Verifying hardware-backed key pairs with Key Attestation

Key Attestation gives you more confidence that the keys you use in your app are stored in a device's hardware-backed keystore. The following sections describe how to verify the properties of hardware-backed keys and how to interpret the schema of the attestation certificate's extension data.

Note: Before you verify the properties of a device's hardware-backed keys in a production-level environment, you should make sure that the device supports hardware-level key attestation. To do so, you should check that the attestation certificate chain contains a root certificate that is signed with the Google attestation root key and that the attestationSecurityLevel element within the key description data structure is set to the TrustedEnvironment security level.

In addition, it's important to verify the signatures in the certificate chain and to check that none of the keys in the chain have been revoked by checking the Certificate Revocation Status List. Unless all are valid and the root is the Google root key mentioned above, you shouldn't fully trust the attestation. Note, however, that devices containing revoked certificates are still at least as trustworthy as devices that only support software attestation. Having a fully-valid attestation is a strong positive indicator. Not having one is a neutral—not negative—indicator.

Retrieve and verify a hardware-backed key pair

During key attestation, you specify the alias of a key pair. The attestation tool, in return, provides a certificate chain, which you can use to verify the properties of that key pair.

If the device supports hardware-level key attestation, the root certificate within this chain is signed using an attestation root key, which the device manufacturer injects into the device's hardware-backed keystore at the factory.

Note: On devices that ship with hardware-level key attestation, Android 7.0 (API level 24) or higher, and Google Play services, the root certificate is signed with the Google attestation root key. You should verify that this root certificate is among those listed below.

To implement key attestation, complete the following steps:

Use a KeyStore object's getCertificateChain() method to get a reference to the chain of X.509 certificates associated with the hardware-backed keystore.
Check each certificate's validity using a X509Certificate object's checkValidity() method. Also verify that the root certificate is trustworthy.

Caution: Although you can complete this process within your app directly, it's safer to check the certificates' revocation status on a separate server that you trust.
On a separate server that you trust, obtain a reference to the ASN.1 parser library that is most appropriate for your toolset. Use this parser to extract the attestation certificate extension data, which appears within the first element of the certificate chain.

The Key Attestation sample uses the ASN.1 parser from Bouncy Castle to extract an attestation certificate's extension data. You can use this sample as a reference for creating your own parser.

For more details about the schema of the extension data, see Certificate Extension Data Schema.
Compare the extension data that you've retrieved from your ASN.1 parser with the set of values that you expect the hardware-backed key to contain.

Caution: Although you can complete this process within your app directly, it's safer to check the certificate's extension data on a separate server that you trust.

Root certificates

The trustworthiness of the attestation depends on the root certificate of the chain. Android devices that have passed the testing required to have the Google suite of apps, including Google Play, and which launched with Android 7.0 (API level 24) or higher should use attestation keys signed by the Google Hardware Attestation Root certificate. This certificate is one of the following certificates:

If the root certificate in the attestation chain you receive is one of the above certificates and none of the certificates in the chain have been revoked, you know that:

Your key is in hardware that Google believes to be secure; and
It has the properties described in the attestation certificate.

If the attestation chain has any other root certificate, then Google does not make any claims about the security of the hardware. This does not mean that your key is compromised, just that the attestation does not prove that the key is in security hardware, and you should adjust your security assumptions accordingly.

If the root certificate is not in the above list, there are two likely reasons:

Most likely, the device launched with an Android version less than 7.0 and it does not support hardware attestation. In this case, Android has a software implementation of attestation which produces the same sort of attestation certificate, but signed with a key hardcoded in Android source code. Because this signing key is not a secret, the attestation could have been created by an attacker pretending to provide secure hardware.
The other likely reason is that the device is not a Google Play device. In that case, the device maker is free to create their own root and to make whatever claims they like about what the attestation means. Refer to the device maker's documentation. Note that as of this writing Google is not aware of any device makers who have done this.

