I'm playing with various crypto libraries to encrypt/decrypt in
AES-128-CTR. 2 of the 3 libs inter-operate fine with each other; i.e. I
can encrypt with one and decrypt with the other and vice versa.
However, when I use openssl with any of these libs, I am having
problems. It seems like up to the first 16 bytes is decrypted fine.
But after that, it's messed up. For example, if I use openssl to
encrypt the string, "12345678901234567890", the other end will only
decrypt "1234567890123456" followed by 4 bytes of garbage. And if the
other lib encrypts "12345678901234567890", openssl will only decrypt
"1234567890123456" followed by 4 bytes of garbage. However, if both
ends is openssl, everything seems fine. But because the other 2 libs
seem to work well with each other, I guess I'm not using the openssl
api's correctly? Below is the code for my AES-128-CTR
encrypter/decrypter
class AES128CTR
{
protected:
bool m_bEncrypt; // indicates if this
is used to encrypt or decrypt
AES_KEY m_key;
U8 m_iv[AES_BLOCK_SIZE];
U8 m_ecount_buf[AES_BLOCK_SIZE];
U32 m_num;
public:
AES128CTR(const U8* key, U32 len, const U8* iv, U32
ivlen, bool bEncrypt) : m_bEncrypt(bEncrypt), m_num(0)
{
assert(len >= 16 && ivlen >= 16); //
if (len < 16 || ivlen < 16) return; // key and
iv need to be 128-bits
len = ivlen = 16;
memcpy(m_iv, iv, ivlen);
memset(m_ecount_buf, 0, sizeof(m_ecount_buf));
int ret = AES_set_encrypt_key(key,
len*8/*bits*/, &m_key); // returns 0 for success
assert(!ret);
}
virtual ~AES128CTR()
{
}
bool encrypt(const U8* in, U32 inlen, U8* out, U32*
outlen)
{
if (m_bEncrypt)
{
AES_ctr128_encrypt(in, out, inlen,
&m_key, m_iv, m_ecount_buf, &m_num);
*outlen = inlen;
return true;
}
return false;
}
bool decrypt(const U8* in, U32 inlen, U8* out, U32*
outlen)
{
if (!m_bEncrypt)
{
// NOTE: calling AES_ctr128_encrypt to
decrypt because AES_ctr128_encrypt is its own inverse.
AES_ctr128_encrypt(in, out, inlen,
&m_key, m_iv, m_ecount_buf, &m_num);
*outlen = inlen;
return true;
}
return false;
}
};
....
// this is how I encrypt data
AES128CTR aesEncrypt;
const char* str = "12345678901234567890";
U8 ciphertext[1024];
U32 len = sizeof(ciphertext);
aesEncrypt.encrypt((U8*)str, strlen(str), ciphertext, &len);
...
// this is how I decrypt data
AES128CTR aesDecrypt;
U8 plaintext[1024];
U32 len = sizeof(plaintext);
aesDecrypt.decrypt(in, inlen, plaintext, &len);
