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Allowing unsupported extensions in X.509 certificates

by Gilles Peskine

Hello, This is a discussion about the design of a new feature in X.509 parsing, and a possible deprecation of MBEDTLS_X509_ALLOW_UNSUPPORTED_CRITICAL_EXTENSION. Normally, when mbedtls_x509_crt_parse() (specifically x509_get_crt_ext()) encounters an extension that it doesn't support (unknown OID), it rejects the certificate if the extension is marked as critical and ignores it otherwise. This is usually the right thing. The application can inspect all extensions, including ignored ones, in crt->v3_ext. However there are use cases where it's useful to pass extensions through to the application and let the application decide, even for critical extensions. There's a compilation flag MBEDTLS_X509_ALLOW_UNSUPPORTED_CRITICAL_EXTENSION (default off) which causes critical extensions to be ignored. However the application must then parse the extension list again to check for all critical extensions, including ones that it doesn't support. https://github.com/ARMmbed/mbedtls/pull/1425 proposes to give the application better control. At least the application should be able to specify at runtime which extensions it allows. And perhaps the parser should provide pointers to the extensions that it finds. The design needs to reconcile several aspects, in particular: • With default compile-time and run-time options, unsupported critical extensions must be rejected, both for security and for backward compatibility. • With MBEDTLS_X509_ALLOW_UNSUPPORTED_CRITICAL_EXTENSION and default run-time options, all critical extensions must be accepted, for backward compatibility. • It's nicer if the library includes knowledge about extension OIDs. However this needs to be balanced against the needs of embedded systems where code size is important and the current size of oid.o is already a problem. • Compile-time options increase the testing burden, so it's better not to add one. And non-default compile-time options aren't useful for platforms that use a shared library built by a distribution. With these considerations, how should a runtime mechanism to select pass-through critical extensions be implemented? Can we do it without a new compile-time option and without increasing the code size? Should the new mechanism be present in the default build, only with MBEDTLS_X509_ALLOW_UNSUPPORTED_CRITICAL_EXTENSION or only with a new compile-time option? MBEDTLS_X509_ALLOW_UNSUPPORTED_CRITICAL_EXTENSION has been around since before my time. I don't know who might be using it. Should we deprecate it if there's a more flexible runtime mechanism that doesn't involve turning it on? -- Gilles Peskine Mbed TLS developer

5 years, 2 months

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Re: [mbed-tls] Static Memory allocation Buffer Size for DTLS #3146

by Manuel Pegourie-Gonnard

Hi, > I am not sure how much memory to assign? I don't think there's a simple answer to that. I think the best you can do is measure how much memory is consumed in your workflow, and add a margin. When doing measurements, you should keep in mind that DTLS handshakes may consume more memory when happening over an unreliable transport, as it then needs to cache out-of-order messages, so you might want to use something like our programs/test/udp_proxy to simulate an unreliable link for you measurements. Also, the size of the messages exchanged (and potentially cached) depends on the size of the certificate chain, so you'll want to do you measurements with a configuration as close as possible to the final one. > > Current use: 33 blocks / 2508 bytes, max: 99 blocks / 5392 bytes (total 8560 bytes), alloc / free: 8803 / 8771 > What is blocks here and how many bytes per block? > My understanding is that - out of 99 blocks, 33 blocks are used. Is it right? In this context, a block just means an area of memory that is or was allocated. So for example if you do `malloc(128); malloc(1024);` you'll have two blocks used, the first being 128 bytes (plus overhead) and the second 1024 bytes (plus overhead). There is no fixed number or size of blocks in the allocator, so here the "max" means the peak of memory consumption - at that time, 5392 bytes had been allocated, over 99 blocks. You'll notice our allocator has a pretty large overhead: when adding administrative data used by the allocator, the peak memory consumption was actually 8560 bytes, out of which only 5392 were actually available to the application. > Does the memory fragmentation and de-fragmentation is handled inside mbedTLS itself? No, the provided allocator is very basic and doesn't protect against memory fragmentation. > And also after every handshake, does it release the used memory buffer for the connection? Once the handshake is complete, all the RAM that was allocated just for the handshake is freed, and the only buffers that are kept are those that are still necessary for the rest of the connection. Hope that helps! Best regards, Manuel.

5 years, 2 months

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question: SIGPIPE handling

by Anibal Portero

Hi, SIGPIPE is handled with a signal( SIGPIPE, SIG_IGN ) in mbedtls_net_connect. While the examples in programs/ssl/ssl_client1.c or programs/ssl/ssl_client2.c are calling mbedtls_net_connect, programs/ssl/mini_client.c is not, and therefore not changing the default behavior of SIGPIPE. Our client is based on mini_client.c. What would the best way to handle SIGPIPE? Would it be worth it to add signal( SIGPIPE, SIG_IGN ) to the mini_client.c example for future reference? maybe even make net_prepare() visible from outside so a mini client like application can use it? Thanks!