Certificate Revocation Status List

Attestation keys can be revoked for a number of reasons, including mishandling or suspected extraction by an attacker. For this reason, it's critical that the status of each certificate in an attestation chain be checked against the official certificate revocation status list. This list is maintained by Google and published at: https://android.googleapis.com/attestation/status. This URL returns a JSON file containing the revocation status for any certificates that don't have a normal valid status. The format of the JSON file adheres to the following JSON Schema (draft 07) definition:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "object",
  "properties": {
    "entries": {
      "description" : "Each entry represents the status of an attestation key. The dictionary-key is the certificate serial number in lowercase hex.",
      "type": "object",
      "propertyNames": {
         "pattern": "^[a-f0-9]*$"
      },
      "additionalProperties": {
        "type": "object",
        "properties": {
          "status": {
            "description": "[REQUIRED] Current status of the key.",
            "type": "string",
            "enum": ["REVOKED", "SUSPENDED"]
          },
          "expires": {
            "description": "[OPTIONAL] UTC date when certificate expires in ISO8601 format (YYYY-MM-DD). Can be used to clear expired certificates from the status list.",
            "type": "string",
            "format": "date"
          },
          "reason": {
            "description": "[OPTIONAL] Reason for the current status.",
            "type": "string",
            "enum": ["UNSPECIFIED", "KEY_COMPROMISE", "CA_COMPROMISE", "SUPERSEDED", "SOFTWARE_FLAW"]
          },
          "comment": {
            "description": "[OPTIONAL] Free form comment about the key status.",
            "type": "string",
            "maxLength": 140
          }
        },
        "required": ["status"],
        "additionalProperties": false
      }
    }
  },
  "required": ["entries"],
  "additionalProperties": false
}

Example certificate revocation status list:

{
  "entries": {
    "2c8cdddfd5e03bfc": {
      "status": "REVOKED",
      "expires": "2020-11-13",
      "reason": "KEY_COMPROMISE",
      "comment": "Key stored on unsecure system"
    },
    "c8966fcb2fbb0d7a": {
      "status": "SUSPENDED",
      "reason": "SOFTWARE_FLAW",
      "comment": "Bug in keystore causes this key malfunction b/555555"
    }
  }
}

Legacy CRLs

The CRL urls embedded in legacy attestation certificates will continue to operate. New attestation certificates will no longer contain a CRL url extension. The status of legacy certificates will also be included in the attestation status list, so developers can safely switch to using the attestation status list for both new and legacy certificates. An example of how to correctly verify Android attestation keys is included in the Key Attestation sample.

Certificate extension data schema

Key attestation verifies the extension data that appears in the first certificate within the chain in a device's hardware-backed keystore. The certificate stores the information according to an ASN.1 schema. To view the schema corresponding to the attestation version that you're using, select the appropriate tab in the following schema listing:

Version 3

KeyDescription ::= SEQUENCE {
    attestationVersion  3,
    attestationSecurityLevel  SecurityLevel,
    keymasterVersion  INTEGER,
    keymasterSecurityLevel  SecurityLevel,
    attestationChallenge  OCTET_STRING,
    uniqueId  OCTET_STRING,
    softwareEnforced  AuthorizationList,
    teeEnforced  AuthorizationList,
}

SecurityLevel ::= ENUMERATED {
    Software  (0),
    TrustedEnvironment  (1),
    StrongBox  (2),
}

