This commit extends the RSA interface by import/export calls that can be used to
setup an RSA context from a subset of the core RSA parameters (N,P,Q,D,E).
The intended workflow is the following:
1. Call mbedtls_rsa_import one or multiple times to import the core parameters.
2. Call mbedtls_rsa_complete to deduce remaining core parameters as well as any
implementation-defined internal helper variables.
The RSA context is ready for use after this call.
The import function comes in two variants mbedtls_rsa_import and
mbedtls_rsa_import_raw, the former taking pointers to MPI's as input, the latter
pointers buffers holding to big-endian encoded MPI's.
The reason for this splitting is the following: When only providing an import
function accepting const MPI's, a user trying to import raw binary data into an
RSA context has to convert these to MPI's first which before passing them to the
import function, introducing an unnecessary copy of the data in memory. The
alternative would be to have another MPI-based import-function with
move-semantics, but this would be in contrast to the rest of the library's
interfaces.
Similarly, there are functions mbedtls_rsa_export and mbedtls_rsa_export_raw for
exporting the core RSA parameters, either as MPI's or in big-endian binary
format.
The main import/export functions deliberately do not include the additional
helper values DP, DQ and QP present in ASN.1-encoded RSA private keys. To
nonetheless be able to check whether given parameters DP, DQ and QP are in
accordance with a given RSA private key, the interface is extended by a function
mbedtls_rsa_check_opt (in line with mbedtls_rsa_check_privkey,
mbedtls_rsa_check_pubkey and mbedtls_rsa_check_pub_priv). Exporting the optional
parameters is taken care of by mbedtls_export_opt (currently MPI format only).
This commit adds convenience functions to the RSA module for computing a
complete RSA private key (with fields N, P, Q, D, E, DP, DQ, QP) from a subset
of core parameters, e.g. (N, D, E).
This is the first step towards making verify_chain() iterative. While from a
readability point of view the current recursive version is fine, one of the
goals of this refactoring is to prepare for restartable ECC integration, which
will need the explicit stack anyway.
Besides avoiding near-duplication, this avoids having three generations of
certificate (child, parent, grandparent) in one function, with all the
off-by-one opportunities that come with it.
This also allows to simplify the signature of verify_child(), which will be
done in next commit.
This is from the morally 5th (and soon obsolete) invocation of this function
in verify_top().
Doing this badtime-skipping when we search for a parent in the provided chain
is a change of behaviour, but it's backwards-compatible: it can only cause us
to accept valid chains that we used to reject before. Eg if the peer has a
chain with two version of an intermediate certificate with different validity
periods, the first non valid and the second valid - such cases are probably
rare or users would have complained already, but it doesn't hurt to handle it
properly as it allows for more uniform code.
This may look like a behaviour change because one check has been added to the
function that was previously done in only one of the 3 call sites. However it
is not, because:
- for the 2 call sites in verify(), the test always succeeds as path_cnt is 0.
- for the call site in verify_child(), the same test was done later anyway in
verify_top()
There are 3 instance that were replaced, but 2 instances of variants of this
function exist and will be handled next (the extra parameter that isn't used
so far is in preparation for that):
- one in verify_child() where path_cnt constraint is handled too
- one in verify_top() where there is extra logic to skip parents that are
expired or future, but only if there are better parents to be found
This is a slight change of behaviour in that the previous condition was:
- same subject
- signature matches
while the new condition is:
- exact same certificate
However the documentation for mbedtls_x509_crt_verify() (note on trust_ca)
mentions the new condition, so code that respected the documentation will keep
working.
In addition, this is a bit faster as it doesn't check the self-signature
(which never needs to be checked for certs in the trusted list).
When we're looking for a parent, in trusted CAs, 'top' should be 1.
This only impacted which call site for verify_top() was chosen, and the error
was then fixed inside verify_top() by iterating over CAs again, this time
correctly setting 'top' to 1.
This is the beginning of a series of commits refactoring the chain
building/verification functions in order to:
- make it simpler to understand and work with
- prepare integration of restartable ECC
