96 lines
2.5 KiB
Go
96 lines
2.5 KiB
Go
package crypto
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import (
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"crypto/aes"
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"crypto/cipher"
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"crypto/rand"
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"errors"
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)
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// pad uses the PKCS#7 padding scheme to align the a payload to a specific block size
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func pad(plaintext []byte, bsize int) ([]byte, error) {
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if bsize >= 256 {
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return nil, errors.New("bsize must be < 256")
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}
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pad := bsize - (len(plaintext) % bsize)
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if pad == 0 {
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pad = bsize
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}
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for i := 0; i < pad; i++ {
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plaintext = append(plaintext, byte(pad))
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}
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return plaintext, nil
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}
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// unpad strips the padding previously added using the PKCS#7 padding scheme
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func unpad(paddedtext []byte) ([]byte, error) {
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length := len(paddedtext)
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paddedtext, lbyte := paddedtext[:length-1], paddedtext[length-1]
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pad := int(lbyte)
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if pad >= 256 || pad > length {
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return nil, errors.New("padding malformed")
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}
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return paddedtext[:length-(pad)], nil
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}
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// AESEncrypt encrypts a payloaded with an AES cipher.
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// The returned ciphertext has three notable properties:
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// 1. ciphertext is aligned to the standard AES block size
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// 2. ciphertext is padded using PKCS#7
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// 3. IV is prepended to the ciphertext
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func AESEncrypt(plaintext, key []byte) ([]byte, error) {
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plaintext, err := pad(plaintext, aes.BlockSize)
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if err != nil {
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return nil, err
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}
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block, err := aes.NewCipher(key)
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if err != nil {
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return nil, err
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}
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ciphertext := make([]byte, aes.BlockSize+len(plaintext))
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iv := ciphertext[:aes.BlockSize]
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if _, err := rand.Read(iv); err != nil {
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return nil, err
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}
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mode := cipher.NewCBCEncrypter(block, iv)
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mode.CryptBlocks(ciphertext[aes.BlockSize:], plaintext)
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return ciphertext, nil
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}
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// AESDecrypt decrypts an encrypted payload with an AES cipher.
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// The decryption algorithm makes three assumptions:
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// 1. ciphertext is aligned to the standard AES block size
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// 2. ciphertext is padded using PKCS#7
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// 3. the IV is prepended to ciphertext
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func AESDecrypt(ciphertext, key []byte) ([]byte, error) {
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if len(ciphertext) < aes.BlockSize {
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return nil, errors.New("ciphertext too short")
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}
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iv := ciphertext[:aes.BlockSize]
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ciphertext = ciphertext[aes.BlockSize:]
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if len(ciphertext)%aes.BlockSize != 0 {
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return nil, errors.New("ciphertext is not a multiple of the block size")
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}
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block, err := aes.NewCipher(key)
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if err != nil {
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return nil, err
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}
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mode := cipher.NewCBCDecrypter(block, iv)
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plaintext := make([]byte, len(ciphertext))
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mode.CryptBlocks(plaintext, ciphertext)
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if len(plaintext)%aes.BlockSize != 0 {
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return nil, errors.New("ciphertext is not a multiple of the block size")
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}
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return unpad(plaintext)
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}
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