fix(challenge): urlsafe base64 encoding (#82)

This fixes issue with wrong encoding being used for the challenge.
According to the specs[^1] the challenge should be base64 url-safe
encoded. Until now, the package used std encoding which uses slightly
different set of characters. This was reported in the linked issue
and we also encountered the same issue when testing our implementation
of webauthn e2e using github.com/descope/virtualwebauthn.

The commit removes the custom type for challenge and instead passes
around the challenge url-safe base64 encoded right from the point of
creation. Because the string is fully valid it can be safely used
in json as well as stored in redis or other possible storage
implementations that might be used by the clients of the library.

I tried to keep the tests as close to the original as possible.
In some cases that required transforming the challenge back to bytes
considering the previously used padding to be part of the bytes
and then re-encoding into url-safe base64 string.

[^1]: https://www.w3.org/TR/webauthn-2/#dom-collectedclientdata-challenge

Fix: duo-labs/webauthn#128

protocol/assertion_test.go

@@ -151,7 +151,7 @@ func TestParsedCredentialAssertionData_Verify(t *testing.T) {
  Raw CredentialAssertionResponse
  }
  type args struct {
- storedChallenge Challenge
+ storedChallenge URLEncodedBase64
  relyingPartyID string
  relyingPartyOrigin string
  verifyUser bool

protocol/attestation_test.go

@@ -83,7 +83,7 @@ func TestPackedAttestationVerification(t *testing.T) {
 var testAttestationOptions = []string{
  // Direct Self Attestation with EC256 - MacOS
  `{"publicKey": {
- "challenge": "rWiex8xDOPfiCgyFu4BLW6vVOmXKgPwHrlMCgEs9SBA=",
+ "challenge": "rWiex8xDOPfiCgyFu4BLW6vVOmXKgPwHrlMCgEs9SBA",
  "rp": {
  "name": "http:https://localhost:9005",
  "id": "localhost"
@@ -108,7 +108,7 @@ var testAttestationOptions = []string{
  }}`,
  // Direct Attestation with EC256
  `{"publicKey": {
- "challenge": "+Ri5NZTzJ8b6mvW3TVScLotEoALfgBa2Bn4YSaIObHc=",
+ "challenge": "-Ri5NZTzJ8b6mvW3TVScLotEoALfgBa2Bn4YSaIObHc",
  "rp": {
  "name": "https://webauthn.io",
  "id": "webauthn.io"
@@ -134,7 +134,7 @@ var testAttestationOptions = []string{
  // None Attestation with EC256
  `{
  "publicKey": {
- "challenge": "sVt4ScceMzqFSnfAq8hgLzblvo3fa4/aFVEcIESHIJ0=",
+ "challenge": "sVt4ScceMzqFSnfAq8hgLzblvo3fa4_aFVEcIESHIJ0",
  "rp": {
  "name": "https://webauthn.io",
  "id": "webauthn.io"

protocol/base64.go

@@ -11,6 +11,10 @@ import (
 // decoded into a byte slice.
 type URLEncodedBase64 []byte
 
+func (e URLEncodedBase64) String() string {
+ return base64.RawURLEncoding.EncodeToString(e)
+}
+
 // UnmarshalJSON base64 decodes a URL-encoded value, storing the result in the
 // provided byte slice.
 func (e *URLEncodedBase64) UnmarshalJSON(data []byte) error {

protocol/challenge.go

@@ -2,26 +2,18 @@ package protocol
 
 import (
  "crypto/rand"
- "encoding/base64"
 )
 
 // ChallengeLength - Length of bytes to generate for a challenge
 const ChallengeLength = 32
 
-// Challenge that should be signed and returned by the authenticator
-type Challenge URLEncodedBase64
-
-// Create a new challenge to be sent to the authenticator. The spec recommends using
-// at least 16 bytes with 100 bits of entropy. We use 32 bytes.
-func CreateChallenge() (Challenge, error) {
+// Create a new challenge that should be signed and returned by the authenticator.
+// The spec recommends using at least 16 bytes with 100 bits of entropy. We use 32 bytes.
+func CreateChallenge() (URLEncodedBase64, error) {
  challenge := make([]byte, ChallengeLength)
  _, err := rand.Read(challenge)
  if err != nil {
  return nil, err
  }
- return challenge, nil
-}
-
-func (c Challenge) String() string {
- return base64.RawURLEncoding.EncodeToString(c)
+ return URLEncodedBase64(challenge), nil
 }

protocol/challenge_test.go

@@ -9,12 +9,12 @@ import (
 func TestCreateChallenge(t *testing.T) {
  tests := []struct {
  name string
- want Challenge
+ want URLEncodedBase64
  wantErr bool
  }{
  {
  "Successfull Challenge Create",
- Challenge{},
+ URLEncodedBase64{},
  false,
  },
  }
@@ -42,7 +42,7 @@ func TestChallenge_String(t *testing.T) {
  wantChallenge := base64.RawURLEncoding.EncodeToString(newChallenge)
  tests := []struct {
  name string
- c Challenge
+ c URLEncodedBase64
  want string
  }{
  {

protocol/client_test.go

@@ -1,19 +1,16 @@
 package protocol
 
 import (
- "encoding/base64"
  "net/url"
  "testing"
 )
 
