- mbed-tls - lists.trustedfirmware.org

Re: [mbed-tls] Random Number Generator module in mbed TLS

by Janos Follath

Hi, Hanno described perfectly how it works and I just would like to add a single remark. You mentioned that some of your platforms don’t have a hardware entropy source. In cases like that, depending on your threat model you might need to use the MBEDTLS_ENTROPY_NV_SEED feature to achieve security. (This uses a stored secret value as a seed and on embedded systems without a hardware entropy source it is more secure against a remote adversary than trying to collect the entropy from the environment. On systems like that NV_SEED can have an edge against physical attackers – compared to attempting to collect entropy from the environment – if the seed is stored in secure storage/internal flash.) Regards, Janos From: mbed-tls <mbed-tls-bounces(a)lists.trustedfirmware.org> on behalf of Hanno Becker via mbed-tls <mbed-tls(a)lists.trustedfirmware.org> Reply to: Hanno Becker <Hanno.Becker(a)arm.com> Date: Tuesday, 28 April 2020 at 07:55 To: "mbed-tls(a)lists.trustedfirmware.org" <mbed-tls(a)lists.trustedfirmware.org>, ROSHINI DEVI <roshinilachi(a)gmail.com> Subject: Re: [mbed-tls] Random Number Generator module in mbed TLS Hi, Mbed TLS establishes variable-length access to random data in a three step fashion: 1) At the bottommost layer, there is a variable number of entropy sources external to the library. Such sources are supposed to provide some true randomness, though the exact amount of entropy they contain isn't yet specified (at least to my knowledge). 2) Mbed TLS' entropy module mixes those entropy sources into a single source of randomness. In contrast to the entropy sources themselves, the idea here is that, ideally, the data obtained from the entropy module has full entropy. This is achieved by (a) accumulating random data from available entropy sources and depending on the amount of entropy each of them offers, and (b) 'mixing' them by a application of hash functions. 3) Based on true randomness, Mbed TLS' provides two implementations of pseudo random number generators: CTR-DRBG and HMAC-DRBG. Those build on top of an entropy context and expand the underlying randomness as standardized in NIST SP 800-90. Applications should use the PRNGs from step 3) as their actual source of randomness, and not directly hook into the underlying TRNGs. Take a look at the example programs such as `ssl_client2` or `ssl_server2` to see how this works practically. Also see https://tls.mbed.org/kb/how-to/add-a-random-generator. Now specifically to your question: You should register your STM32 hardware entropy as an entropy source via `mbedtls_entropy_add_source()` but not (need) to change anything else in your code. In particular, steps 2) and 3) above are entirely independent of the exact source of true randomness. Hope this helps, Hanno ________________________________ From: mbed-tls <mbed-tls-bounces(a)lists.trustedfirmware.org> on behalf of ROSHINI DEVI via mbed-tls <mbed-tls(a)lists.trustedfirmware.org> Sent: Tuesday, April 28, 2020 6:06 AM To: mbed-tls(a)lists.trustedfirmware.org <mbed-tls(a)lists.trustedfirmware.org> Subject: Re: [mbed-tls] Random Number Generator module in mbed TLS Hello, Can anyone confirm this? Its urgent. Thanks On Fri, Apr 17, 2020 at 4:50 PM ROSHINI DEVI <roshinilachi(a)gmail.com<mailto:roshinilachi@gmail.com>> wrote: Hello, Is there any random number library available in mbedTLS? Right now, I am using hardware entropy in STM32 boards. If hardware platform changes and if there is no hardware entropy present, then again we need to redefine the API. Thanks IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.

5 years, 4 months

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Re: [mbed-tls] Random Number Generator module in mbed TLS

