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Allow flagship attestation from the Play Integrity API (#4293)
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,223 @@ | ||
| package oauth | ||
|
|
||
| import ( | ||
| "crypto/aes" | ||
| "crypto/cipher" | ||
| "crypto/x509" | ||
| "encoding/base64" | ||
| "errors" | ||
| "fmt" | ||
| "strings" | ||
|
|
||
| "github.com/cozy/cozy-stack/model/instance" | ||
| "github.com/cozy/cozy-stack/pkg/config/config" | ||
| "github.com/cozy/cozy-stack/pkg/crypto" | ||
| "github.com/cozy/cozy-stack/pkg/logger" | ||
| jwt "github.com/golang-jwt/jwt/v5" | ||
| ) | ||
|
|
||
| // checkPlayIntegrityAttestation will check an attestation made by the Play | ||
| // Integrity API. | ||
| // https://developer.android.com/google/play/integrity | ||
| func (c *Client) checkPlayIntegrityAttestation(inst *instance.Instance, req AttestationRequest) error { | ||
| store := GetStore() | ||
| if ok := store.CheckAndClearChallenge(inst, c.ID(), req.Challenge); !ok { | ||
| return errors.New("invalid challenge") | ||
| } | ||
|
|
||
| token, err := decryptPlayIntegrityToken(req) | ||
| if err != nil { | ||
| inst.Logger().Debugf("cannot decrypt the play integrity token: %s", err) | ||
| return fmt.Errorf("cannot parse attestation: %s", err) | ||
| } | ||
| claims, ok := token.Claims.(jwt.MapClaims) | ||
| if !ok { | ||
| return errors.New("invalid claims type") | ||
| } | ||
| inst.Logger().Debugf("checkPlayIntegrityAttestation claims = %#v", claims) | ||
|
|
||
| nonce, ok := getFromClaims(claims, "requestDetails.nonce").(string) | ||
| if !ok || len(nonce) == 0 { | ||
| return errors.New("missing nonce") | ||
| } | ||
| if req.Challenge != nonce { | ||
| return errors.New("invalid nonce") | ||
| } | ||
|
|
||
| if err := checkPlayIntegrityPackageName(claims); err != nil { | ||
| return err | ||
| } | ||
| if err := checkPlayIntegrityCertificateDigest(claims); err != nil { | ||
| return err | ||
| } | ||
| return nil | ||
| } | ||
|
|
||
| // CheckPlayIntegrityAttestationForTestingPurpose is only used for testing | ||
| // purpose. It is a simplified version of checkPlayIntegrityAttestation. In | ||
| // particular, it doesn't return an error for invalid package name with a test | ||
| // attestation. | ||
| func CheckPlayIntegrityAttestationForTestingPurpose(req AttestationRequest) error { | ||
| token, err := decryptPlayIntegrityToken(req) | ||
| if err != nil { | ||
| return fmt.Errorf("cannot parse attestation: %s", err) | ||
| } | ||
| claims, ok := token.Claims.(jwt.MapClaims) | ||
| if !ok { | ||
| return errors.New("invalid claims type") | ||
| } | ||
|
|
||
| nonce, ok := getFromClaims(claims, "requestDetails.nonce").(string) | ||
| if !ok || len(nonce) == 0 { | ||
| return errors.New("missing nonce") | ||
| } | ||
| if req.Challenge != nonce { | ||
| return errors.New("invalid nonce") | ||
| } | ||
| return nil | ||
| } | ||
|
|
||
| func decryptPlayIntegrityToken(req AttestationRequest) (*jwt.Token, error) { | ||
| lastErr := errors.New("no decryption key") | ||
| for _, key := range config.GetConfig().Flagship.PlayIntegrityDecryptionKeys { | ||
| decrypted, err := decryptPlayIntegrityJWE(req.Attestation, key) | ||
| if err == nil { | ||
| return parsePlayIntegrityToken(decrypted) | ||
| } | ||
| lastErr = err | ||
| } | ||
| return nil, lastErr | ||
| } | ||
|
|
||
| func decryptPlayIntegrityJWE(attestation string, rawKey string) ([]byte, error) { | ||
| parts := strings.Split(attestation, ".") | ||
| if len(parts) != 5 { | ||
| return nil, errors.New("invalid integrity token") | ||
| } | ||
| header := []byte(parts[0]) | ||
| encryptedKey, err := base64.RawURLEncoding.DecodeString(parts[1]) | ||
| // AES Key wrap works with 64 bits block, and the wrapped version has n+1 | ||
| // blocks (for integrity check). The kek key is 256bits, thus the | ||
| // encryptedKey is 320bits => 40bytes. | ||
| if err != nil || len(encryptedKey) != 40 { | ||
| return nil, fmt.Errorf("invalid encrypted key: %w", err) | ||
| } | ||
| initVector, err := base64.RawURLEncoding.DecodeString(parts[2]) | ||
| if err != nil { | ||
| return nil, fmt.Errorf("invalid initialization vector: %w", err) | ||
| } | ||
| cipherText, err := base64.RawURLEncoding.DecodeString(parts[3]) | ||
| if err != nil { | ||
