internal override bool Decrypt(int recordType, int version,
byte[] data, ref int off, ref int len)
{
int blen = bc.BlockSize;
int hlen = hm.MACSize;
/*
* Grab a copy of the last encrypted block; this is
* the "saved IV" for the next record.
*/
Array.Copy(data, off + len - blen, ivTmp, 0, blen);
/*
* Decrypt the data. The length has already been
* checked. If there is an explicit IV, it gets
* "decrypted" as well, which is not a problem.
*/
bc.CBCDecrypt(iv, data, off, len);
Array.Copy(ivTmp, 0, iv, 0, blen);
if (explicitIV)
{
off += blen;
len -= blen;
}
/*
* Compute minimum and maximum length of plaintext + MAC.
* These can be inferred from the observable record length,
* and thus are not secret.
*/
int minLen = (hlen + 256 < len) ? len - 256 : hlen;
int maxLen = len - 1;
/*
* Get the actual padding length and check padding. The
* padding length must match the minLen/maxLen range.
*/
int padLen = data[off + len - 1];
int good = ~(((maxLen - minLen) - padLen) >> 31);
int lenWithMAC = minLen ^ (good & (minLen ^ (maxLen - padLen)));
int dbb = 0;
for (int i = minLen; i < maxLen; i++)
{
dbb |= ~((i - lenWithMAC) >> 31)
& (data[off + i] ^ padLen);
}
good &= ~((dbb | -dbb) >> 31);
/*
* Extract the MAC value; this is done in one pass, but
* results in a "rotate" MAC value. The rotation count
* is kept in 'rotCount': this is the offset of the
* first MAC value byte in tmp1[].
*/
int lenNoMAC = lenWithMAC - hlen;
minLen -= hlen;
int rotCount = 0;
for (int i = 0; i < hlen; i++)
{
tmp1[i] = 0;
}
int v = 0;
for (int i = minLen; i < maxLen; i++)
{
int m = ~((i - lenNoMAC) >> 31)
& ((i - lenWithMAC) >> 31);
tmp1[v] |= (byte)(m & data[off + i]);
m = i - lenNoMAC;
rotCount |= ~((m | -m) >> 31) & v;
if (++v == hlen)
{
v = 0;
}
}
maxLen -= hlen;
/*
* Rotate back the MAC value. We do it bit by bit, with
* 6 iterations; this is good for all MAC value up to
* and including 64 bytes.
*/
for (int i = 5; i >= 0; i--)
{
int rc = 1 << i;
if (rc >= hlen)
{
continue;
}
int ctl = -((rotCount >> i) & 1);
for (int j = 0, k = rc; j < hlen; j++)
{
int b1 = tmp1[j];
int b2 = tmp1[k];
tmp2[j] = (byte)(b1 ^ (ctl & (b1 ^ b2)));
if (++k == hlen)
{
k = 0;
}
}
Array.Copy(tmp2, 0, tmp1, 0, hlen);
rotCount &= ~rc;
}
/*
* Recompute the HMAC value. At that point, minLen and
* maxLen have been adjusted to match the plaintext
* without the MAC.
*/
IO.Enc64be(seq++, tmp2, 0);
IO.WriteHeader(recordType, version, lenNoMAC, tmp2, 8);
hm.Update(tmp2, 0, 13);
hm.ComputeCT(data, off, lenNoMAC, minLen, maxLen, tmp2, 0);
/*
* Compare MAC values.
*/
dbb = 0;
for (int i = 0; i < hlen; i++)
{
dbb |= tmp1[i] ^ tmp2[i];
}
good &= ~((dbb | -dbb) >> 31);
/*
* We must also check that the plaintext length fits in
* the maximum allowed by the standard (previous check
* was on the encrypted length).
*/
good &= (lenNoMAC - 16385) >> 31;
len = lenNoMAC;
return(good != 0);
}
