After mitigating the 'triple handshake attack' by checking that
the peer's end-CRT didn't change during renegotation, the current
code avoids re-parsing the CRT by moving the CRT-pointer from the
old session to the new one. While efficient, this will no longer
work once only the hash of the peer's CRT is stored beyond the
handshake.
This commit removes the code-path moving the old CRT, and instead
frees the entire peer CRT chain from the initial handshake as soon
as the 'triple handshake attack' protection has completed.
Additional work done as part of merge:
- Run ./tests/scripts/check-generated-files.sh and check in the
resulting changes to programs/ssl/query_config.c
- Populate the ECDH private key slot with a fresh private EC key
designated for the correct algorithm.
- Export the public part of the ECDH private key from PSA and
reformat it to suite the format of the ClientKeyExchange message.
- Perform the PSA-based ECDH key agreement and store the result
as the premaster secret for the connection.
- Reformat the server's ECDH public key to make it suitable
for the PSA key agreement API. Currently, the key agreement
API needs a full SubjectPublicKeyInfo structure, while the
TLS ServerKeyExchange message only contains a ECPoint structure.
This is the first in a series of commits adding client-side
support for PSA-based ECDHE.
Previously, the state of an ECDHE key agreement was maintained
in the field mbedtls_ssl_handshake_params::ecdh_ctx, of type
::mbedtls_ecdh_context and manipulated through the ECDH API.
The ECDH API will be superseeded by the PSA Crypto API for key
agreement, which needs the following data:
(a) A raw buffer holding the public part of the key agreement
received from our peer.
(b) A key slot holding the private part of the key agreement.
(c) The algorithm to use.
The commit adds fields to ::mbedtls_ssl_handshake_params
representing these three inputs to PSA-based key agreement.
Specifically, it adds a field for the key slot holding the
ECDH private key, a field for the EC curve identifier, and
a buffer holding the peer's public key.
Note: Storing the peer's public key buffer is slightly
inefficient, as one could perform the ECDH computation
as soon as the peer sends its public key, either working
with in-place or using a stack-buffer to reformat the
public key before passing it to PSA. This optimization
is left for a later commit.
Silence a compiler warning about implicit fallthrough by using a comment
format the compiler understand to mean that the fallthrough is
intentional.
In file included from library/cipher.c:63:0:
include/mbedtls/psa_util.h: In function ‘mbedtls_psa_translate_cipher_mode’:
include/mbedtls/psa_util.h:91:15: error: this statement may fall through [-Werror=implicit-fallthrough=]
if( taglen == 0 )
^
include/mbedtls/psa_util.h:94:9: note: here
default:
^~~~~~~
cc1: all warnings being treated as errors
$ gcc --version
gcc (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
When using PSA with MBEDTLS_ENTROPY_NV_SEED, some test suites
require the seed file for PSA initialization, which was normally generated
later, when entropy tests were run. This change creates an initial seedfile
in all.sh.
Don't unconditionally enable PSA Crypto for all tests. Only enable it in
tests that require it. This allows crypto tests to check that
psa_crypto_init() fails when it is supposed to fail, since we want to
perform some action in a test, and then call psa_crypto_init() and check
the result without it having been called previously.
Enable handling of zero-length null output in PKCS1 v1.5 decryption.
Prevent undefined behavior by avoiding a memcpy() to zero-length null
output buffers.
In mbedtls_rsa_rsaes_oaep_encrypt and
mbedtls_rsa_rsaes_pkcs1_v15_encrypt, if the input length is 0 (which
is unusual and mostly useless, but permitted) then it is fine for the
input pointer to be NULL. Don't return an error in this case.
When `input` is NULL, `memcpy( p, input, ilen )` has undefined behavior
even if `ilen` is zero. So skip the `memcpy` call in this case.
Likewise, in `mbedtls_rsa_rsaes_oaep_decrypt`, skip the `memcpy` call if
`*olen` is zero.
Context: During a handshake, the SSL/TLS handshake logic constructs
an instance of ::mbedtls_ssl_session representing the SSL session
being established. This structure contains information such as the
session's master secret, the peer certificate, or the session ticket
issues by the server (if applicable).
During a renegotiation, the new session is constructed aside the existing
one and destroys and replaces the latter only when the renegotiation is
complete. While conceptually clear, this means that during the renegotiation,
large pieces of information such as the peer's CRT or the session ticket
exist twice in memory, even though the original versions are removed
eventually.
This commit removes the simultaneous presence of two peer CRT chains
in memory during renegotiation, in the following way:
- Unlike in the case of SessionTickets handled in the previous commit,
we cannot simply free the peer's CRT chain from the previous handshake
before parsing the new one, as we need to verify that the peer's end-CRT
hasn't changed to mitigate the 'Triple Handshake Attack'.
- Instead, we perform a binary comparison of the original peer end-CRT
with the one presented during renegotiation, and if it succeeds, we
avoid re-parsing CRT by moving the corresponding CRT pointer from the
old to the new session structure.
- The remaining CRTs in the peer's chain are not affected by the triple
handshake attack protection, and for them we may employ the canonical
approach of freeing them before parsing the remainder of the new chain.
Note that this commit intends to not change any observable behavior
of the stack. In particular:
- The peer's CRT chain is still verified during renegotiation.
- The tail of the peer's CRT chain may change during renegotiation.
Context: During a handshake, the SSL/TLS handshake logic constructs
an instance of ::mbedtls_ssl_session representing the SSL session
being established. This structure contains information such as the
session's master secret, the peer certificate, or the session ticket
issues by the server (if applicable).
During a renegotiation, the new session is constructed aside the existing
one and destroys and replaces the latter only when the renegotiation is
complete. While conceptually clear, this means that during the renegotiation,
large pieces of information such as the peer's CRT or the session ticket
exist twice in memory, even though the original versions are removed
eventually.
This commit starts removing this memory inefficiency by freeing the old
session's SessionTicket before the one for the new session is allocated.