by Hanno Becker

Hi, Mbed TLS establishes variable-length access to random data in a three step fashion: 1) At the bottommost layer, there is a variable number of entropy sources external to the library. Such sources are supposed to provide some true randomness, though the exact amount of entropy they contain isn't yet specified (at least to my knowledge). 2) Mbed TLS' entropy module mixes those entropy sources into a single source of randomness. In contrast to the entropy sources themselves, the idea here is that, ideally, the data obtained from the entropy module has full entropy. This is achieved by (a) accumulating random data from available entropy sources and depending on the amount of entropy each of them offers, and (b) 'mixing' them by a application of hash functions. 3) Based on true randomness, Mbed TLS' provides two implementations of pseudo random number generators: CTR-DRBG and HMAC-DRBG. Those build on top of an entropy context and expand the underlying randomness as standardized in NIST SP 800-90. Applications should use the PRNGs from step 3) as their actual source of randomness, and not directly hook into the underlying TRNGs. Take a look at the example programs such as `ssl_client2` or `ssl_server2` to see how this works practically. Also see https://tls.mbed.org/kb/how-to/add-a-random-generator. Now specifically to your question: You should register your STM32 hardware entropy as an entropy source via `mbedtls_entropy_add_source()` but not (need) to change anything else in your code. In particular, steps 2) and 3) above are entirely independent of the exact source of true randomness. Hope this helps, Hanno ________________________________ From: mbed-tls <mbed-tls-bounces(a)lists.trustedfirmware.org> on behalf of ROSHINI DEVI via mbed-tls <mbed-tls(a)lists.trustedfirmware.org> Sent: Tuesday, April 28, 2020 6:06 AM To: mbed-tls(a)lists.trustedfirmware.org <mbed-tls(a)lists.trustedfirmware.org> Subject: Re: [mbed-tls] Random Number Generator module in mbed TLS Hello, Can anyone confirm this? Its urgent. Thanks On Fri, Apr 17, 2020 at 4:50 PM ROSHINI DEVI <roshinilachi(a)gmail.com<mailto:roshinilachi@gmail.com>> wrote: Hello, Is there any random number library available in mbedTLS? Right now, I am using hardware entropy in STM32 boards. If hardware platform changes and if there is no hardware entropy present, then again we need to redefine the API. Thanks IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.

5 years, 4 months

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Random Number Generator module in mbed TLS

by ROSHINI DEVI

Hello, Is there any random number library available in mbedTLS? Right now, I am using hardware entropy in STM32 boards. If hardware platform changes and if there is no hardware entropy present, then again we need to redefine the API. Thanks

5 years, 4 months

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Edwards curve support (#3245): fixing mbedtls_ecp_get_type

by Aurelien Jarno

Hi all, I have submitted PR#3245 in order to add Edwards curves support to Mbed TLS, with the eventual goal to add support for the EdDSA algorithm. There are still a few things to fix that require discussion. Let's start the discussion with the first one. Adding a new curve type requires to add a new entry to the mbedtls_ecp_curve_type enum. The curve type used by a group is returned by the mbedtls_ecp_get_type function. It currently uses the coordinates type of the base point to determine the curve type. Montgomery curves are lacking the y coordinate, and the Short Weierstrass curves use the three x, y and z coordinates. The Edwards arithmetic implementation in this PR uses the projective coordinates. As such it also uses the x, y and z coordinates and this gives no way to differentiate a Short Weierstrass from an Edwards curve. I have currently implemented that by checking if the curves are the Ed25519 or Ed448 ones using the group id [1]. I am not sure it's very clean and it won't scale if more curves are added later. Another alternative would be to add another entry to mbedtls_ecp_group to hold the curve type. What do you think is the best option? Any other idea? Thanks, Aurelien [1] https://github.com/ARMmbed/mbedtls/pull/3245/commits/aa20cf122a1a54cfa23624… -- Aurelien Jarno GPG: 4096R/1DDD8C9B aurelien(a)aurel32.net http://www.aurel32.net

5 years, 5 months

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Re: [mbed-tls] Development environment: minimum tool versions

by Steffan Karger

Hi, I maintain OpenVPN-NL (https://openvpn.fox-it.com), which uses mbedtls as it's crypto library. On 24-04-2020 00:25, Gilles Peskine via mbed-tls wrote: > 1. Is there any interest in the community for making sure that the build > keeps (or starts) working on RHEL 6? Yes, we support RHEL6 until it's retirement (30 November 2020). > 2. Is there any interest in the community for making sure that the build > keeps working on Ubuntu 16.04? Yes, we support all Ubuntu LTS versions until their standard support ends (April 2021 for 16.04). > 3. Are there any other "old" platforms that we should consider? All Debian and SLES versions that are still in regular maintance (ie, not including any "extended security maintenance" support). It would be nice if it's at least possible to build mbedtls on/for these platforms until they go out of regular support. Preferably with their default packages compilers. SLES/RHEL are the most demanding, with their 10 year support cycle. Thanks, -Steffan

