/* The input and output encrypted as though 128bit cfb mode is being
* used. The extra state information to record how much of the
* 128bit block we have used is contained in *num;
*/
void CRYPTO_cfb128_encrypt(const unsigned char *in, unsigned char *out,
size_t len, const void *key,
unsigned char ivec[16], int *num,
int enc, block128_f block)
{
unsigned int n;
size_t l = 0;
assert(in && out && key && ivec && num);
n = *num;
if (enc) 加密时
{
if (16%sizeof(size_t) == 0) /* always true actually */
{
do
{
while (n && len)
{
*(out++) = ivec[n] ^= *(in++);
--len;
n = (n+1) % 16;
}
上述代码段中n最多在n,(n+1)%16,...,15之间，len最多在len,len-1,...,len-14之间变动，具体变动范围还要看16-n与len的关系。
16-n>len时 依次执行的是ivec[n...n+(len-1)]和明文in[0...(len-1)]的异或，之后len的值变为0了，即明文已用完。
假设len为6， n=3的话是
ivec [0 1 2 3 4 5 6 7 8 9 0 a b c d e f]
in [0 1 2 3 4 5]
的异或得到out，此时得到的out后后面没机会执行了，没加密意义。
16-n<=len时依次执行的是ivec[n...n+(len-1)]和明文in[0...(15-n)]的异或，之后n的值变为0了。
假设len为>=13， n=3的话是
ivec [0 1 2 3 4 5 6 7 8 9 a b c d e f]
in [0 1 2 3 4 5 6 7 8 9 a b c]
的异或得到out及新的ivec，如
out[0]=ivec[3]^in[0];out[c]=ivec[f]^in[c],out[d]=0;
ivec[0]=ivec[0],ivec[5]=ivec[5]^in[2];
如上生成的out均算是成品了，
if (((size_t)in|(size_t)out|(size_t)ivec)%sizeof(size_t) != 0)
break;
while (len>=16)
{
对于剩余的明文长度大于等于16时进行迭代，即用密钥key加密上面或上一次的本处循环生成的生成ivec，ivec的密文继续作为ivec，并与明文分组进行异或。如此循环知道len<16了
(*block)(ivec, ivec, key);
for (n=0; n<16; n+=sizeof(size_t))
{
*(size_t*)(out+n) =
*(size_t*)(ivec+n) ^= *(size_t*)(in+n);
}
len -= 16;
out += 16;
in += 16;
}
n = 0;
if (len)
{这里意味着len大于0小于16了，那么生成新的一组流ivec，并将前len长度字节与剩余的len长度明文进行异或，得到最后不足一分组的密文输出。
(*block)(ivec, ivec, key);
while (len--)
{
out[n] = ivec[n] ^= in[n];
++n;
}
}
*num = n; 设置num值，这里n为最后一个ivec那个使用的字节数。
设明文长度为len，入参*num值为num_in可以推断出，出参*num值为(len+(num_in-1))%16
return;
} while (0);
}
/* the rest would be commonly eliminated by x86* compiler */
while (l<len)
{这里根本就不会执行
if (n == 0)
{
(*block)(ivec, ivec, key);
}
out[l] = ivec[n] ^= in[l];
++l;
n = (n+1) % 16;
}
*num = n;
}
else
{
//解密 原理同上，只是一个逆过程，不再赘述。
if (16%sizeof(size_t) == 0) do
{ /* always true actually */
while (n && len)
{
unsigned char c;
*(out++) = ivec[n] ^ (c = *(in++)); ivec[n] = c;
--len;
n = (n+1) % 16;
}
if (((size_t)in|(size_t)out|(size_t)ivec)%sizeof(size_t) != 0)
break;
{
(*block)(ivec, ivec, key);
for (n=0; n<16; n+=sizeof(size_t))
{
size_t t = *(size_t*)(in+n);
*(size_t*)(out+n) = *(size_t*)(ivec+n) ^ t;
*(size_t*)(ivec+n) = t;
}
len -= 16;
out += 16;
in += 16;
}
n = 0;
if (len)
{
(*block)(ivec, ivec, key);
while (len--)
{
unsigned char c;
out[n] = ivec[n] ^ (c = in[n]); ivec[n] = c;
++n;
}
}
*num = n;
return;
} while (0);
/* the rest would be commonly eliminated by x86* compiler */
{
unsigned char c;
if (n == 0)
{
(*block)(ivec, ivec, key);
}
