Unfortunately it's not that simple - encryption is not integrity. First, as a simple example
consider a stream cipher such as RC4, which was for a long time used as the standard encryption
for all wireless and internet (https) communication. RC4 by itself provides no integrity protection
whatsoever, and it is trivial for an attacker to "bit-twiddle" any desired bits in the encrypted
stream. Even with block ciphers, with the current standard chaining methods, encrypted blocks
can be cut and pasted and replayed between files and sessions with a good probability of success.
Second, there are significant performance penalties for encrypting system files, where integrity,
not confidentiality is all that is required.
In IPSEC, a deliberate decision was made to allow integrity (AH - authentication headers)
without encryption, but not encryption without integrity.
The bottom line is that encryption by itself does not guarantee integrity, and you really want
both.
One of the hardest problems in confidentiality is key management. It does no good to have
strong encryption with a key system that makes it easy for an attacker to steal the keys.
The trusted key component does two things to help with secure key management on Linux. First,
it provides a kernel key ring service in which the symmetric encryption keys are never visible in
plain text to userspace. The keys are created in the kernel, and sealed by a hardware device such
as a TPM, with userspace seeing only the sealed blobs. Malicious or compromised applications
cannot steal a trusted key, since only the kernel can see the unsealed blobs. Secondly, the trusted
keys can tie key unsealing to the integrity measurements, so that keys cannot be stolen in an
offline attack, such as by booting an unlocked Linux image from CD or USB. As the
measurements will be different, the TPM chip will refuse to unseal the keys, even for the kernel.
In this way, Linux ties confidentiality and integrity together. The integrity of the system is a
prerequisite for decrypting confidential files.
Threat Models:
There are many different types of attacks on file integrity, authenticity, and confidentiality, and
there are significant tradeoffs between security and cost and performance, depending on which of
the threats need to be defended against.
Remote Attacks
The most common attack is the remote software attack, in which the attacker attempts either to
trick a user into running the attacker's malicious code (trojan), or in which the attacker sends
malicious data in an attempt to exploit a vulnerability in the system's software (injection,
overflow).
Local Attacks
Local attacks assume that the attacker has physical access to the system, such as with a malicious
insider (operator), or in the case of theft of the system. Local attacks can be software based, such
as in an offline attack, or can be hardware based, such as with simple JTAG memory probes,
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