5 years, 5 months

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Re: [mbed-tls] RSA key generation issue

by Gilles Peskine

Hi Nazar, On 24/04/2020 10:22, Nazar Chornenkyy via mbed-tls wrote: > > On the PSoC6 device we are using mbedtls_rsa_gen_key function with a > good hardware TRNG passed FIPS 140-2 verification. > > This function verifies generated random pair and requests new pair if > they don’t follow FIPS 186-4 criteria. > > The number of verification loops is between 1 and 6 in worst case. > > Is it expected behavior? > > The arithmetic of one loop takes a lots of time. > > How can we improve the random number to have always pass FIPS 186-4 in > a one loop? > I'm not sure what you're doing. Are you calling mbedtls_rsa_gen_key() multiple times and rejecting the key if it doesn't pass additional checks? Or are you just calling it once? As far as I know, mbedtls_rsa_gen_key() follows the procedure described in FIPS 186-4 (which is compatible with previous versions of FIPS 186), and no additional checks should be needed. If you're calling it once and observing that it tries multiple values of p and q before finding a suitable one, this is expected behavior. There's no way to "improve" the random numbers to make it faster: the probability that a pair of (probabilistic) primes can work together as an RSA key is less than 1. I don't know the math off the top of my head, but 1–6 tries seems reasonable. Key generation is the slowest of the RSA operations. If that's a problem in your application, consider using elliptic curve cryptography instead. ECC has very fast key generation, faster private key operations (signature, decryption) than RSA, and smaller keys. The main benefit of RSA is that public key operations (verification, encryption) are faster. -- Gilles Peskine Mbed TLS developer IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.

5 years, 5 months

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Development environment: minimum tool versions

by Gilles Peskine

Hello, I'm starting this thread to get input from the community about what versions of compilers and other build tools Mbed TLS should support. We of course strive to remain compatible with the latest versions, so this discussion is about support for older versions. We currently don't really have an official version range for the build dependencies of Mbed TLS: * C compilers (GCC, Clang, Arm Compiler, IAR, Visual Studio) * Build tools (GNU make, CMake) * Scripting tools (shell, Perl, Python) In practice, for the past three years or so, most of our continuous integration has been running on Ubuntu 16.04, so we mostly test with GCC 5.4.0, Clang 3.8, GNU make 4.1, CMake 3.5.1 and Python 3.5. (For shell and Perl, even pretty ancient version should work.) For Arm Compiler (armcc/armclang) and IAR, we use the same version as Mbed OS. On Windows, we currently test Visual Studio 2013, 2015 and 2017, as well as a version of Mingw that's changed more quickly than most of the rest of our CI. We also run some tests on FreeBSD (a bit more recent that the Ubuntu machines). The top-level CMakeLists.txt has conditionals that detect much older versions of CMake, GCC and Clang, but we haven't tested those in ages and we don't really know if, for example, the library still builds with CMake 2.8 and GCC 4.2. As part of the move of the project from Arm to Trusted Firmware, we'll change our CI infrastructure, and this may or may not mean we end up testing with different versions. In https://github.com/ARMmbed/mbedtls/issues/2896 I looked at the software versions in some common "slow updating" operating systems: RHEL/Centos, Ubuntu LTS. 1. Is there any interest in the community for making sure that the build keeps (or starts) working on RHEL 6? 2. Is there any interest in the community for making sure that the build keeps working on Ubuntu 16.04? 3. Are there any other "old" platforms that we should consider? 4. Should we consider older compilers (maybe some platforms are stuck with older versions of GCC)? 5. On Windows, what's the oldest version of Visual Studio we should keep compatibility with? -- Gilles Peskine Mbed TLS developer IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.

5 years, 5 months

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RSA key generation issue

by Nazar Chornenkyy

Hi I found this email on ARM MBED support forum. Could you help to resolve our issue? On the PSoC6 device we are using mbedtls_rsa_gen_key function with a good hardware TRNG passed FIPS 140-2 verification. This function verifies generated random pair and requests new pair if they don't follow FIPS 186-4 criteria. The number of verification loops is between 1 and 6 in worst case. Is it expected behavior? The arithmetic of one loop takes a lots of time. How can we improve the random number to have always pass FIPS 186-4 in a one loop? Thanks Nazar This message and any attachments may contain confidential information from Cypress or its subsidiaries. If it has been received in error, please advise the sender and immediately delete this message.

