Image encryption/decryption support in skopeo

Signed-off-by: Harshal Patil <harshal.patil@in.ibm.com>
Signed-off-by: Brandon Lum <lumjjb@gmail.com>

vendor/github.com/containers/ocicrypt/reader.go

@@ -0,0 +1,40 @@
+/*
+   Copyright The ocicrypt Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package ocicrypt
+
+import (
+	"io"
+)
+
+type readerAtReader struct {
+	r   io.ReaderAt
+	off int64
+}
+
+// ReaderFromReaderAt takes an io.ReaderAt and returns an io.Reader
+func ReaderFromReaderAt(r io.ReaderAt) io.Reader {
+	return &readerAtReader{
+		r:   r,
+		off: 0,
+	}
+}
+
+func (rar *readerAtReader) Read(p []byte) (n int, err error) {
+	n, err = rar.r.ReadAt(p, rar.off)
+	rar.off += int64(n)
+	return n, err
+}