// jwsHasher indicates suitable JWS algorithm name and a hash function
// to use for signing a digest with the provided key.
// It returns ("", 0) if the key is not supported.
func jwsHasher(key crypto.Signer) (string, crypto.Hash) {
switch key := key.(type) {
case *rsa.PrivateKey:
return "RS256", crypto.SHA256
case *ecdsa.PrivateKey:
switch key.Params().Name {
case "P-256":
return "ES256", crypto.SHA256
case "P-384":
return "ES384", crypto.SHA384
case "P-512":
return "ES512", crypto.SHA512
}
}
return "", 0
}
// jwsSign signs the digest using the given key.
// It returns ErrUnsupportedKey if the key type is unknown.
// The hash is used only for RSA keys.
func jwsSign(key crypto.Signer, hash crypto.Hash, digest []byte) ([]byte, error) {
switch key := key.(type) {
case *rsa.PrivateKey:
return key.Sign(rand.Reader, digest, hash)
case *ecdsa.PrivateKey:
r, s, err := ecdsa.Sign(rand.Reader, key, digest)
if err != nil {
return nil, err
}
rb, sb := r.Bytes(), s.Bytes()
size := key.Params().BitSize / 8
if size%8 > 0 {
size++
}
sig := make([]byte, size*2)
copy(sig[size-len(rb):], rb)
copy(sig[size*2-len(sb):], sb)
return sig, nil
}
return nil, ErrUnsupportedKey
}