If encryption buffer size is not a multiple of the block size (in AES
that's always 16 bytes) then the ENCRYPTION ONLY will write data outside
of the caller's buffer when it encrypts the last block.
See code below for problem and fix.
==================================
void AES_cbc_encrypt(const unsigned char *in, unsigned char *out,
const unsigned long length, const AES_KEY *key,
unsigned char *ivec, const int enc) {
unsigned long n;
unsigned long len = length;
unsigned char tmp[AES_BLOCK_SIZE];
const unsigned char *iv = ivec;
assert(in && out && key && ivec);
assert((AES_ENCRYPT == enc)||(AES_DECRYPT == enc));
if (AES_ENCRYPT == enc) {
while (len >= AES_BLOCK_SIZE) {
for(n=0; n < AES_BLOCK_SIZE; ++n)
out[n] = in[n] ^ iv[n];
AES_encrypt(out, out, key);
iv = out;
len -= AES_BLOCK_SIZE;
in += AES_BLOCK_SIZE;
out += AES_BLOCK_SIZE;
}
#if 0 /* BUFFER OVERRUN - REMOVE CODE */
if (len) {
for(n=0; n < len; ++n)
out[n] = in[n] ^ iv[n];
for(n=len; n < AES_BLOCK_SIZE; ++n)
out[n] = iv[n];
AES_encrypt(out, out, key);
iv = out;
}
#else /* BUFFER OVERRUN - FIX FOLLOWS */
if (len)
{
memcpy(tmp, in, len);
memcpy(tmp + len, iv + len, AES_BLOCK_SIZE - len);
for (n = 0; n < len; n++)
tmp[n] = in[n] ^ iv[n];
AES_encrypt(tmp, tmp, key);
memcpy(out, iv = tmp, len);
}
#endif /* BUFFER OVERRUN - END OF FIX */
memcpy(ivec,iv,AES_BLOCK_SIZE);
} else if (in != out) {
while (len >= AES_BLOCK_SIZE) {
AES_decrypt(in, out, key);
for(n=0; n < AES_BLOCK_SIZE; ++n)
out[n] ^= iv[n];
iv = in;
len -= AES_BLOCK_SIZE;
in += AES_BLOCK_SIZE;
out += AES_BLOCK_SIZE;
}
if (len) {
AES_decrypt(in,tmp,key);
for(n=0; n < len; ++n)
out[n] = tmp[n] ^ iv[n];
iv = in;
}
memcpy(ivec,iv,AES_BLOCK_SIZE);
} else {
while (len >= AES_BLOCK_SIZE) {
memcpy(tmp, in, AES_BLOCK_SIZE);
AES_decrypt(in, out, key);
for(n=0; n < AES_BLOCK_SIZE; ++n)
out[n] ^= ivec[n];
memcpy(ivec, tmp, AES_BLOCK_SIZE);
len -= AES_BLOCK_SIZE;
in += AES_BLOCK_SIZE;
out += AES_BLOCK_SIZE;
}
if (len) {
memcpy(tmp, in, AES_BLOCK_SIZE);
AES_decrypt(tmp, out, key);
for(n=0; n < len; ++n)
out[n] ^= ivec[n];
for(n=len; n < AES_BLOCK_SIZE; ++n)
out[n] = tmp[n];
memcpy(ivec, tmp, AES_BLOCK_SIZE);
}
}
}
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