code review 3069041: crypto/cipher: add package

src/pkg/crypto/cipher/cbc.go

 // Copyright 2009 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.
 
 // Cipher block chaining (CBC) mode.
 
 // CBC provides confidentiality by xoring (chaining) each plaintext block
 // with the previous ciphertext block before applying the block cipher.
 
 // See NIST SP 800-38A, pp 10-11
 
 package cipher
 
 type cbc struct {
         b         Block
         blockSize int
         iv        []byte
         tmp       []byte
 }
 
 func newCBC(b Block, iv []byte) *cbc {
         return &cbc{
                 b:         b,
                 blockSize: b.BlockSize(),
                 iv:        dup(iv),
                 tmp:       make([]byte, b.BlockSize()),
         }
 }
 
-type CBCEncrypter cbc
+type cbcEncrypter cbc
 
-func NewCBCEncrypter(b Block, iv []byte) *CBCEncrypter {
-        return (*CBCEncrypter)(newCBC(b, iv))
+// NewCBCEncrypter returns a BlockMode which encrypts in cipher block chaining
+// mode, using the given Block. The length of iv must be the same as the
+// Block's block size.
+func NewCBCEncrypter(b Block, iv []byte) BlockMode {
+        return (*cbcEncrypter)(newCBC(b, iv))
 }
 
-func (x *CBCEncrypter) BlockSize() int { return x.blockSize }
+func (x *cbcEncrypter) BlockSize() int { return x.blockSize }
 
-func (x *CBCEncrypter) CryptBlocks(dst, src []byte) {
+func (x *cbcEncrypter) CryptBlocks(dst, src []byte) {
         for len(src) > 0 {
                 for i := 0; i < x.blockSize; i++ {
                         x.iv[i] ^= src[i]
                 }
                 x.b.Encrypt(x.iv, x.iv)
                 for i := 0; i < x.blockSize; i++ {
                         dst[i] = x.iv[i]
                 }
                 src = src[x.blockSize:]
                 dst = dst[x.blockSize:]
         }
 }
 
-type CBCDecrypter cbc
+type cbcDecrypter cbc
 
-func NewCBCDecrypter(b Block, iv []byte) *CBCDecrypter {
-        return (*CBCDecrypter)(newCBC(b, iv))
+// NewCBCDecrypter returns a BlockMode which decrypts in cipher block chaining
+// mode, using the given Block. The length of iv must be the same as the
+// Block's block size as must match the iv used to encrypt the data.
+func NewCBCDecrypter(b Block, iv []byte) *cbcDecrypter {
+        return (*cbcDecrypter)(newCBC(b, iv))
 }
 
-func (x *CBCDecrypter) BlockSize() int { return x.blockSize }
+func (x *cbcDecrypter) BlockSize() int { return x.blockSize }
 
-func (x *CBCDecrypter) CryptBlocks(dst, src []byte) {
+func (x *cbcDecrypter) CryptBlocks(dst, src []byte) {
         for len(src) > 0 {
                 x.b.Decrypt(x.tmp, src[:x.blockSize])
                 for i := 0; i < x.blockSize; i++ {
                         x.tmp[i] ^= x.iv[i]
                         x.iv[i] = src[i]
                         dst[i] = x.tmp[i]
                 }
 
                 src = src[x.blockSize:]
                 dst = dst[x.blockSize:]
         }
 }