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// Package cipher is a package for common encrypt/decrypt of symmetric key messages.
package cipher
import (
"bytes"
"crypto/aes"
"crypto/cipher"
"errors"
)
// Decrypt will use the given key, iv, and ciphertext and return
// the plaintext bytes.
func Decrypt(iv, key, ciphertext []byte) ([]byte, error) {
block, err := aes.NewCipher(key)
if err != nil {
return nil, err
}
if len(ciphertext) < aes.BlockSize {
return nil, errors.New("ciphertext too short")
}
cbc := cipher.NewCBCDecrypter(block, iv)
cbc.CryptBlocks(ciphertext, ciphertext)
unpaddedText, err := pkcs7Unpad(ciphertext, aes.BlockSize)
if err != nil {
return nil, err
}
return unpaddedText, nil
}
// Encrypt will use the given iv, key, and plaintext bytes
// and return ciphertext bytes.
func Encrypt(iv, key, plaintext []byte) ([]byte, error) {
block, err := aes.NewCipher(key)
if err != nil {
return nil, err
}
paddedText, err := pkcs7Pad(plaintext, block.BlockSize())
if err != nil {
return nil, err
}
ciphertext := make([]byte, len(paddedText))
mode := cipher.NewCBCEncrypter(block, iv)
mode.CryptBlocks(ciphertext, paddedText)
return ciphertext, nil
}
// PKCS7 padding.
// PKCS7 errors.
var (
// ErrInvalidBlockSize indicates hash blocksize <= 0.
ErrInvalidBlockSize = errors.New("invalid blocksize")
// ErrInvalidPKCS7Data indicates bad input to PKCS7 pad or unpad.
ErrInvalidPKCS7Data = errors.New("invalid PKCS7 data (empty or not padded)")
// ErrInvalidPKCS7Padding indicates PKCS7 unpad fails to bad input.
ErrInvalidPKCS7Padding = errors.New("invalid padding on input")
)
// pkcs7Pad right-pads the given byte slice with 1 to n bytes, where
// n is the block size. The size of the result is x times n, where x
// is at least 1.
func pkcs7Pad(b []byte, blocksize int) ([]byte, error) {
if blocksize <= 0 {
return nil, ErrInvalidBlockSize
}
if b == nil || len(b) == 0 {
return nil, ErrInvalidPKCS7Data
}
n := blocksize - (len(b) % blocksize)
pb := make([]byte, len(b)+n)
copy(pb, b)
copy(pb[len(b):], bytes.Repeat([]byte{byte(n)}, n))
return pb, nil
}
// pkcs7Unpad validates and unpads data from the given bytes slice.
// The returned value will be 1 to n bytes smaller depending on the
// amount of padding, where n is the block size.
func pkcs7Unpad(b []byte, blocksize int) ([]byte, error) {
if blocksize <= 0 {
return nil, ErrInvalidBlockSize
}
if b == nil || len(b) == 0 {
return nil, ErrInvalidPKCS7Data
}
if len(b)%blocksize != 0 {
return nil, ErrInvalidPKCS7Padding
}
c := b[len(b)-1]
n := int(c)
if n == 0 || n > len(b) {
return nil, ErrInvalidPKCS7Padding
}
for i := 0; i < n; i++ {
if b[len(b)-n+i] != c {
return nil, ErrInvalidPKCS7Padding
}
}
return b[:len(b)-n], nil
}
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