out[l] = ivec[n] ^ (c = in[l]); ivec[n] = c;
++l;
n = (n+1) % 16;
}
*num=n;
}
}
/* This expects a single block of size nbits for both in and out. Note that
it corrupts any extra bits in the last byte of out */
static void cfbr_encrypt_block(const unsigned char *in,unsigned char *out,
int nbits,const void *key,
unsigned char ivec[16],int enc,
block128_f block)
{
int n,rem,num;
unsigned char ovec[16*2 + 1]; /* +1 because we dererefence (but don't use) one byte off the end */
if (nbits<=0 || nbits>128) return;
/* fill in the first half of the new IV with the current IV */
memcpy(ovec,ivec,16);
//向量变化:ovec=ivec[0...15]|NULL
/* construct the new IV */
(*block)(ivec,ivec,key); //Ek(ivec)
num = (nbits+7)/8; // nbits位对应的字节数
if (enc) /* encrypt the input */
{
// 对应加密过程
for(n=0 ; n < num ; ++n)
out[n] = (ovec[16+n] = in[n] ^ ivec[n]);
// 设1<=nbits<=8 则num为1。则此处仅
// out[0]=ovec[16]=in[0]^ivec[0]=in[0]^(Ek(ivec))[0];
//向量变化:ovec=ivec[0...15]|in[0]^(Ek(ivec))[0];
}
else /* decrypt the input */
{
// 对应解密过程
for(n=0 ; n < num ; ++n)
out[n] = (ovec[16+n] = in[n]) ^ ivec[n];
// 与上完全一样，不过in代表密文，out代表输出的明文了
//向量变化:ovec=ivec[0...15]|in[0]^(Ek(ivec))[0];
}
/* shift ovec left... */
rem = nbits%8; //ovec要左移的位数
num = nbits/8;
if(rem==0)
{
// 可见 1<=num<=16
memcpy(ivec,ovec+num,16);
//如果在nbits为8的整倍数的时候，如8时用ivec[1...15]|(Ek(ivec)[0]^in[0]) 作为下一轮的ivec
//如nbits为16时用ivec[2...15]|(Ek(ivec)[0]^in[0])|(Ek(ivec)[1]^in[1]) 作为下一轮的ivec
//向量变化:ivec=ivec[num...15]|in[0...(num-1)]^(Ek(ivec))[0...(num-1)];
}
else
{
// 可见 0<=num<=15
for(n=0 ; n < 16 ; ++n)
{
ivec[n] = ovec[n+num]<<rem | ovec[n+num+1]>>(8-rem);
}
//如果nbits不为8的倍数，则需要将ivec[num]字节的右边低8-rem位直到ivec[16+num]的左边高rem位共计128位作为下一轮的ivec
}
/* it is not necessary to cleanse ovec, since the IV is not secret */
}
/* N.B. This expects the input to be packed, MS bit first */
void CRYPTO_cfb128_1_encrypt(const unsigned char *in, unsigned char *out,
size_t bits, const void *key,
unsigned char ivec[16], int *num,
int enc, block128_f block)
{
size_t n;
unsigned char c[1],d[1];
assert(in && out && key && ivec && num);
assert(*num == 0);
for(n=0 ; n<bits ; ++n)
{
c[0]=(in[n/8]&(1 << (7-n%8))) ? 0x80 : 0;
// in的第n位为1 则 c[0]=0x80 否则 c[0]=0;即最高位为in的第n位的值，其他位为0.
cfbr_encrypt_block(c,d,1,key,ivec,enc,block);
out[n/8]=(out[n/8]&~(1 << (unsigned int)(7-n%8))) |
((d[0]&0x80) >> (unsigned int)(n%8));
//设置out结果的第n位为d[0]最高位的值。
}
}
void CRYPTO_cfb128_8_encrypt(const unsigned char *in, unsigned char *out,
size_t length, const void *key,
unsigned char ivec[16], int *num,
int enc, block128_f block)
{
size_t n;
assert(in && out && key && ivec && num);
assert(*num == 0);
for(n=0 ; n<length ; ++n)
cfbr_encrypt_block(&in[n],&out[n],8,key,ivec,enc,block);
//对于每次偏移位数为8时，为整字节运算，本函数的第3参数为字节数。
}
