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package protocol
import (
"bytes"
"crypto/x509"
"encoding/asn1"
"fmt"
"github.com/duo-labs/webauthn/protocol/webauthncose"
)
var androidAttestationKey = "android-key"
func init() {
RegisterAttestationFormat(androidAttestationKey, verifyAndroidKeyFormat)
}
// From §8.4. https://www.w3.org/TR/webauthn/#android-key-attestation
// The android-key attestation statement looks like:
// $$attStmtType //= (
// fmt: "android-key",
// attStmt: androidStmtFormat
// )
// androidStmtFormat = {
// alg: COSEAlgorithmIdentifier,
// sig: bytes,
// x5c: [ credCert: bytes, * (caCert: bytes) ]
// }
func verifyAndroidKeyFormat(att AttestationObject, clientDataHash []byte) (string, []interface{}, error) {
// Given the verification procedure inputs attStmt, authenticatorData and clientDataHash, the verification procedure is as follows:
// §8.4.1. Verify that attStmt is valid CBOR conforming to the syntax defined above and perform CBOR decoding on it to extract
// the contained fields.
// Get the alg value - A COSEAlgorithmIdentifier containing the identifier of the algorithm
// used to generate the attestation signature.
alg, present := att.AttStatement["alg"].(int64)
if !present {
return androidAttestationKey, nil, ErrAttestationFormat.WithDetails("Error retreiving alg value")
}
// Get the sig value - A byte string containing the attestation signature.
sig, present := att.AttStatement["sig"].([]byte)
if !present {
return androidAttestationKey, nil, ErrAttestationFormat.WithDetails("Error retreiving sig value")
}
// If x5c is not present, return an error
x5c, x509present := att.AttStatement["x5c"].([]interface{})
if !x509present {
// Handle Basic Attestation steps for the x509 Certificate
return androidAttestationKey, nil, ErrAttestationFormat.WithDetails("Error retreiving x5c value")
}
// §8.4.2. Verify that sig is a valid signature over the concatenation of authenticatorData and clientDataHash
// using the public key in the first certificate in x5c with the algorithm specified in alg.
attCertBytes, valid := x5c[0].([]byte)
if !valid {
return androidAttestationKey, nil, ErrAttestation.WithDetails("Error getting certificate from x5c cert chain")
}
signatureData := append(att.RawAuthData, clientDataHash...)
attCert, err := x509.ParseCertificate(attCertBytes)
if err != nil {
return androidAttestationKey, nil, ErrAttestationFormat.WithDetails(fmt.Sprintf("Error parsing certificate from ASN.1 data: %+v", err))
}
coseAlg := webauthncose.COSEAlgorithmIdentifier(alg)
sigAlg := webauthncose.SigAlgFromCOSEAlg(coseAlg)
err = attCert.CheckSignature(x509.SignatureAlgorithm(sigAlg), signatureData, sig)
if err != nil {
return androidAttestationKey, nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Signature validation error: %+v\n", err))
}
// Verify that the public key in the first certificate in x5c matches the credentialPublicKey in the attestedCredentialData in authenticatorData.
pubKey, err := webauthncose.ParsePublicKey(att.AuthData.AttData.CredentialPublicKey)
if err != nil {
return androidAttestationKey, nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Error parsing public key: %+v\n", err))
}
e := pubKey.(webauthncose.EC2PublicKeyData)
valid, err = e.Verify(signatureData, sig)
if err != nil || valid != true {
return androidAttestationKey, nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Error parsing public key: %+v\n", err))
}
// §8.4.3. Verify that the attestationChallenge field in the attestation certificate extension data is identical to clientDataHash.
// attCert.Extensions
var attExtBytes []byte
for _, ext := range attCert.Extensions {
if ext.Id.Equal([]int{1, 3, 6, 1, 4, 1, 11129, 2, 1, 17}) {
attExtBytes = ext.Value
}
}
if len(attExtBytes) == 0 {
return androidAttestationKey, nil, ErrAttestationFormat.WithDetails("Attestation certificate extensions missing 1.3.6.1.4.1.11129.2.1.17")
}
// As noted in §8.4.1 (https://w3c.github.io/webauthn/#key-attstn-cert-requirements) the Android Key Attestation attestation certificate's
// android key attestation certificate extension data is identified by the OID "1.3.6.1.4.1.11129.2.1.17".
decoded := keyDescription{}
_, err = asn1.Unmarshal([]byte(attExtBytes), &decoded)
if err != nil {
return androidAttestationKey, nil, ErrAttestationFormat.WithDetails("Unable to parse Android key attestation certificate extensions")
}
// Verify that the attestationChallenge field in the attestation certificate extension data is identical to clientDataHash.
if 0 != bytes.Compare(decoded.AttestationChallenge, clientDataHash) {
return androidAttestationKey, nil, ErrAttestationFormat.WithDetails("Attestation challenge not equal to clientDataHash")
}
// The AuthorizationList.allApplications field is not present on either authorization list (softwareEnforced nor teeEnforced), since PublicKeyCredential MUST be scoped to the RP ID.
if nil != decoded.SoftwareEnforced.AllApplications || nil != decoded.TeeEnforced.AllApplications {
return androidAttestationKey, nil, ErrAttestationFormat.WithDetails("Attestation certificate extensions contains all applications field")
}
// For the following, use only the teeEnforced authorization list if the RP wants to accept only keys from a trusted execution environment, otherwise use the union of teeEnforced and softwareEnforced.