AuthorizationList ::= SEQUENCE {
    purpose  [1] EXPLICIT SET OF INTEGER OPTIONAL,
    algorithm  [2] EXPLICIT INTEGER OPTIONAL,
    keySize  [3] EXPLICIT INTEGER OPTIONAL,
    digest  [5] EXPLICIT SET OF INTEGER OPTIONAL,
    padding  [6] EXPLICIT SET OF INTEGER OPTIONAL,
    ecCurve  [10] EXPLICIT INTEGER OPTIONAL,
    rsaPublicExponent  [200] EXPLICIT INTEGER OPTIONAL,
    rollbackResistance  [303] EXPLICIT NULL OPTIONAL,
    activeDateTime  [400] EXPLICIT INTEGER OPTIONAL,
    originationExpireDateTime  [401] EXPLICIT INTEGER OPTIONAL,
    usageExpireDateTime  [402] EXPLICIT INTEGER OPTIONAL,
    noAuthRequired  [503] EXPLICIT NULL OPTIONAL,
    userAuthType  [504] EXPLICIT INTEGER OPTIONAL,
    authTimeout  [505] EXPLICIT INTEGER OPTIONAL,
    allowWhileOnBody  [506] EXPLICIT NULL OPTIONAL,
    trustedUserPresenceRequired  [507] EXPLICIT NULL OPTIONAL,
    trustedConfirmationRequired  [508] EXPLICIT NULL OPTIONAL,
    unlockedDeviceRequired  [509] EXPLICIT NULL OPTIONAL,
    allApplications  [600] EXPLICIT NULL OPTIONAL,
    applicationId  [601] EXPLICIT OCTET_STRING OPTIONAL,
    creationDateTime  [701] EXPLICIT INTEGER OPTIONAL,
    origin  [702] EXPLICIT INTEGER OPTIONAL,
    rootOfTrust  [704] EXPLICIT RootOfTrust OPTIONAL,
    osVersion  [705] EXPLICIT INTEGER OPTIONAL,
    osPatchLevel  [706] EXPLICIT INTEGER OPTIONAL,
    attestationApplicationId  [709] EXPLICIT OCTET_STRING OPTIONAL,
    attestationIdBrand  [710] EXPLICIT OCTET_STRING OPTIONAL,
    attestationIdDevice  [711] EXPLICIT OCTET_STRING OPTIONAL,
    attestationIdProduct  [712] EXPLICIT OCTET_STRING OPTIONAL,
    attestationIdSerial  [713] EXPLICIT OCTET_STRING OPTIONAL,
    attestationIdImei  [714] EXPLICIT OCTET_STRING OPTIONAL,
    attestationIdMeid  [715] EXPLICIT OCTET_STRING OPTIONAL,
    attestationIdManufacturer  [716] EXPLICIT OCTET_STRING OPTIONAL,
    attestationIdModel  [717] EXPLICIT OCTET_STRING OPTIONAL,
    vendorPatchLevel  [718] EXPLICIT INTEGER OPTIONAL,
    bootPatchLevel  [719] EXPLICIT INTEGER OPTIONAL,
}

RootOfTrust ::= SEQUENCE {
    verifiedBootKey  OCTET_STRING,
    deviceLocked  BOOLEAN,
    verifiedBootState  VerifiedBootState,
    verifiedBootHash OCTET_STRING,
}

VerifiedBootState ::= ENUMERATED {
    Verified  (0),
    SelfSigned  (1),
    Unverified  (2),
    Failed  (3),
}

Version 2

KeyDescription ::= SEQUENCE {
    attestationVersion  2,
    attestationSecurityLevel  SecurityLevel,
    keymasterVersion  INTEGER,
    keymasterSecurityLevel  SecurityLevel,
    attestationChallenge  OCTET_STRING,
    uniqueId  OCTET_STRING,
    softwareEnforced  AuthorizationList,
    teeEnforced  AuthorizationList,
}

