaces.PrefixList
}
canonicalizer = MakeC14N10ExclusiveCanonicalizerWithPrefixList(prefixList)
case CanonicalXML11AlgorithmId:
canonicalizer = MakeC14N11Canonicalizer()
case CanonicalXML10RecAlgorithmId:
canonicalizer = MakeC14N10RecCanonicalizer()
case CanonicalXML10CommentAlgorithmId:
canonicalizer = MakeC14N10CommentCanonicalizer()
default:
return nil, nil, errors.New("Unknown Transform Algorithm: " + algo)
}
}
if canonicalizer == nil {
return nil, nil, errors.New("Expected canonicalization transform")
}
return el, 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(sig *types.Signature, canonicalizer Canonicalizer, signatureMethodId string, cert *x509.Certificate, decodedSignature []byte) error {
signatureElement := sig.UnderlyingElement()
nsCtx, err := etreeutils.NSBuildParentContext(signatureElement)
if err != nil {
return err
}
signedInfo, err := etreeutils.NSFindOneChildCtx(nsCtx, signatureElement, Namespace, SignedInfoTag)
if err != nil {
return err
}
if signedInfo == nil {
return errors.New("Missing SignedInfo")
}
// Canonicalize the xml
canonical, err := canonicalSerialize(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[:], decodedSignature)
if err != nil {
return err
}
return nil
}
func (ctx *ValidationContext) validateSignature(el *etree.Element, sig *types.Signature, cert *x509.Certificate) (*etree.Element, error) {
idAttr := el.SelectAttr(ctx.IdAttribute)
if idAttr == nil || idAttr.Value == "" {
return nil, errors.New("Missing ID attribute")
}
var ref *types.Reference
// Find the first reference which references the top-level element
for _, _ref := range sig.SignedInfo.References {
if _ref.URI == "" || _ref.URI[1:] == idAttr.Value {
ref = &_ref
}
}
// Perform all transformations listed in the 'SignedInfo'
// Basically, this means removing the 'SignedInfo'
transformed, canonicalizer, err := ctx.transform(el, sig, ref)
if err != nil {
return nil, err
}
digestAlgorithm := ref.DigestAlgo.Algorithm
// Digest the transformed XML and compare it to the 'DigestValue' from the 'SignedInfo'
digest, err := ctx.digest(transformed, digestAlgorithm, canonicalizer)
if err != nil {
return nil, err
}
decodedDigestValue, err := base64.StdEncoding.DecodeString(ref.DigestValue)
if err != nil {
return nil, err
}
if !bytes.Equal(digest, decodedDigestValue) {
return nil, errors.New("Signature could not be verified")
}
if sig.SignatureValue == nil {
return nil, errors.New("Signature could not be verified")
}
// Decode the 'SignatureValue' so we can compare against it
decodedSignature, err := base64.StdEncoding.DecodeString(sig.SignatureValue.Data)
if err != nil {
return nil, errors.New("Could not decode signature")
}
// Actually verify the 'SignedInfo' was signed by a trusted source
signatureMethod := sig.SignedInfo.SignatureMethod.Algorithm
err = ctx.verifySignedInfo(sig, canonicalizer, signatureMethod, 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
}
// In most places, we use etree Elements, but while deserializing the Signature, we use
// encoding/xml unmarshal directly to convert to a convenient go struct. This presents a problem in some cases because
// when an xml element repeats under the parent, the last element will win and/or be appended. We need to assert that
// the Signature object matches the expected shape of a Signature object.
func validateShape(signatureEl *etree.Element) error {
children := signatureEl.ChildElements()
childCounts := map[string]int{}
for _, child := range children {
childCounts[child.Tag]++
}
validateCount := childCounts[SignedInfoTag] == 1 && childCounts[KeyInfoTag] <= 1 && childCounts[SignatureValueTag] == 1
if !validateCount {
return ErrInvalidSignature
}
return nil
}
// findSignature searches for a Signature element referencing the passed root element.
