add UpdateKey method to KMS interface (#7974)

This commit adds a new method `UpdateKey` to the KMS
interface.

The purpose of `UpdateKey` is to re-wrap an encrypted
data key (the key generated & encrypted with a master key by e.g.
Vault).
For example, consider Vault with a master key ID: `master-key-1`
and an encrypted data key `E(dk)` for a particular object. The
data key `dk` has been generated randomly when the object was created.
Now, the KMS operator may "rotate" the master key `master-key-1`.
However, the KMS cannot forget the "old" value of that master key
since there is still an object that requires `dk`, and therefore,
the `D(E(dk))`.
With the `UpdateKey` method call MinIO can ask the KMS to decrypt
`E(dk)` with the old key (internally) and re-encrypted `dk` with
the new master key value: `E'(dk)`.

However, this operation only works for the same master key ID.
When rotating the data key (replacing it with a new one) then
we perform a `UnsealKey` operation with the 1st master key ID
and then a `GenerateKey` operation with the 2nd master key ID.

This commit also updates the KMS documentation and removes
the `encrypt` policy entry (we don't use `encrypt`) and
add a policy entry for `rewarp`.

cmd/crypto/kms.go

@@ -86,6 +86,19 @@ type KMS interface {
 	// referenced by the keyID. The provided context must
 	// match the context used to generate the sealed key.
 	UnsealKey(keyID string, sealedKey []byte, context Context) (key [32]byte, err error)
+
+	// UpdateKey re-wraps the sealedKey if the master key, referenced by
+	// `keyID`, has changed in the meantime. This usually happens when the
+	// KMS operator performs a key-rotation operation of the master key.
+	// UpdateKey fails if the provided sealedKey cannot be decrypted using
+	// the master key referenced by keyID.
+	//
+	// UpdateKey makes no guarantees whatsoever about whether the returned
+	// rotatedKey is actually different from the sealedKey. If nothing has
+	// changed at the KMS or if the KMS does not support updating generated
+	// keys this method may behave like a NOP and just return the sealedKey
+	// itself.
+	UpdateKey(keyID string, sealedKey []byte, context Context) (rotatedKey []byte, err error)
 }
 
 type masterKeyKMS struct {
@@ -126,6 +139,13 @@ func (kms *masterKeyKMS) UnsealKey(keyID string, sealedKey []byte, ctx Context)
 	return key, nil
 }
 
+func (kms *masterKeyKMS) UpdateKey(keyID string, sealedKey []byte, ctx Context) ([]byte, error) {
+	if _, err := kms.UnsealKey(keyID, sealedKey, ctx); err != nil {
+		return nil, err
+	}
+	return sealedKey, nil // The master key cannot update data keys -> Do nothing.
+}
+
 func (kms *masterKeyKMS) deriveKey(keyID string, context Context) (key [32]byte) {
 	if context == nil {
 		context = Context{}