SecurityLevel ::= ENUMERATED {
    Software  (0),
    TrustedEnvironment  (1),
}

AuthorizationList ::= SEQUENCE {
    purpose  [1] EXPLICIT SET OF INTEGER OPTIONAL,
    algorithm  [2] EXPLICIT INTEGER OPTIONAL,
    keySize  [3] EXPLICIT INTEGER OPTIONAL,
    digest  [5] EXPLICIT SET OF INTEGER OPTIONAL,
    padding  [6] EXPLICIT SET OF INTEGER OPTIONAL,
    ecCurve  [10] EXPLICIT INTEGER OPTIONAL,
    rsaPublicExponent  [200] EXPLICIT INTEGER OPTIONAL,
    activeDateTime  [400] EXPLICIT INTEGER OPTIONAL,
    originationExpireDateTime  [401] EXPLICIT INTEGER OPTIONAL,
    usageExpireDateTime  [402] EXPLICIT INTEGER OPTIONAL,
    noAuthRequired  [503] EXPLICIT NULL OPTIONAL,
    userAuthType  [504] EXPLICIT INTEGER OPTIONAL,
    authTimeout  [505] EXPLICIT INTEGER OPTIONAL,
    allowWhileOnBody  [506] EXPLICIT NULL OPTIONAL,
    allApplications  [600] EXPLICIT NULL OPTIONAL,
    applicationId  [601] EXPLICIT OCTET_STRING OPTIONAL,
    creationDateTime  [701] EXPLICIT INTEGER OPTIONAL,
    origin  [702] EXPLICIT INTEGER OPTIONAL,
    rollbackResistant  [703] EXPLICIT NULL OPTIONAL,
    rootOfTrust  [704] EXPLICIT RootOfTrust OPTIONAL,
    osVersion  [705] EXPLICIT INTEGER OPTIONAL,
    osPatchLevel  [706] EXPLICIT INTEGER OPTIONAL,
    attestationApplicationId  [709] EXPLICIT OCTET_STRING OPTIONAL,
    attestationIdBrand  [710] EXPLICIT OCTET_STRING OPTIONAL,
    attestationIdDevice  [711] EXPLICIT OCTET_STRING OPTIONAL,
    attestationIdProduct  [712] EXPLICIT OCTET_STRING OPTIONAL,
    attestationIdSerial  [713] EXPLICIT OCTET_STRING OPTIONAL,
    attestationIdImei  [714] EXPLICIT OCTET_STRING OPTIONAL,
    attestationIdMeid  [715] EXPLICIT OCTET_STRING OPTIONAL,
    attestationIdManufacturer  [716] EXPLICIT OCTET_STRING OPTIONAL,
    attestationIdModel  [717] EXPLICIT OCTET_STRING OPTIONAL,
}

RootOfTrust ::= SEQUENCE {
    verifiedBootKey  OCTET_STRING,
    deviceLocked  BOOLEAN,
    verifiedBootState  VerifiedBootState,
}

VerifiedBootState ::= ENUMERATED {
    Verified  (0),
    SelfSigned  (1),
    Unverified  (2),
    Failed  (3),
}

Version 1

KeyDescription ::= SEQUENCE {
    attestationVersion  1,
    attestationSecurityLevel  SecurityLevel,
    keymasterVersion  INTEGER,
    keymasterSecurityLevel  SecurityLevel,
    attestationChallenge  OCTET_STRING,
    uniqueId  OCTET_STRING,
    softwareEnforced  AuthorizationList,
    teeEnforced  AuthorizationList,
}

SecurityLevel ::= ENUMERATED {
    Software  (0),
    TrustedEnvironment  (1),
}

AuthorizationList ::= SEQUENCE {
    purpose  [1] EXPLICIT SET OF INTEGER OPTIONAL,
    algorithm  [2] EXPLICIT INTEGER OPTIONAL,
    keySize  [3] EXPLICIT INTEGER OPTIONAL,
    digest  [5] EXPLICIT SET OF INTEGER OPTIONAL,
    padding  [6] EXPLICIT SET OF INTEGER OPTIONAL,
    ecCurve  [10] EXPLICIT INTEGER OPTIONAL,
    rsaPublicExponent  [200] EXPLICIT INTEGER OPTIONAL,
    activeDateTime  [400] EXPLICIT INTEGER OPTIONAL,
    originationExpireDateTime  [401] EXPLICIT INTEGER OPTIONAL,
    usageExpireDateTime  [402] EXPLICIT INTEGER OPTIONAL,
    noAuthRequired  [503] EXPLICIT NULL OPTIONAL,
    userAuthType  [504] EXPLICIT INTEGER OPTIONAL,
    authTimeout  [505] EXPLICIT INTEGER OPTIONAL,
    allowWhileOnBody  [506] EXPLICIT NULL OPTIONAL,
    allApplications  [600] EXPLICIT NULL OPTIONAL,
    applicationId  [601] EXPLICIT OCTET_STRING OPTIONAL,
    creationDateTime  [701] EXPLICIT INTEGER OPTIONAL,
    origin  [702] EXPLICIT INTEGER OPTIONAL,
    rollbackResistant  [703] EXPLICIT NULL OPTIONAL,
    rootOfTrust  [704] EXPLICIT RootOfTrust OPTIONAL,
    osVersion  [705] EXPLICIT INTEGER OPTIONAL,
    osPatchLevel  [706] EXPLICIT INTEGER OPTIONAL,
}