// The value in the AuthorizationList.origin field is equal to KM_ORIGIN_GENERATED. (which == 0)
if KM_ORIGIN_GENERATED != decoded.SoftwareEnforced.Origin || KM_ORIGIN_GENERATED != decoded.TeeEnforced.Origin {
return androidAttestationKey, nil, ErrAttestationFormat.WithDetails("Attestation certificate extensions contains authorization list with origin not equal KM_ORIGIN_GENERATED")
}
// The value in the AuthorizationList.purpose field is equal to KM_PURPOSE_SIGN. (which == 2)
if !contains(decoded.SoftwareEnforced.Purpose, KM_PURPOSE_SIGN) && !contains(decoded.TeeEnforced.Purpose, KM_PURPOSE_SIGN) {
return androidAttestationKey, nil, ErrAttestationFormat.WithDetails("Attestation certificate extensions contains authorization list with purpose not equal KM_PURPOSE_SIGN")
}
return androidAttestationKey, x5c, err
}
func contains(s []int, e int) bool {
for _, a := range s {
if a == e {
return true
}
}
return false
}
type keyDescription struct {
AttestationVersion int
AttestationSecurityLevel asn1.Enumerated
KeymasterVersion int
KeymasterSecurityLevel asn1.Enumerated
AttestationChallenge []byte
UniqueID []byte
SoftwareEnforced authorizationList
TeeEnforced authorizationList
}
type authorizationList struct {
Purpose []int `asn1:"tag:1,explicit,set,optional"`
Algorithm int `asn1:"tag:2,explicit,optional"`
KeySize int `asn1:"tag:3,explicit,optional"`
Digest []int `asn1:"tag:5,explicit,set,optional"`
Padding []int `asn1:"tag:6,explicit,set,optional"`
EcCurve int `asn1:"tag:10,explicit,optional"`
RsaPublicExponent int `asn1:"tag:200,explicit,optional"`
RollbackResistance interface{} `asn1:"tag:303,explicit,optional"`
ActiveDateTime int `asn1:"tag:400,explicit,optional"`
OriginationExpireDateTime int `asn1:"tag:401,explicit,optional"`
UsageExpireDateTime int `asn1:"tag:402,explicit,optional"`
NoAuthRequired interface{} `asn1:"tag:503,explicit,optional"`
UserAuthType int `asn1:"tag:504,explicit,optional"`
AuthTimeout int `asn1:"tag:505,explicit,optional"`
AllowWhileOnBody interface{} `asn1:"tag:506,explicit,optional"`
TrustedUserPresenceRequired interface{} `asn1:"tag:507,explicit,optional"`
TrustedConfirmationRequired interface{} `asn1:"tag:508,explicit,optional"`
UnlockedDeviceRequired interface{} `asn1:"tag:509,explicit,optional"`
AllApplications interface{} `asn1:"tag:600,explicit,optional"`
ApplicationID interface{} `asn1:"tag:601,explicit,optional"`
CreationDateTime int `asn1:"tag:701,explicit,optional"`
Origin int `asn1:"tag:702,explicit,optional"`
RootOfTrust rootOfTrust `asn1:"tag:704,explicit,optional"`
OsVersion int `asn1:"tag:705,explicit,optional"`
OsPatchLevel int `asn1:"tag:706,explicit,optional"`
AttestationApplicationID []byte `asn1:"tag:709,explicit,optional"`
AttestationIDBrand []byte `asn1:"tag:710,explicit,optional"`
AttestationIDDevice []byte `asn1:"tag:711,explicit,optional"`
AttestationIDProduct []byte `asn1:"tag:712,explicit,optional"`
AttestationIDSerial []byte `asn1:"tag:713,explicit,optional"`
AttestationIDImei []byte `asn1:"tag:714,explicit,optional"`
AttestationIDMeid []byte `asn1:"tag:715,explicit,optional"`
AttestationIDManufacturer []byte `asn1:"tag:716,explicit,optional"`
AttestationIDModel []byte `asn1:"tag:717,explicit,optional"`
VendorPatchLevel int `asn1:"tag:718,explicit,optional"`
BootPatchLevel int `asn1:"tag:719,explicit,optional"`
}
type rootOfTrust struct {
verifiedBootKey []byte
deviceLocked bool
verifiedBootState verifiedBootState
verifiedBootHash []byte
}
type verifiedBootState int
const (
Verified verifiedBootState = iota
SelfSigned
Unverified
Failed
)
/**
* The origin of a key (or pair), i.e. where it was generated. Note that KM_TAG_ORIGIN can be found
* in either the hardware-enforced or software-enforced list for a key, indicating whether the key
* is hardware or software-based. Specifically, a key with KM_ORIGIN_GENERATED in the
* hardware-enforced list is guaranteed never to have existed outide the secure hardware.
*/
type KM_KEY_ORIGIN int
const (
KM_ORIGIN_GENERATED = iota /* Generated in keymaster. Should not exist outside the TEE. */
KM_ORIGIN_DERIVED /* Derived inside keymaster. Likely exists off-device. */
KM_ORIGIN_IMPORTED /* Imported into keymaster. Existed as cleartext in Android. */
KM_ORIGIN_UNKNOWN /* Keymaster did not record origin. This value can only be seen on
* keys in a keymaster0 implementation. The keymaster0 adapter uses
* this value to document the fact that it is unkown whether the key
* was generated inside or imported into keymaster. */
)
/**
* Possible purposes of a key (or pair).
*/
type KM_PURPOSE int
const (
KM_PURPOSE_ENCRYPT = iota /* Usable with RSA, EC and AES keys. */
KM_PURPOSE_DECRYPT /* Usable with RSA, EC and AES keys. */
KM_PURPOSE_SIGN /* Usable with RSA, EC and HMAC keys. */
KM_PURPOSE_VERIFY /* Usable with RSA, EC and HMAC keys. */
KM_PURPOSE_DERIVE_KEY /* Usable with EC keys. */
KM_PURPOSE_WRAP /* Usable with wrapped keys. */
)
|
