discourse/lib/discourse_webauthn/registration_service.rb

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# frozen_string_literal: true
require "cbor"
require "cose"
module DiscourseWebauthn
class RegistrationService < BaseValidationService
##
# See https://w3c.github.io/webauthn/#sctn-registering-a-new-credential for
# the registration steps followed here. Memoized methods are called in their
# place in the step flow to make the process clearer.
def register_security_key
# 4. Verify that the value of C.type is webauthn.create.
validate_webauthn_type(::DiscourseWebauthn::ACCEPTABLE_REGISTRATION_TYPE)
# 5. Verify that the value of C.challenge equals the base64url encoding of options.challenge.
validate_challenge
# 6. Verify that the value of C.origin matches the Relying Party's origin.
validate_origin
# 7. Verify that the value of C.tokenBinding.status matches the state of Token Binding for the TLS
# connection over which the assertion was obtained. If Token Binding was used on that TLS connection,
# also verify that C.tokenBinding.id matches the base64url encoding of the Token Binding ID for the connection.
# Not using this right now.
# 8. Let hash be the result of computing a hash over response.clientDataJSON using SHA-256.
client_data_hash
# 9. Perform CBOR decoding on the attestationObject field of the AuthenticatorAttestationResponse
# structure to obtain the attestation statement format fmt, the authenticator data authData,
# and the attestation statement attStmt.
attestation
# 10. Verify that the rpIdHash in authData is the SHA-256 hash of the RP ID expected by the Relying Party.
# check the SHA256 hash of the rpId is the same as the authData bytes 0..31
validate_rp_id_hash
# 11. Verify that the User Present bit of the flags in authData is set.
# https://blog.bigbinary.com/2011/07/20/ruby-pack-unpack.html
#
validate_user_presence
#
# 12. If user verification is required for this registration, verify that
# the User Verified bit of the flags in authData is set.
validate_user_verification if @factor_type == UserSecurityKey.factor_types[:first_factor]
# 13. Verify that the "alg" parameter in the credential public key in authData matches the alg
# attribute of one of the items in options.pubKeyCredParams.
# https://w3c.github.io/webauthn/#table-attestedCredentialData
# See https://www.iana.org/assignments/cose/cose.xhtml#algorithms for supported algorithm
# codes.
credential_public_key, credential_public_key_bytes, credential_id =
extract_public_key_and_credential_from_attestation(auth_data)
if ::DiscourseWebauthn::SUPPORTED_ALGORITHMS.exclude?(credential_public_key.alg)
raise(
UnsupportedPublicKeyAlgorithmError,
I18n.t("webauthn.validation.unsupported_public_key_algorithm_error"),
)
end
# 14. Verify that the values of the client extension outputs in clientExtensionResults and the authenticator
# extension outputs in the extensions in authData are as expected, considering the client extension input
# values that were given in options.extensions. In particular, any extension identifier values in the
# clientExtensionResults and the extensions in authData MUST also be present as extension identifier values
# in options.extensions, i.e., no extensions are present that were not requested. In the general case, the
# meaning of "are as expected" is specific to the Relying Party and which extensions are in use.
# Not using this right now.
# 15. Determine the attestation statement format by performing a USASCII case-sensitive match on fmt against the
# set of supported WebAuthn Attestation Statement Format Identifier values. An up-to-date list of registered
# WebAuthn Attestation Statement Format Identifier values is maintained in the IANA registry of the same
# name [WebAuthn-Registries].
# 16. Verify that attStmt is a correct attestation statement, conveying a valid attestation signature,
# by using the attestation statement format fmts verification procedure given attStmt, authData and hash.
if ::DiscourseWebauthn::VALID_ATTESTATION_FORMATS.exclude?(attestation["fmt"]) ||
attestation["fmt"] != "none"
raise(
UnsupportedAttestationFormatError,
I18n.t("webauthn.validation.unsupported_attestation_format_error"),
)
end
#==================================================
# ONLY APPLIES IF fmt !== none, this is all to do with
# verifying attestation. May want to come back to this at
# some point for additional security.
#==================================================
#
# 17. If validation is successful, obtain a list of acceptable trust anchors (attestation root certificates or
# ECDAA-Issuer public keys) for that attestation type and attestation statement format fmt, from a trusted
# source or from policy. For example, the FIDO Metadata Service [FIDOMetadataService] provides one way
# to obtain such information, using the aaguid in the attestedCredentialData in authData.
#
# 18. Assess the attestation trustworthiness using the outputs of the verification procedure in step 16, as follows:
# If no attestation was provided, verify that None attestation is acceptable under Relying Party policy.
#==================================================
# 19. Check that the credentialId is not yet registered to any other user. If registration
# is requested for a credential that is already registered to a different user,
# the Relying Party SHOULD fail this registration ceremony, or it MAY decide to accept
# the registration, e.g. while deleting the older registration.
encoded_credential_id = Base64.strict_encode64(credential_id)
encoded_public_key = Base64.strict_encode64(credential_public_key_bytes)
if UserSecurityKey.exists?(credential_id: encoded_credential_id)
raise(CredentialIdInUseError, I18n.t("webauthn.validation.credential_id_in_use_error"))
end
# 20. If the attestation statement attStmt verified successfully and is found to be trustworthy,
# then register the new credential with the account that was denoted in options.user, by
# associating it with the credentialId and credentialPublicKey in the attestedCredentialData
# in authData, as appropriate for the Relying Party's system.
UserSecurityKey.create!(
user: @current_user,
credential_id: encoded_credential_id,
public_key: encoded_public_key,
name: @params[:name],
factor_type: @factor_type,
)
rescue CBOR::UnpackError, CBOR::TypeError, CBOR::MalformedFormatError, CBOR::StackError
raise MalformedAttestationError, I18n.t("webauthn.validation.malformed_attestation_error")
end
private
def attestation
@attestation ||= CBOR.decode(Base64.decode64(@params[:attestation]))
end
def auth_data
@auth_data ||= attestation["authData"]
end
def extract_public_key_and_credential_from_attestation(auth_data)
# see https://w3c.github.io/webauthn/#authenticator-data for lengths
# of authdata for extraction
rp_id_length = 32
flags_length = 1
sign_count_length = 4
attested_credential_data_start_position = rp_id_length + flags_length + sign_count_length # 37
attested_credential_data_length = auth_data.size - attested_credential_data_start_position
attested_credential_data =
auth_data[
attested_credential_data_start_position..(
attested_credential_data_start_position + attested_credential_data_length - 1
)
]
# see https://w3c.github.io/webauthn/#attested-credential-data for lengths
# of data for extraction
aa_guid = attested_credential_data[0..15]
credential_id_length = attested_credential_data[16..17].unpack("n*")[0]
credential_id = attested_credential_data[18..(18 + credential_id_length - 1)]
public_key_start_position = 18 + credential_id_length
public_key_bytes =
attested_credential_data[
public_key_start_position..(public_key_start_position + attested_credential_data.size - 1)
]
public_key = COSE::Key.deserialize(public_key_bytes)
[public_key, public_key_bytes, credential_id]
end
end
end