学習目的で、単純な AES-256-GCM 暗号化と復号化を実装しました。文字列の長さを 6 の倍数で入力した場合にコードをテストすると、正しい出力が得られますが、それ以外の場合は、復号化されたデータにガベージ文字が追加されます。
Case1:
Enter string: abcdef
Enter key: sdasdasdsa
-^%�
abcdef
6
Case2:
Enter string: abcdefghi
Enter key: sadsadsad
\h�,�[�
abcdefghi�\�
-1
今、私はhttps://www.openssl.org/docs/crypto/EVP_EncryptFinal_ex.htmlを読みました
EVP_DecryptFinal() will return an error code if padding is enabled and the
final block is not correctly formatted.
しかし、この場合、パディングはデフォルトで有効になっているため、問題は最終ブロックの正しいフォーマットにあると推測しています。以下にコードを添付しました。
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <openssl/evp.h>
void handleErrors()
{
printf("Some error occured\n");
}
int encrypt(unsigned char *plaintext, int plaintext_len, unsigned char *aad,
int aad_len, unsigned char *key, unsigned char *iv,
unsigned char *ciphertext, unsigned char *tag)
{
EVP_CIPHER_CTX *ctx;
int len, ciphertext_len=0;
/* Create and initialise the context */
if(!(ctx = EVP_CIPHER_CTX_new()))
handleErrors();
/* Initialise the encryption operation. */
if(1 != EVP_EncryptInit_ex(ctx, EVP_aes_256_gcm(), NULL, NULL, NULL))
handleErrors();
/* Set IV length if default 12 bytes (96 bits) is not appropriate */
if(1 != EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_IVLEN, 16, NULL))
handleErrors();
/* Initialise key and IV */
if(1 != EVP_EncryptInit_ex(ctx, NULL, NULL, key, iv)) handleErrors();
/* Provide any AAD data. This can be called zero or more times as
* required
*/
if(1 != EVP_EncryptUpdate(ctx, NULL, &len, aad, aad_len))
handleErrors();
/* Provide the message to be encrypted, and obtain the encrypted output.
* EVP_EncryptUpdate can be called multiple times if necessary
*/
if(1 != EVP_EncryptUpdate(ctx, ciphertext, &len, plaintext, plaintext_len))
handleErrors();
ciphertext_len+= len;
/* Finalise the encryption. Normally ciphertext bytes may be written at
* this stage, but this does not occur in GCM mode
*/
if(1 != EVP_EncryptFinal_ex(ctx, ciphertext + len, &len)) handleErrors();
ciphertext_len += len;
/* Get the tag */
if(1 != EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_GET_TAG, 16, tag))
handleErrors();
/* Clean up */
EVP_CIPHER_CTX_free(ctx);
return ciphertext_len;
}
int decrypt(unsigned char *ciphertext, int ciphertext_len, unsigned char *aad,
int aad_len, unsigned char *tag, unsigned char *key, unsigned char *iv,
unsigned char *plaintext)
{
EVP_CIPHER_CTX *ctx;
int len, plaintext_len=0, ret;
/* Create and initialise the context */
if(!(ctx = EVP_CIPHER_CTX_new()))
handleErrors();
/* Initialise the decryption operation. */
if(!EVP_DecryptInit_ex(ctx, EVP_aes_256_gcm(), NULL, NULL, NULL))
handleErrors();
/* Set IV length. Not necessary if this is 12 bytes (96 bits) */
if(!EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_IVLEN, 16, NULL))
handleErrors();
/* Initialise key and IV */
if(!EVP_DecryptInit_ex(ctx, NULL, NULL, key, iv)) handleErrors();
/* Provide any AAD data. This can be called zero or more times as
* required
*/
if(!EVP_DecryptUpdate(ctx, NULL, &len, aad, aad_len))
handleErrors();
/* Provide the message to be decrypted, and obtain the plaintext output.
* EVP_DecryptUpdate can be called multiple times if necessary
*/
if(!EVP_DecryptUpdate(ctx, plaintext, &len, ciphertext, ciphertext_len))
handleErrors();
plaintext_len+= len;
/* Set expected tag value. Works in OpenSSL 1.0.1d and later */
if(!EVP_CIPHER_CTX_ctrl(ctx, EVP_CTRL_GCM_SET_TAG, 16, tag))
handleErrors();
/* Finalise the decryption. A positive return value indicates success,
* anything else is a failure - the plaintext is not trustworthy.
*/
ret = EVP_DecryptFinal_ex(ctx, plaintext + len, &len);
/* Clean up */
EVP_CIPHER_CTX_free(ctx);
if(ret > 0)
{
/* Success */
plaintext_len += len;
return plaintext_len;
}
else
{
/* Verify failed */
return -1;
}
}
int main (void)
{
unsigned char str[1024],key[10],ciphertext[1024+EVP_MAX_BLOCK_LENGTH],tag[100],pt[1024];
unsigned char iv[]="1234567890abcdef";
unsigned char aad[]="1234567890123456";
int k;
printf("Enter string: ");
scanf("%s",str);
printf("Enter key: ");
scanf("%s",key);
encrypt(str, strlen(str), aad, strlen(aad), key, iv, ciphertext, tag);
printf("%s\n",ciphertext);
k = decrypt(ciphertext, strlen(ciphertext), aad, strlen(aad), tag, key, iv, pt);
printf("%s\n%d\n",pt,k);
}