Implement WebAuthn protocol

- Add a CBOR decoder - Add public key credential options - Add COSE key decoder - Add authenticator attestation and assertion - Add credentials - Add exclude_credentials
2026-02-04 12:39:50 +01:00
parent 768e7a3a6e
commit 4bb0a2929c
24 changed files with 2626 additions and 0 deletions
@@ -0,0 +1,141 @@
+# = Action Pack WebAuthn COSE Key
+#
+# Parses COSE (CBOR Object Signing and Encryption) public keys as specified in
+# RFC 9053[https://datatracker.ietf.org/doc/html/rfc9053]. WebAuthn authenticators
+# return public keys in COSE format, which must be converted to a standard format
+# for signature verification.
+#
+# == Usage
+#
+#   # Decode a COSE key from CBOR bytes (e.g., from authenticator data)
+#   cose_key = ActionPack::WebAuthn::CoseKey.decode(cbor_bytes)
+#
+#   # Convert to OpenSSL key for signature verification
+#   openssl_key = cose_key.to_openssl_key
+#   openssl_key.verify("SHA256", signature, signed_data)
+#
+# == Supported Algorithms
+#
+# [ES256]
+#   ECDSA with P-256 curve and SHA-256. The most common algorithm for WebAuthn.
+#
+# [RS256]
+#   RSASSA-PKCS1-v1_5 with SHA-256. Used by some security keys and platforms.
+#
+# == Attributes
+#
+# [+key_type+]
+#   The COSE key type (2 for EC2, 3 for RSA).
+#
+# [+algorithm+]
+#   The COSE algorithm identifier (-7 for ES256, -257 for RS256).
+#
+# [+parameters+]
+#   The full COSE key parameters map, including curve and coordinate data.
+class ActionPack::WebAuthn::CoseKey
+  # Raised when the key type, algorithm, or curve is not supported.
+  class UnsupportedKeyTypeError < StandardError; end
+
+  # COSE key labels
+  KEY_TYPE_LABEL = 1
+  ALGORITHM_LABEL = 3
+  EC2_CURVE_LABEL = -1
+  EC2_X_LABEL = -2
+  EC2_Y_LABEL = -3
+  RSA_N_LABEL = -1
+  RSA_E_LABEL = -2
+
+  # COSE key types
+  EC2 = 2
+  RSA = 3
+
+  # COSE algorithms
+  ES256 = -7
+  RS256 = -257
+
+  # COSE EC2 curves
+  P256 = 1
+
+  # OpenSSL types
+  UNCOMPRESSED_POINT_MARKER = 0x04
+
+  attr_reader :key_type, :algorithm, :parameters
+
+  class << self
+    # Decodes a COSE key from CBOR-encoded bytes.
+    #
+    #   cose_key = ActionPack::WebAuthn::CoseKey.decode(cbor_bytes)
+    #   cose_key.algorithm # => -7 (ES256)
+    def decode(bytes)
+      data = ActionPack::WebAuthn::CborDecoder.decode(bytes)
+      new(
+        key_type: data[KEY_TYPE_LABEL],
+        algorithm: data[ALGORITHM_LABEL],
+        parameters: data
+      )
+    end
+  end
+
+  def initialize(key_type:, algorithm:, parameters:) # :nodoc:
+    @key_type = key_type
+    @algorithm = algorithm
+    @parameters = parameters
+  end
+
+  # Converts the COSE key to an OpenSSL public key object.
+  #
+  # Returns an +OpenSSL::PKey::EC+ for EC2 keys or +OpenSSL::PKey::RSA+ for
+  # RSA keys, suitable for use with +OpenSSL::PKey#verify+.
+  #
+  # Raises +UnsupportedKeyTypeError+ if the key type, algorithm, or curve
+  # is not supported.
+  def to_openssl_key
+    case [ key_type, algorithm ]
+    when [ EC2, ES256 ] then build_ec2_es256_key
+    when [ RSA, RS256 ] then build_rsa_rs256_key
+    else raise UnsupportedKeyTypeError, "Unsupported COSE key type/algorithm: #{key_type}/#{algorithm}"
+    end
+  end
+
+  private
+    def build_ec2_es256_key
+      curve = parameters[EC2_CURVE_LABEL]
+      raise UnsupportedKeyTypeError, "Unsupported EC curve: #{curve}" unless curve == P256
+
+      x = parameters[EC2_X_LABEL]
+      y = parameters[EC2_Y_LABEL]
+
+      # Uncompressed point format: 0x04 || x || y
+      public_key_bytes = [ UNCOMPRESSED_POINT_MARKER, *x.bytes, *y.bytes ].pack("C*")
+
+      asn1 = OpenSSL::ASN1::Sequence([
+        OpenSSL::ASN1::Sequence([
+          OpenSSL::ASN1::ObjectId("id-ecPublicKey"),
+          OpenSSL::ASN1::ObjectId("prime256v1")
+        ]),
+        OpenSSL::ASN1::BitString(public_key_bytes)
+      ])
+
+      OpenSSL::PKey::EC.new(asn1.to_der)
+    end
+
+    def build_rsa_rs256_key
+      n = OpenSSL::BN.new(parameters[RSA_N_LABEL], 2)
+      e = OpenSSL::BN.new(parameters[RSA_E_LABEL], 2)
+
+      asn1 = OpenSSL::ASN1::Sequence([
+        OpenSSL::ASN1::Sequence([
+          OpenSSL::ASN1::ObjectId("rsaEncryption"),
+          OpenSSL::ASN1::Null.new(nil)
+        ]),
+        OpenSSL::ASN1::BitString(
+          OpenSSL::ASN1::Sequence([
+            OpenSSL::ASN1::Integer(n),
+            OpenSSL::ASN1::Integer(e)
+          ]).to_der
+        )
+      ])
+
+      OpenSSL::PKey::RSA.new(asn1.to_der)
+    end
+end