On Fri, May 15, 2020 at 01:41:41PM -0700, Eric Biggers wrote: > From: Eric Biggers <ebiggers@xxxxxxxxxx> > > The eMMC inline crypto standard will only specify 32 DUN bits (a.k.a. IV > bits), unlike UFS's 64. IV_INO_LBLK_64 is therefore not applicable, but > an encryption format which uses one key per policy and permits the > moving of encrypted file contents (as f2fs's garbage collector requires) > is still desirable. > > To support such hardware, add a new encryption format IV_INO_LBLK_32 > that makes the best use of the 32 bits: the IV is set to > 'SipHash-2-4(inode_number) + file_logical_block_number mod 2^32', where > the SipHash key is derived from the fscrypt master key. We hash only > the inode number and not also the block number, because we need to > maintain contiguity of DUNs to merge bios. > > Unlike with IV_INO_LBLK_64, with this format IV reuse is possible; this > is unavoidable given the size of the DUN. This means this format should > only be used where the requirements of the first paragraph apply. > However, the hash spreads out the IVs in the whole usable range, and the > use of a keyed hash makes it difficult for an attacker to determine > which files use which IVs. > > Besides the above differences, this flag works like IV_INO_LBLK_64 in > that on ext4 it is only allowed if the stable_inodes feature has been > enabled to prevent inode numbers and the filesystem UUID from changing. > > Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx> Reviewed-by: Theodore Ts'o <tytso@xxxxxxx> I kind of wish we had Kunit tests with test vectors, but that's for another commit I think. - Ted
