Add COSE_Key support

Add support for COSE_Key structure, and associated enums for key type,
ECC curve, and key ops, as defined in RFC8152.

Signed-off-by: Sergei Trofimov <sergei.trofimov@arm.com>

errors.go

@@ -14,4 +14,8 @@ var (
 	ErrUnavailableHashFunc   = errors.New("hash function is not available")
 	ErrVerification          = errors.New("verification error")
 	ErrInvalidPubKey         = errors.New("invalid public key")
+	ErrInvalidPrivKey        = errors.New("invalid private key")
+	ErrNotPrivKey            = errors.New("not a private key")
+	ErrSignOpNotSupported    = errors.New("sign key_op not supported by key")
+	ErrVerifyOpNotSupported  = errors.New("verify key_op not supported by key")
 )

go.mod

@@ -10,5 +10,6 @@ require (
 	github.com/stretchr/objx v0.5.0 // indirect
 	github.com/stretchr/testify v1.8.3 // indirect
 	github.com/x448/float16 v0.8.4 // indirect
+	golang.org/x/exp v0.0.0-20230522175609-2e198f4a06a1 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )

go.sum

@@ -15,6 +15,8 @@ github.com/stretchr/testify v1.8.3 h1:RP3t2pwF7cMEbC1dqtB6poj3niw/9gnV4Cjg5oW5gt
 github.com/stretchr/testify v1.8.3/go.mod h1:sz/lmYIOXD/1dqDmKjjqLyZ2RngseejIcXlSw2iwfAo=
 github.com/x448/float16 v0.8.4 h1:qLwI1I70+NjRFUR3zs1JPUCgaCXSh3SW62uAKT1mSBM=
 github.com/x448/float16 v0.8.4/go.mod h1:14CWIYCyZA/cWjXOioeEpHeN/83MdbZDRQHoFcYsOfg=
+golang.org/x/exp v0.0.0-20230522175609-2e198f4a06a1 h1:k/i9J1pBpvlfR+9QsetwPyERsqu1GIbi967PQMq3Ivc=
+golang.org/x/exp v0.0.0-20230522175609-2e198f4a06a1/go.mod h1:V1LtkGg67GoY2N1AnLN78QLrzxkLyJw7RJb1gzOOz9w=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=