5 years, 5 months

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Re: [mbed-tls] 3.0 plans - move net.c out of the library

by Gilles Peskine

Like with timing (see my message in the timing thread, https://lists.trustedfirmware.org/pipermail/mbed-tls/2020-April/000061.html), I have a strong preference for the library to be usable out of the box on typical platforms. This means keeping the abstractions that are currently in the net_sockets module, or similar abstractions. This doesn't mean keeping the net_sockets module as such: these abstractions could be in platform.c. I lean towards having a platform_posix.c and a platform_windows.c (and any other we care to support), chosen at build time based on the target platform. I'm not sure exactly what they should look like. In particular, if you have an embedded platform that's sort of POSIX-like, but doesn't have every POSIX feature and does a few differently, should you be able to customize platform_posix.c through #defines, or should you copy it and modify it to suit your platform? -- Gilles Peskine Mbed TLS developer On 20/04/2020 11:52, Manuel Pegourie-Gonnard via mbed-tls wrote: > Hi all, > > [ Context: https://developer.trustedfirmware.org/w/mbed-tls/tech-plans-3.0/ ] > > Currently the SSL/TLS library includes a module called net_sockets.c (formerly > net.c), enabled by default in config.h (but disabled by config.py baremetal), > that contains some networking functions based on POSIX or Windows sockets, > including functions suitable for use as I/O callbacks with our SSL/TLS > modules. > > Those functions are used only in example/testing programs, but nowhere in the > library itself. > > In Mbed TLS 3.0, as part of our effort to clean up and minimize our API, > we're considering removing (parts of) this module from the library, and > moving its parts to a variety of other places, such as example programs (or > a library/file shared by them, like the current query_config mechanism). > > Reasons for removing this module from the library include: > > - overall the module is less portable and perhaps of lesser quality than the > rest of the library > - it's currently entirely untested on Windows (despite being very > platform-specific) > - there are often confusions about whether the module is meant to be a > general-purpose networking library, or just provide basic support for our > example and test, as well a simple prototypes > - it's not our core area of expertise, let's do one thing and to it well > > Reasons for keeping (parts of) it in the library include: > > - having mbedtls_net_recv{,_timeout}() and mbedtls_net_send() available right > in the library makes it easier to test and develop prototype > - this module is listed as a component in our high-level design document [2], > so perhaps removing it from the library can be seen as a bigger change? > > [2]: https://tls.mbed.org/high-level-design > > What do you think? Should we remove the entire module from the library, keep > it all, or just keep some parts? In that case, which parts and where? > > Regards, > Manuel. IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.

5 years, 5 months

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Re: [mbed-tls] 3.0 plans - move timing.c out of the library

by Gilles Peskine

I have a strong preference for the library, the tests and companion programs to build out of the box on officially supported platforms. On platforms that don't work out of the box, users should be able to drop in platform support code in the form of #defines or extra modules to link with. Ok, we don't have an official list of supported platforms. But inasmuch as we do have officially supported platforms, they include Linux and other modern Unix-like or POSIX environments, Windows, and Mbed OS. Mbed OS is a special case because Mbed TLS is part of the build tree there, so the mbedtls tree doesn't have to contain Mbed OS support. This doesn't mean that the library should have a timing module. I think the timing wrappers that the library needs, i.e. mbedtls_timing_{get,set}_delay, should be in platform.c. For platform features that are only used by test or sample programs, I think we should have a separate platform support file, and separate #defines so that you can build the library even if you can't build all the programs. For example, it makes sense to build DTLS support without having the timing functions used by the benchmark program. -- Gilles Peskine Mbed TLS developer On 21/04/2020 10:46, Manuel Pegourie-Gonnard via mbed-tls wrote: > Hi all, > > So personally I'm quite strongly inclined to remove timing.c from the > library, and move most of its content elsewhere, with one possible > exception: > >> - mbedtls_timing_set_delay() and mbedtls_timing_get_delay() are an example >> implementation (only for Unix and Windows) of timer callbacks for DTLS, only >> used in programs/ssl/*.c > Since timer callbacks are a hard requirement for using DTLS, and it seems > quite desirable to be able to support DTLS out of the box at least on > some platforms, I was thinking this pair of function (and the associated > context type) could be kept in the library, in a new module that would > be called something like ssl_dtls_timer.c. > > This would be somewhat similar to ssl_cookies.c, ssl_tickets.c and > ssl_cache.c: they all provide implementations of callbacks that can be > used with the main SSL/(D)TLS module, but users are obviously free to > compile them out and use their own implementation if the one we provide > does not meet their needs. > > As it happens, all three of these support modules work best if > MBEDTLS_HAVE_TIME is defined, but can work without it. > > For the new ssl_dtls_timer.c I'm suggesting, the situation would be > different: the module could have a hard dependency on MBEDTLS_HAVE_TIME, > but work better on selected platforms (say, C11, POSIX and > Windows) where we know how to access sub-second timing information. (Or > alternatively, have a hard dependency on C11-or-Posix-or-Windows.) > > For the record, mbedtls_ssl_conf_handshake_timeout() accepts timeout > values in milliseconds, but recommended values for use over the general > internet start at 1 second: > https://tools.ietf.org/html/rfc6347#section-4.2.4.1 > So it might make sense to provide this module even when we only have > second resolution - we'd just have to work out how the timer function > would behave when passed sub-second values. (My first thought it > rounding up to the next second would be quite OK.) > > What do you'all think? Personnally, I don't have a strong opinion > between the three following options, though I have a slight preference > for the first one: > > 1. Provide ssl_dtls_timer.c in the library with hard dep on HAVE_TIME > and enhanced features with C11/Posix/Windows. > 2. Provide ssl_dtls_timer.c in the library with hard dep on > C11/Posix/Windows. > 3. Move it all out of the library and let the thing live somewhere under > programs/ as it's only used by example/testing programs. > > Note though that we could also choose to go with option 3 for Mbed TLS > 3.0, see how it goes, and later switch to option 2 or 1 if we want, as > adding modules can be done at any time. (In the same vein, we could > start with 2 and switch to 1 at any time as well.) > > Regards, > Manuel. IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.

