Revert "Encode private key using COSE instead."

This reverts commit cf614d4.

index.bs

@@ -5708,13 +5708,8 @@ The <dfn>Credential Parameters</dfn> is a JSON [=Object=] passed to the [=remote
             <tr>
                 <td>|privateKey|</td>
                 <td>
-                    The [=public key credential source/privateKey|private key=] encoded in COSE_Key format,
-                    as defined in [=Section 7=] of [[!RFC8152]], using the [=CTAP2 canonical CBOR encoding form=].
-                    The COSE_Key-encoded [=public key credential source/privateKey|private key=] MUST contain the "alg"
-                    parameter and MUST NOT contain any other OPTIONAL parameters. The "alg" parameter MUST contain a
-                    {{COSEAlgorithmIdentifier}} value. The encoded [=credential public key=] MUST also contain any
-                    additional REQUIRED parameters stipulated by the relevant key type specification, i.e., REQUIRED for
-                    the key type "kty" and algorithm "alg" (see Section 8 of [[!RFC8152]]).
+                    An asymmetric key package containing a single [=public key credential source/privateKey|private key=]
+                    per [[RFC5958]], encoded using [=Base64url Encoding=].
                 </td>
                 <td>string</td>
             </tr>
@@ -5747,8 +5742,8 @@ The [=remote end steps=] are:
  1. If |rpId| is not a valid [=RP ID=], return a [=WebDriver error=] with [=WebDriver error code=] [=invalid argument=].
  1. Let |privateKey| be the result of decoding [=Base64url Encoding=] on the |parameters|' |privateKey| property.
  1. If |privateKey| is failure, return a [=WebDriver error=] with [=WebDriver error code=] [=invalid argument=].
- 1. If |privateKey| is not a validly-encoded COSE_Key private key per [[RFC8152]], return a [=WebDriver error=] with
-     [=WebDriver error code=] [=invalid argument=].
+ 1. If |privateKey| is not a validly-encoded asymmetric key package containing a single private key per [[RFC5958]],
+     return a [=WebDriver error=] with [=WebDriver error code=] [=invalid argument=].
  1. If the |parameters|' |userHandle| property is defined:
      1. Let |userHandle| be the result of decoding [=Base64url Encoding=] on the |parameters|' |userHandle| property.
      1. If |userHandle| is failure, return a [=WebDriver error=] with [=WebDriver error code=] [=invalid argument=].