5 years, 2 months

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Static Memory allocation Buffer Size for DTLS #3146

by ROSHINI DEVI

I am directing the issue - https://github.com/ARMmbed/mbedtls/issues/3146 The content of the issue is - Hello, Currently I am using mbed tls 2.16.3 and I am using it on STM32L4xx platform. Before I was using dynamic memory allocation (heap), now planning to shift to static memory allocation. I am using x509 ecc based certificate. I am using DTLS handshake. I am also using "mbedtls_memory_buffer_alloc_status" to print the status of static buffer. The queries are - 1. I am not sure how much memory to assign? 2. I am not able to understand the message - *Current use: 33 blocks / 2508 bytes, max: 99 blocks / 5392 bytes (total 8560 bytes), alloc / free: 8803 / 8771* What is blocks here and how many bytes per block? My understanding is that - out of 99 blocks, 33 blocks are used. Is it right? 1. Does the memory fragmentation and de-fragmentation is handled inside mbedTLS itself? And also after every handshake, does it release the used memory buffer for the connection? I need the values correct to make sure not to get the memory allocation failure Please help me in these queries.

5 years, 2 months

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TLS context serialization: can it be done?

by Kuvaiskii, Dmitrii

Dear all, I have the following question. mbedTLS v2.21.0 has support for TLS context serialization in the form of two functions: `mbedtls_ssl_context_save()` and `mbedtls_ssl_context_load()`. I'm trying to use these functions in another project (Graphene, an Intel SGX framework). Slightly oversimplifying, I want to establish a secure communication channel between two different Linux processes. I'd like to persist one of them and then re-spawn it again with the communication channel intact (so that there is no need for a new TLS handshake). However, I notice that currently these functions support only DTLS 1.2, see e.g.: https://github.com/ARMmbed/mbedtls/blob/aaabe86ac1f47193f4fc499846a0b3abeae… But I want to use a normal TLS channel, in particular with a ciphersuite `MBEDTLS_TLS_PSK_WITH_AES_128_GCM_SHA256`. I commented out the checks on DTLS in these functions just to see what will happen. As expected, both functions serialized and then deserialized the context, but when doing a `write(ssl_ctx)` in one (not-persisted) process and a `read(loaded_ssl_ctx)` in another (re-spawned) process, I get an error in `mbedtls_cipher_auth_decrypt()`. Clearly, my deserialized context didn't restore some vital information on the TLS session, and this led to failure in decryption. Thus, I have two questions: 1. Is there any version of this code that also works on TLS? 2. What are the additional internal objects that must be serialized for TLS (if it makes things easier, in my particular case with AES-GCM and a pre-shared key)? I looked at the code and tried to dump more fields in `mbedtls_ssl_transform`, but it didn't help much. If you'd provide me with some pointers, I could tinker more with mbedTLS code and hopefully make it work. Thanks in advance for any pointers! -- Dmitrii

5 years, 2 months

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Stability of the development environment for long-time support branches

by Gilles Peskine

Hello, In Mbed TLS long-time support branches (currently 2.7.x and 2.16.x), we make bug fixes, but we preserve backward compatibility as much as possible. This means that as much as possible, we don't change existing behavior unless it's definitely wrong, we don't add new APIs, we don't increase the code size. We also preserve backward compatibility with build and deployment processes for projects that use the library. We don't create new source files, we don't change the name of makefile targets, we don't add new tools requirements or tool version requirements to build the library and its tests, etc. For example: * We added Python as a requirement to build the tests shortly before the 2.16 release, but kept the existing Perl script in the 2.7 LTS. * We've changed config.pl to a Python script in development, but kept it in Perl in LTS branches. * We've dropped support for older Visual Studio version in development, but kept it in LTS branches. How far should this extend to non-core files, such as documentation and higher-level test scripts? For example: * We're considering tweaking the format of the ChangeLog file. Should we preserve the format strictly in LTS branches? * How about renaming ChangeLog to CHANGES or CHANGES.md? * We're considering rewriting some multi-configuration test scripts and maintainer sanity checks such as tests/scripts/depends-*.pl, tests/scripts/all.sh, tests/scripts/check-names.sh, etc., in Python. (Note that we've already done that with new scripts added to all.sh: Python was not a requirement to pass all.sh in 2.7.0, but it is now.) -- Gilles Peskine Mbed TLS developer

5 years, 2 months

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Welcome to Mbed TLS @ TrustedFirmware.org

by Dan Handley

Welcome to the Mbed TLS list @ TrustedFirmware.org! This mailing list is the primary channel for public discussion, questions and announcements about the Mbed TLS project. Please use this where possible in preference to the GitHub issue tracker or private support channels. The GitHub issue tracker should still be used for raising bugs and enhancement requests (after first checking the issue doesn't already exist). Thanks and regards Dan.

5 years, 2 months

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Test message - please ignore

by Bill Fletcher

You may safely ignore this message -- [image: Linaro] <http://www.linaro.org/> *Bill Fletcher* | *Field Engineering* T: +44 7833 498336 <+44+7833+498336> bill.fletcher(a)linaro.org | Skype: billfletcher2020

5 years, 2 months

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Welcome to Mbed TLS @ TrustedFirmware.org

by Dan Handley

Welcome to the Mbed TLS list @ TrustedFirmware.org! This mailing list is the primary channel for public discussion, questions and announcements about the Mbed TLS project. Please use this where possible in preference to the GitHub issue tracker or private support channels. The GitHub issue tracker should still be used for raising bugs and enhancement requests (after first checking the issue doesn't already exist). Thanks and regards Dan.

5 years, 2 months

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