-func setupCollectedClientData(challenge []byte) *CollectedClientData {
- ccd := &CollectedClientData{
- Type: CreateCeremony,
- Origin: "example.com",
+func setupCollectedClientData(challenge URLEncodedBase64) *CollectedClientData {
+ return &CollectedClientData{
+ Type: CreateCeremony,
+ Origin: "example.com",
+ Challenge: challenge.String(),
  }
-
- ccd.Challenge = base64.RawURLEncoding.EncodeToString(challenge)
- return ccd
 }
 
 func TestVerifyCollectedClientData(t *testing.T) {
@@ -28,7 +25,7 @@ func TestVerifyCollectedClientData(t *testing.T) {
  originURL, _ := url.Parse(ccd.Origin)
  err = ccd.Verify(storedChallenge.String(), ccd.Type, FullyQualifiedOrigin(originURL))
  if err != nil {
- t.Fatalf("error verifying challenge: expected %#v got %#v", Challenge(ccd.Challenge), storedChallenge)
+ t.Fatalf("error verifying challenge: expected %#v got %#v", ccd.Challenge, storedChallenge)
  }
 }
 
@@ -42,9 +39,8 @@ func TestVerifyCollectedClientDataIncorrectChallenge(t *testing.T) {
  if err != nil {
  t.Fatalf("error creating challenge: %s", err)
  }
- storedChallenge := Challenge(bogusChallenge)
- err = ccd.Verify(storedChallenge.String(), ccd.Type, ccd.Origin)
+ err = ccd.Verify(bogusChallenge.String(), ccd.Type, ccd.Origin)
  if err == nil {
- t.Fatalf("error expected but not received. expected %#v got %#v", Challenge(ccd.Challenge), storedChallenge)
+ t.Fatalf("error expected but not received. expected %#v got %#v", ccd.Challenge, bogusChallenge)
  }
 }

protocol/credential_test.go

@@ -162,7 +162,7 @@ func TestParsedCredentialCreationData_Verify(t *testing.T) {
  Raw CredentialCreationResponse
  }
  type args struct {
- storedChallenge Challenge
+ storedChallenge URLEncodedBase64
  verifyUser bool
  relyingPartyID string
  relyingPartyOrigin string
@@ -221,7 +221,7 @@ func TestParsedCredentialCreationData_Verify(t *testing.T) {
  },
  },
  args: args{
- storedChallenge: byteChallenge,
+ storedChallenge: URLEncodedBase64(byteChallenge),
  verifyUser: false,
  relyingPartyID: `webauthn.io`,
  relyingPartyOrigin: `https://webauthn.io`,

protocol/options.go

@@ -15,7 +15,7 @@ type CredentialAssertion struct {
 // In order to create a Credential via create(), the caller specifies a few parameters in a CredentialCreationOptions object.
 // See §5.4. Options for Credential Creation https://www.w3.org/TR/webauthn/#dictionary-makecredentialoptions
 type PublicKeyCredentialCreationOptions struct {
- Challenge Challenge  `json:"challenge"`
+ Challenge URLEncodedBase64 `json:"challenge"`
  RelyingParty RelyingPartyEntity `json:"rp"`
  User UserEntity `json:"user"`
  Parameters []CredentialParameter `json:"pubKeyCredParams,omitempty"`
@@ -30,7 +30,7 @@ type PublicKeyCredentialCreationOptions struct {
 // Its challenge member MUST be present, while its other members are OPTIONAL.
 // See §5.5. Options for Assertion Generation https://www.w3.org/TR/webauthn/#assertion-options
 type PublicKeyCredentialRequestOptions struct {
- Challenge Challenge  `json:"challenge"`
+ Challenge URLEncodedBase64 `json:"challenge"`
  Timeout int `json:"timeout,omitempty"`
  RelyingPartyID string `json:"rpId,omitempty"`
  AllowedCredentials []CredentialDescriptor `json:"allowCredentials,omitempty"`

protocol/options_test.go

@@ -7,7 +7,7 @@ import (
 
 func TestPublicKeyCredentialRequestOptions_GetAllowedCredentialIDs(t *testing.T) {
  type fields struct {
- Challenge Challenge
+ Challenge URLEncodedBase64
  Timeout int
  RelyingPartyID string
  AllowedCredentials []CredentialDescriptor
@@ -22,7 +22,7 @@ func TestPublicKeyCredentialRequestOptions_GetAllowedCredentialIDs(t *testing.T)
  {
  "Correct Credential IDs",
  fields{
- Challenge: Challenge([]byte{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00}),
+ Challenge: URLEncodedBase64([]byte{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00}),
  Timeout: 60,
  AllowedCredentials: []CredentialDescriptor{
  {

webauthn/login.go

@@ -2,7 +2,6 @@ package webauthn
 
 import (
  "bytes"
- "encoding/base64"
  "net/http"
 
  "github.com/go-webauthn/webauthn/protocol"
@@ -64,7 +63,7 @@ func (webauthn *WebAuthn) beginLogin(userID []byte, allowedCredentials []protoco
  }
 
  sessionData := SessionData{
- Challenge: base64.RawURLEncoding.EncodeToString(challenge),
+ Challenge: challenge.String(),
  UserID: userID,
  AllowedCredentialIDs: requestOptions.GetAllowedCredentialIDs(),
  UserVerification: requestOptions.UserVerification,

webauthn/registration.go

@@ -2,7 +2,6 @@ package webauthn
 
 import (
  "bytes"
- "encoding/base64"
  "net/http"
 
  "github.com/go-webauthn/webauthn/protocol"
@@ -57,7 +56,7 @@ func (webauthn *WebAuthn) BeginRegistration(user User, opts ...RegistrationOptio
 
  response := protocol.CredentialCreation{Response: creationOptions}
  newSessionData := SessionData{
- Challenge: base64.RawURLEncoding.EncodeToString(challenge),
+ Challenge: challenge.String(),
  UserID: user.WebAuthnID(),
  UserVerification: creationOptions.AuthenticatorSelection.UserVerification,
  }