| return nil, fmt.Errorf("invalid ciphertext: %w", err) | ||
| } | ||
| authTag, err := base64.RawURLEncoding.DecodeString(parts[4]) | ||
| if err != nil || len(authTag) != 16 { // GCM uses 128bits => 16bytes | ||
| return nil, fmt.Errorf("invalid authentication tag: %w", err) | ||
| } | ||
|
|
||
| kek, err := base64.StdEncoding.DecodeString(rawKey) // kek means Key-encryption key, cf RFC-3394 | ||
| if err != nil { | ||
| return nil, fmt.Errorf("invalid decryption key: %w", err) | ||
| } | ||
| block, err := aes.NewCipher(kek) | ||
| if err != nil { | ||
| return nil, fmt.Errorf("invalid decryption key: %w", err) | ||
| } | ||
| contentKey, err := crypto.UnwrapA256KW(block, encryptedKey) | ||
| if err != nil { | ||
| return nil, fmt.Errorf("cannot unwrap the key: %w", err) | ||
| } | ||
| if len(contentKey) != 32 { // AES256 means 256bits => 32bytes | ||
| return nil, fmt.Errorf("invalid encrypted key: %w", err) | ||
| } | ||
|
|
||
| cek, err := aes.NewCipher(contentKey) | ||
| if err != nil { | ||
| return nil, fmt.Errorf("cannot load the cek: %w", err) | ||
| } | ||
| aesgcm, err := cipher.NewGCM(cek) | ||
| if err != nil { | ||
| return nil, fmt.Errorf("cannot initialize AES-GCM: %w", err) | ||
| } | ||
| if len(initVector) != aesgcm.NonceSize() { | ||
| return nil, fmt.Errorf("invalid initialization vector: %w", err) | ||
| } | ||
| decrypted, err := aesgcm.Open(nil, initVector, append(cipherText, authTag...), header) | ||
| if err != nil { | ||
| return nil, fmt.Errorf("cannot decrypt: %w", err) | ||
| } | ||
|
|
||
| return decrypted, nil | ||
| } | ||
|
|
||
| func parsePlayIntegrityToken(decrypted []byte) (*jwt.Token, error) { | ||
| lastErr := errors.New("no verification key") | ||
| for _, key := range config.GetConfig().Flagship.PlayIntegrityVerificationKeys { | ||
| token, err := parsePlayIntegrityJWT(decrypted, key) | ||
| if err == nil { | ||
| return token, err | ||
| } | ||
| lastErr = err | ||
| } | ||
| return nil, lastErr | ||
| } | ||
|
|
||
| func parsePlayIntegrityJWT(decrypted []byte, rawKey string) (*jwt.Token, error) { | ||
| return jwt.Parse(string(decrypted), func(token *jwt.Token) (interface{}, error) { | ||
| if _, ok := token.Method.(*jwt.SigningMethodECDSA); !ok { | ||
| return nil, fmt.Errorf("unexpected signing method: %v", token.Header["alg"]) | ||
| } | ||
| key, err := base64.StdEncoding.DecodeString(rawKey) | ||
| if err != nil { | ||
| return nil, fmt.Errorf("invalid verification key: %w", err) | ||
| } | ||
| pubKey, err := x509.ParsePKIXPublicKey(key) | ||
| if err != nil { | ||
| return nil, fmt.Errorf("invalid verification key: %w", err) | ||
| } | ||
| return pubKey, nil | ||
| }) | ||
| } | ||
|
|
||
| func checkPlayIntegrityPackageName(claims jwt.MapClaims) error { | ||
| packageName, ok := getFromClaims(claims, "appIntegrity.packageName").(string) | ||
| if !ok || len(packageName) == 0 { | ||
| return errors.New("missing appIntegrity.packageName") | ||
| } | ||
| names := config.GetConfig().Flagship.APKPackageNames | ||
| for _, name := range names { | ||
| if name == packageName { | ||
| return nil | ||
| } | ||
| } | ||
| return fmt.Errorf("%s is not the package name of the flagship app", packageName) | ||
| } | ||
|
|
||
| func checkPlayIntegrityCertificateDigest(claims jwt.MapClaims) error { | ||
| certDigest, ok := getFromClaims(claims, "appIntegrity.certificateSha256Digest").([]interface{}) | ||
| if !ok || len(certDigest) == 0 { | ||
| return errors.New("missing appIntegrity.certificateSha256Digest") | ||
| } | ||
| digests := config.GetConfig().Flagship.APKCertificateDigests | ||
| for _, digest := range digests { | ||
| if digest == certDigest[0] { | ||
| return nil | ||
| } | ||
| } | ||
| logger.WithNamespace("oauth"). | ||
| Debugf("Invalid certificate digest, expected %s, got %s", digests[0], certDigest) | ||
| return errors.New("invalid certificate digest") | ||
| } | ||
|
|
||
| func getFromClaims(claims jwt.MapClaims, key string) interface{} { | ||
| parts := strings.Split(key, ".") | ||
| var obj interface{} = map[string]interface{}(claims) | ||
| for _, part := range parts { | ||
| m, ok := obj.(map[string]interface{}) | ||
| if !ok { | ||
| return nil | ||
| } | ||
| obj = m[part] | ||
| } | ||
| return obj | ||
| } |
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