Julian,
Reading into it more, I am suggesting the following minimalist correction:
A) The definintion of 'length' Section 6 should be:
length
The size of the StoredData structure excluding 'length' and 'signature'.
B) In Section 6.1, the input to the signature algorithm should be:
resource_id + kind + stored_data + SignerIdentity
Where these values are:
...
stored_data
The contents of the StoredData structure excluding the 'signature' as
described in the previous sections, with the lifetime set to 0.
SignerIdentity
The signer identity as defined in Section 5.3.4, which MUST include
a hash of the signer's certificate.
It is an obvious style of this draft that a length value of a structure
never includes the bytes occupied by the length itself.
Thanks
--Michael
jc wrote:
On Oct 30, 2009, at 6:03 PM, Michael Chen wrote:
Cullen,
The definition of StoredData the opening of Section 6 and its
Signature in Section 6.1 are confusing:
struct {
uint32 length;
uint64 storage_time;
uint32 lifetime;
StoredDataValue value;
Signature signature;
} StoredData;
The contents of this structure are as follows:
length
The length of the rest of the structure in octets.
...
value
The data value itself, as described in Section 6.2.
signature
A signature over the data value. Section 6.1 describes the
signature computation. The element is formatted as described in
Section 5.3.4
6.1. Data Signature Computation
Each StoredData .... The input to the signature algorithm is:
resource_id + kind + StoredData + SignerIdentity
Where these values are:
...
StoredData
The contents of the stored data value, as described in the
previous sections, with the lifetime set to 0.
1) "length" seems to suggest it equals (sizeof(StoredData) -
sizeof(length)), which means the size of StoredDataValue and
Signature must be obtained before signing. No big deal, but the text
should more explicit and precise. Suggestion:
length
The size of the StoredData structure in octets excluding the size
of length itself.
The wording is wrong and it should be simply "length =
sizeof(StoredData)" because "the length is the size in octets of
StoredData excluding all other fields". Please correct me if I am
wrong, it's late.
2) The big problem is the signature input. In Section 6 the term
"data value" appears in both the definition of "value" and
"signature", seems to suggest that the 3rd input block for the
signature only refer to the "value" field. In Section 6.1, the
instruction of setting "lifetime" to 0 before signing suggest the
signature includes the rest of the fields PLUS the signature itself
(the 3rd input block is "+ StoredData +").
If I am not mistaken, the correct definition for the signature input
should be:
resource_id + kind + storage_time + lifetime + value + SignerIdentity
The fact that "length" includes the size of the signature is somewhat
awkward to be in the input. Excluding it does not degrade the
security of the signature. If it is up to me, instead of setting
"lifetime" to 0 before signing, why not exclude it all together?
Where does the length field include the sizeof(signature)? See above.
3) "SignerIdentity" in this section should refer to its definition in
Section 5.3.4 to prevent people from reading it as simply a 20-byte
SHA1 hash value.
SignerIdentity
The signer identity as defined in Section 5.3.4, which MUST include
a hash of the signer's certificate.
Thanks
--Michael
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