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reverse: Derive IV from inode number

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The truncated SHA1 hash of the inode number is used as the IV
for files encrypted in reverse mode.
Passes the test suite.

Closes #24.
This commit is contained in:
Jakob Unterwurzacher 2014-11-23 21:20:06 +01:00
parent 3fce933f65
commit 73a5accb8e
1 changed files with 39 additions and 9 deletions
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48
encfs/CipherFileIO.cpp
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@ -30,6 +30,8 @@
#include <cerrno>
#include <string.h>
#include <openssl/sha.h>
/*
- Version 2:0 adds support for a per-file initialization vector with a
fixed 8 byte header. The headers are enabled globally within a
@ -265,24 +267,52 @@ bool CipherFileIO::writeHeader() {
}
/**
* Generate the file IV header bytes in reverse mode
* Generate the file IV header bytes for reverse mode
* (truncated SHA1 hash of the inode number)
*
* The kernel guarantees that the inode number is unique for one file
* system. SHA1 spreads out the values over the whole 64-bit space.
* Without this step, the XOR with the block number (see readOneBlock)
* may lead to duplicate IVs.
* SSL_Cipher::setIVec does an additional HMAC before using
* the IV. This guarantees unpredictability and prevents watermarking
* attacks.
*/
void CipherFileIO::generateReverseHeader(unsigned char* buf) {
rDebug("generating file IV header (reverse mode)");
void CipherFileIO::generateReverseHeader(unsigned char* headerBuf) {
// TODO derive from inode
for (int i = 0; i < HEADER_SIZE; ++i) {
buf[i]=i;
struct stat stbuf;
int res = getAttr(&stbuf);
rAssert( res == 0 );
ino_t ino = stbuf.st_ino;
rAssert( ino != 0 );
rDebug("generating reverse file IV header from ino=%lu", (unsigned long)ino);
// Serialize the inode number into inoBuf
unsigned char inoBuf[sizeof(ino_t)];
for (unsigned int i = 0; i < sizeof(ino_t); ++i) {
inoBuf[i] = (unsigned char)(ino & 0xff);
ino >>= 8;
}
// Save plain-text IV
/* Take the SHA1 hash of the inode number so the values are spread out
* over the whole 64-bit space. Otherwise, the XOR with the block number
* may lead to duplicate IVs (see readOneBlock) */
unsigned char md[20];
SHA1(inoBuf, sizeof(ino), md);
rAssert( HEADER_SIZE <= 20 );
memcpy(headerBuf, md, HEADER_SIZE);
// Save the IV in fileIV for internal use
fileIV = 0;
for (int i = 0; i < HEADER_SIZE; ++i) {
fileIV = (fileIV << 8) | (uint64_t)buf[i];
fileIV = (fileIV << 8) | (uint64_t)headerBuf[i];
}
rDebug("fileIV=%" PRIx64, fileIV);
// Encrypt externally-visible header
cipher->streamEncode(buf, HEADER_SIZE, externalIV, key);
cipher->streamEncode(headerBuf, HEADER_SIZE, externalIV, key);
}
/**