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package protocol
import (
"bytes"
"crypto/ecdsa"
"crypto/elliptic"
"crypto/sha256"
"crypto/x509"
"encoding/asn1"
"fmt"
"math/big"
"github.com/go-webauthn/webauthn/metadata"
"github.com/go-webauthn/webauthn/protocol/webauthncose"
)
var appleAttestationKey = "apple"
func init() {
RegisterAttestationFormat(appleAttestationKey, verifyAppleFormat)
}
// The apple attestation statement looks like:
// $$attStmtType //= (
//
// fmt: "apple",
// attStmt: appleStmtFormat
//
// )
//
// appleStmtFormat = {
// x5c: [ credCert: bytes, * (caCert: bytes) ]
// }
//
// Specification: ยง8.8. Apple Anonymous Attestation Statement Format (https://www.w3.org/TR/webauthn/#sctn-apple-anonymous-attestation)
func verifyAppleFormat(att AttestationObject, clientDataHash []byte) (string, []interface{}, error) {
// Step 1. Verify that attStmt is valid CBOR conforming to the syntax defined
// above and perform CBOR decoding on it to extract the contained fields.
// 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 "", nil, ErrAttestationFormat.WithDetails("Error retrieving x5c value")
}
credCertBytes, valid := x5c[0].([]byte)
if !valid {
return "", nil, ErrAttestation.WithDetails("Error getting certificate from x5c cert chain")
}
credCert, err := x509.ParseCertificate(credCertBytes)
if err != nil {
return "", nil, ErrAttestationFormat.WithDetails(fmt.Sprintf("Error parsing certificate from ASN.1 data: %+v", err))
}
// Step 2. Concatenate authenticatorData and clientDataHash to form nonceToHash.
nonceToHash := append(att.RawAuthData, clientDataHash...)
// Step 3. Perform SHA-256 hash of nonceToHash to produce nonce.
nonce := sha256.Sum256(nonceToHash)
// Step 4. Verify that nonce equals the value of the extension with OID 1.2.840.113635.100.8.2 in credCert.
var attExtBytes []byte
for _, ext := range credCert.Extensions {
if ext.Id.Equal([]int{1, 2, 840, 113635, 100, 8, 2}) {
attExtBytes = ext.Value
}
}
if len(attExtBytes) == 0 {
return "", nil, ErrAttestationFormat.WithDetails("Attestation certificate extensions missing 1.2.840.113635.100.8.2")
}
decoded := AppleAnonymousAttestation{}
if _, err = asn1.Unmarshal(attExtBytes, &decoded); err != nil {
return "", nil, ErrAttestationFormat.WithDetails("Unable to parse apple attestation certificate extensions")
}
if !bytes.Equal(decoded.Nonce, nonce[:]) || err != nil {
return "", nil, ErrInvalidAttestation.WithDetails("Attestation certificate does not contain expected nonce")
}
// Step 5. Verify that the credential public key equals the Subject Public Key of credCert.
// TODO: Probably move this part to webauthncose.go
pubKey, err := webauthncose.ParsePublicKey(att.AuthData.AttData.CredentialPublicKey)
if err != nil {
return "", nil, ErrInvalidAttestation.WithDetails(fmt.Sprintf("Error parsing public key: %+v\n", err))
}
credPK := pubKey.(webauthncose.EC2PublicKeyData)
subjectPK := credCert.PublicKey.(*ecdsa.PublicKey)
credPKInfo := &ecdsa.PublicKey{
Curve: elliptic.P256(),
X: big.NewInt(0).SetBytes(credPK.XCoord),
Y: big.NewInt(0).SetBytes(credPK.YCoord),
}
if !credPKInfo.Equal(subjectPK) {
return "", nil, ErrInvalidAttestation.WithDetails("Certificate public key does not match public key in authData")
}
// Step 6. If successful, return implementation-specific values representing attestation type Anonymization CA and attestation trust path x5c.
return string(metadata.AnonCA), x5c, nil
}
// Apple has not yet publish schema for the extension(as of JULY 2021.)
type AppleAnonymousAttestation struct {
Nonce []byte `asn1:"tag:1,explicit"`
}
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