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diff --git a/src/crypto/cipher/cipher.go b/src/crypto/cipher/cipher.go
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+++ b/src/crypto/cipher/cipher.go
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+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package cipher implements standard block cipher modes that can be wrapped
+// around low-level block cipher implementations.
+// See https://csrc.nist.gov/groups/ST/toolkit/BCM/current_modes.html
+// and NIST Special Publication 800-38A.
+package cipher
+
+// A Block represents an implementation of block cipher
+// using a given key. It provides the capability to encrypt
+// or decrypt individual blocks. The mode implementations
+// extend that capability to streams of blocks.
+type Block interface {
+	// BlockSize returns the cipher's block size.
+	BlockSize() int
+
+	// Encrypt encrypts the first block in src into dst.
+	// Dst and src must overlap entirely or not at all.
+	Encrypt(dst, src []byte)
+
+	// Decrypt decrypts the first block in src into dst.
+	// Dst and src must overlap entirely or not at all.
+	Decrypt(dst, src []byte)
+}
+
+// A Stream represents a stream cipher.
+type Stream interface {
+	// XORKeyStream XORs each byte in the given slice with a byte from the
+	// cipher's key stream. Dst and src must overlap entirely or not at all.
+	//
+	// If len(dst) < len(src), XORKeyStream should panic. It is acceptable
+	// to pass a dst bigger than src, and in that case, XORKeyStream will
+	// only update dst[:len(src)] and will not touch the rest of dst.
+	//
+	// Multiple calls to XORKeyStream behave as if the concatenation of
+	// the src buffers was passed in a single run. That is, Stream
+	// maintains state and does not reset at each XORKeyStream call.
+	XORKeyStream(dst, src []byte)
+}
+
+// A BlockMode represents a block cipher running in a block-based mode (CBC,
+// ECB etc).
+type BlockMode interface {
+	// BlockSize returns the mode's block size.
+	BlockSize() int
+
+	// CryptBlocks encrypts or decrypts a number of blocks. The length of
+	// src must be a multiple of the block size. Dst and src must overlap
+	// entirely or not at all.
+	//
+	// If len(dst) < len(src), CryptBlocks should panic. It is acceptable
+	// to pass a dst bigger than src, and in that case, CryptBlocks will
+	// only update dst[:len(src)] and will not touch the rest of dst.
+	//
+	// Multiple calls to CryptBlocks behave as if the concatenation of
+	// the src buffers was passed in a single run. That is, BlockMode
+	// maintains state and does not reset at each CryptBlocks call.
+	CryptBlocks(dst, src []byte)
+}