RootOfTrust ::= SEQUENCE {
    verifiedBootKey  OCTET_STRING,
    deviceLocked  BOOLEAN,
    verifiedBootState  VerifiedBootState,
}

VerifiedBootState ::= ENUMERATED {
    Verified  (0),
    SelfSigned  (1),
    Unverified  (2),
    Failed  (3),
}

The following list presents a description of each element within the schema:

KeyDescription

This sequence of values presents general information about the key pair being verified through key attestation and provides easy access to additional details.

attestationVersion

The version of the key attestation feature.

attestationSecurityLevel

The security level of the attestation.

Warning: Although it is possible to attest keys that are stored in the Android system—that is, if the value of attestationSecurityLevel is set to Software—you cannot trust these attestations if the Android system becomes compromised.

keymasterVersion

The version of the Keymaster hardware abstraction layer (HAL). Use 0 to represent version 0.2 or 0.3, 1 to represent version 1.0, and 2 to represent version 2.0.

keymasterSecurityLevel

The security level of the Keymaster implementation.

uniqueId

This value identifies the device—but only for a limited period of time. It is computed and is only used by system apps. In all other apps, uniqueId is empty.

softwareEnforced

Optional. The Keymaster authorization list that is enforced by the Android system, not by the device's TEE.

teeEnforced

Optional. The Keymaster authorization list that is enforced by the device's TEE.

SecurityLevel

This data structure indicates the extent to which a software feature, such as a key pair, is protected based on its location within the device.

Because the data structure is an enumeration, it takes on exactly one of the following values:

Software: The logic for creating and managing the feature is implemented in the Android system. For the purposes of creating and storing key pairs, this location is less secure than the TEE but is more secure than your app's process space.
TrustedEnvironment: The logic for creating and managing the feature is implemented in secure hardware, such as a TEE. For the purposes of creating and storing key pairs, this location is more secure because secure hardware is highly resistant to remote compromise.
StrongBox: The logic for creating and managing the feature is implemented in a dedicated hardware security module. For the purposes of creating and storing key pairs, this location is more secure because it is highly resistant to remote compromise and hardware attacks against the module.

AuthorizationList

This data structure contains the key pair's properties themselves, as defined in the Keymaster hardware abstraction layer (HAL). You compare these values to the device's current state or to a set of expected values to verify that a key pair is still valid for use in your app.

Each field name corresponds to a similarly-named Keymaster authorization tag. For example, the keySize field in an authorization list corresponds to the Tag::KEY_SIZE Keymaster authorization tag.

Each field in the following list is optional:

purpose

Corresponds to the Tag::PURPOSE Keymaster authorization tag, which uses a tag ID value of 1.

algorithm

Corresponds to the Tag::ALGORITHM Keymaster authorization tag, which uses a tag ID value of 2.

In an attestation AuthorizationList object, the algorithm value is always RSA or EC.

keySize

Corresponds to the Tag::KEY_SIZE Keymaster authorization tag, which uses a tag ID value of 3.

digest

Corresponds to the Tag::DIGEST Keymaster authorization tag, which uses a tag ID value of 5.

padding

Corresponds to the Tag::PADDING Keymaster authorization tag, which uses a tag ID value of 6.

ecCurve

Corresponds to the Tag::EC_CURVE Keymaster authorization tag, which uses a tag ID value of 10.

The set of parameters used to generate an elliptic curve (EC) key pair, which uses ECDSA for signing and verification, within the Android system keystore.

rsaPublicExponent

Corresponds to the Tag::RSA_PUBLIC_EXPONENT Keymaster authorization tag, which uses a tag ID value of 200.

rollbackResistance

Present only in key attestation version 3.

