On 5/23/2017 10:48 PM, Ken Goldman wrote:
On 5/18/2017 5:38 AM, Roberto Sassu wrote:
On 5/17/2017 6:28 PM, Ken Goldman wrote:
On 5/17/2017 3:25 AM, Roberto Sassu wrote:
The format of digestN is: <algo name>:\0<digest value>, the same used
for the file digest.
Since the format is changing from the SHA-1 log format anyway ...
How do people feel about the colon and null terminated string format for
algorithm identifiers?
The TCG standard enumerations are uint16_t, and there is a registry of
hash algorithms.
As a consuming parser, it feels nice to know it's always 2 bytes and not
have to worry about a missing colon or a missing nul terminator risking
a buffer overflow.
There cannot be buffer overflow, because the length of each digest
field is known.
Roberto
I was not referring to the digest, but the digest algorithm.
I wanted opinions on the colon and null terminated string format for
algorithm identifiers.
The TCG standard log uses the TCG standard enumerations. They're always
exactly 2 bytes. Parsing is trivial.
I have two concerns regarding this:
is there a standard way to convert TPM_ALG_ to strings, like a function
exposed by the TSS? If not, suppose that a parser uses openssl to verify
the integrity of event data, by calculating the digest. Then,
the parser should maintain an association table between TPM_ALG_
and a string (the string will be passed to EVP_get_digestbyname()).
When a new TPM algorithm is added to the TCG registry, all parsers
should be modified to update the association table. If IMA sends
a string, only the crypto subsystem has to be updated.
The format I'm proposing for event data digests would be the same
of that used for file digests. Should IMA provide a list with
two different formats?
Roberto
If IMA uses strings, the attacker can send, e.g., sha1: and not null
terminate it. A careful parser can go a byte at a time until it reaches
a maximum length - if you specify a maximum length. But it is an attack
surface. Is there a corresponding advantage?
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