func (ctx *ValidationContext) findSignature(root *etree.Element) (*types.Signature, error) {
idAttr := root.SelectAttr(ctx.IdAttribute)
if idAttr == nil || idAttr.Value == "" {
return nil, errors.New("Missing ID attribute")
}
var sig *types.Signature
// Traverse the tree looking for a Signature element
err := etreeutils.NSFindIterate(root, Namespace, SignatureTag, func(ctx etreeutils.NSContext, signatureEl *etree.Element) error {
err := validateShape(signatureEl)
if err != nil {
return err
}
found := false
err = etreeutils.NSFindChildrenIterateCtx(ctx, signatureEl, Namespace, SignedInfoTag,
func(ctx etreeutils.NSContext, signedInfo *etree.Element) error {
detachedSignedInfo, err := etreeutils.NSDetatch(ctx, signedInfo)
if err != nil {
return err
}
c14NMethod, err := etreeutils.NSFindOneChildCtx(ctx, detachedSignedInfo, Namespace, CanonicalizationMethodTag)
if err != nil {
return err
}
if c14NMethod == nil {
return errors.New("missing CanonicalizationMethod on Signature")
}
c14NAlgorithm := c14NMethod.SelectAttrValue(AlgorithmAttr, "")
var canonicalSignedInfo *etree.Element
switch AlgorithmID(c14NAlgorithm) {
case CanonicalXML10ExclusiveAlgorithmId:
err := etreeutils.TransformExcC14n(detachedSignedInfo, "")
if err != nil {
return err
}
// NOTE: TransformExcC14n transforms the element in-place,
// while canonicalPrep isn't meant to. Once we standardize
// this behavior we can drop this, as well as the adding and
// removing of elements below.
canonicalSignedInfo = detachedSignedInfo
case CanonicalXML11AlgorithmId:
canonicalSignedInfo = canonicalPrep(detachedSignedInfo, map[string]struct{}{})
case CanonicalXML10RecAlgorithmId:
canonicalSignedInfo = canonicalPrep(detachedSignedInfo, map[string]struct{}{})
case CanonicalXML10CommentAlgorithmId:
canonicalSignedInfo = canonicalPrep(detachedSignedInfo, map[string]struct{}{})
default:
return fmt.Errorf("invalid CanonicalizationMethod on Signature: %s", c14NAlgorithm)
}
signatureEl.RemoveChild(signedInfo)
signatureEl.AddChild(canonicalSignedInfo)
found = true
return etreeutils.ErrTraversalHalted
})
if err != nil {
return err
}
if !found {
return errors.New("Missing SignedInfo")
}
// Unmarshal the signature into a structured Signature type
_sig := &types.Signature{}
err = etreeutils.NSUnmarshalElement(ctx, signatureEl, _sig)
if err != nil {
return err
}
// Traverse references in the signature to determine whether it has at least
// one reference to the top level element. If so, conclude the search.
for _, ref := range _sig.SignedInfo.References {
if ref.URI == "" || ref.URI[1:] == idAttr.Value {
sig = _sig
return etreeutils.ErrTraversalHalted
}
}
return nil
})
if err != nil {
return nil, err
}
if sig == nil {
return nil, ErrMissingSignature
}
return sig, nil
}
func (ctx *ValidationContext) verifyCertificate(sig *types.Signature) (*x509.Certificate, error) {
now := ctx.Clock.Now()
roots, err := ctx.CertificateStore.Certificates()
if err != nil {
return nil, err
}
var cert *x509.Certificate
if sig.KeyInfo != nil {
// If the Signature includes KeyInfo, extract the certificate from there
if len(sig.KeyInfo.X509Data.X509Certificates) == 0 || sig.KeyInfo.X509Data.X509Certificates[0].Data == "" {
return nil, errors.New("missing X509Certificate within KeyInfo")
}
certData, err := base64.StdEncoding.DecodeString(
whiteSpace.ReplaceAllString(sig.KeyInfo.X509Data.X509Certificates[0].Data, ""))
if err != nil {
return nil, errors.New("Failed to parse certificate")
}
cert, err = x509.ParseCertificate(certData)
if err != nil {
return nil, err
}
} else {
// If the Signature doesn't have KeyInfo, Use the root certificate if there is only one
if len(roots) == 1 {
cert = roots[0]
} else {
return nil, errors.New("Missing x509 Element")
}
}
// 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
}
// Validate verifies that the passed element contains a valid enveloped signature
// matching a currently-valid certificate in the context's CertificateStore.
func (ctx *ValidationContext) Validate(el *etree.Element) (*etree.Element, error) {
// Make a copy of the element to avoid mutating the one we were passed.
el = el.Copy()
sig, err := ctx.findSignature(el)
if err != nil {
return nil, err
}
cert, err := ctx.verifyCertificate(sig)
if err != nil {
return nil, err
}
return ctx.validateSignature(el, sig, cert)
}
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