Corresponds to the Tag::ROLLBACK_RESISTANT Keymaster authorization tag, which uses a tag ID value of 303.

activeDateTime

Corresponds to the Tag::ACTIVE_DATETIME Keymaster authorization tag, which uses a tag ID value of 400.

originationExpireDateTime

Corresponds to the Tag::ORIGINATION_EXPIRE_DATETIME Keymaster authorization tag, which uses a tag ID value of 401.

usageExpireDateTime

Corresponds to the Tag::USAGE_EXPIRE_DATETIME Keymaster authorization tag, which uses a tag ID value of 402.

noAuthRequired

Corresponds to the Tag::NO_AUTH_REQUIRED Keymaster authorization tag, which uses a tag ID value of 503.

userAuthType

Corresponds to the Tag::USER_AUTH_TYPE Keymaster authorization tag, which uses a tag ID value of 504.

authTimeout

Corresponds to the Tag::AUTH_TIMEOUT Keymaster authorization tag, which uses a tag ID value of 505.

allowWhileOnBody

Corresponds to the Tag::ALLOW_WHILE_ON_BODY Keymaster authorization tag, which uses a tag ID value of 506.

Allows the key to be used after its authentication timeout period if the user is still wearing the device on their body. Note that a secure on-body sensor determines whether the device is being worn on the user's body.

trustedUserPresenceRequired

Present only in key attestation version 3.

Corresponds to the Tag::TRUSTED_USER_PRESENCE_REQUIRED Keymaster authorization tag, which uses a tag ID value of 507.

Specifies that this key is usable only if the user has provided proof of physical presence. Several examples include the following:

For a StrongBox key, a hardware button hardwired to a pin on the StrongBox device.
For a TEE key, fingerprint authentication provides proof of presence as long as the TEE has exclusive control of the scanner and performs the fingerprint matching process.

trustedConfirmationRequired

Present only in key attestation version 3.

Corresponds to the Tag::TRUSTED_CONFIRMATION_REQUIRED Keymaster authorization tag, which uses a tag ID value of 508.

Specifies that the key is usable only if the user provides confirmation of the data to be signed using an approval token. For more information about how to obtain user confirmation, see Android Protected Confirmation.

Note: This tag is only applicable to keys that use the SIGN purpose.

unlockedDeviceRequired

Present only in key attestation version 3.

Corresponds to the Tag::UNLOCKED_DEVICE_REQUIRED Keymaster authorization tag, which uses a tag ID value of 509.

allApplications

Corresponds to the Tag::ALL_APPLICATIONS Keymaster authorization tag, which uses a tag ID value of 600.

Indicates whether all apps on a device can access the key pair.

applicationId

Corresponds to the Tag::APPLICATION_ID Keymaster authorization tag, which uses a tag ID value of 601.

creationDateTime

Corresponds to the Tag::CREATION_DATETIME Keymaster authorization tag, which uses a tag ID value of 701.

origin

Corresponds to the Tag::ORIGIN Keymaster authorization tag, which uses a tag ID value of 702.

rollbackResistant

Present only in key attestation versions 1 and 2.

Corresponds to the Tag::ROLLBACK_RESISTANT Keymaster authorization tag, which uses a tag ID value of 703.

rootOfTrust

Corresponds to the Tag::ROOT_OF_TRUST Keymaster authorization tag, which uses a tag ID value of 704.

For more details, see the section describing the RootOfTrust data structure.

osVersion

Corresponds to the Tag::OS_VERSION Keymaster authorization tag, which uses a tag ID value of 705.

The version of the Android operating system associated with the Keymaster, specified as a six-digit integer. For example, version 8.1.0 is represented as 080100.

Only Keymaster version 1.0 or higher includes this value in the authorization list.

osPatchLevel

Corresponds to the Tag::PATCHLEVEL Keymaster authorization tag, which uses a tag ID value of 706.

The month and year associated with the security patch that is being used within the Keymaster, specified as a six-digit integer. For example, the August 2018 patch is represented as 201808.

Only Keymaster version 1.0 or higher includes this value in the authorization list.

attestationApplicationId