5 years, 5 months

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Re: [mbed-tls] 3.0 plans - move timing.c out of the library

by Manuel Pegourie-Gonnard

Hi, > [I'm assuming didn't mean to reply on the list and not just to me, so I'm replying on the list.] Oops, I misinterpreted the information presented by my mail client, you did reply on the list. Sorry for the noise (and for breaking the thread by not replying to the correct email.) Manuel.

5 years, 5 months

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Re: [mbed-tls] 3.0 plans - move net.c out of the library

by Manuel Pegourie-Gonnard

Hi again, In the first email of the thread I tried giving a fair exposition of the question, now I'm going to give my personal opinion. Similarly to what I suggested for timing.c, I'm inclined to remove net.c as such, move most of its content (net_bind(), net_accept(), etc.) out of the library (most probably into a new file in programs/ssl, shared by SSL examples), and keep in the library only the 3 functions that can be used as SSL callbacks (and a supporting context type), but in a new module named for example ssl_io_sockets.c. This would also be similar to the other SSL modules that provide a reference/example implementation of some SSL callback but can easily be disabled (or jsut disregarded) in favour of a custom implementation for custom needs or unsupported platforms. Just like the current net_sockets.c, this new reduced module would probably only be supported on platforms with sockets, which is Posix platforms and Windows. Advantages include: - Reduce the amount of code and API surface in the library. Things like binding and accepting are hardly related to (D)TLS so there is little justification for having them in the library. Also, things in the library are bound by API stability rules which are sometimes a constraint. - It would clarify that we're not providing a general-purpose socket abstraction, as general socket management would be out of the library, and only the bits for plugging a socket into our SSL layer would be in the library. - (By the way currently net_sockets.h includes ssl.h, so it's already quite specific to our SSL module rather than generic, but moving the remaining bits of it into the SSL namespace would make that clearer.) - User would still have the convenience (on supported platforms) of ready-for-use callbacks. If you have an application already working with sockets without TLS, then we provide all the bits needed to turn that into an application using TLS on top of those sockets. I think that would be a good compromise between convenience for users, and reduced surface for the library. Regards, Manuel.

5 years, 5 months

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Re: [mbed-tls] 3.0 plans - move certs.c out of the library

by Hanno Becker

Hi, I support this. Could certs.c live in `tests/data_files`? We should also make sure this file is automatically synced with the actual CRT and key files in that directory. There is a script which does that, but IIRC it's not called as part of the CI, which lead to certs.c and the data files get out of sync multiple times in the past. Best Hanno ________________________________ From: mbed-tls <mbed-tls-bounces(a)lists.trustedfirmware.org> on behalf of Manuel Pegourie-Gonnard via mbed-tls <mbed-tls(a)lists.trustedfirmware.org> Sent: Monday, April 20, 2020 10:49 AM To: mbed-tls(a)lists.trustedfirmware.org <mbed-tls(a)lists.trustedfirmware.org> Cc: nd <nd(a)arm.com> Subject: [mbed-tls] 3.0 plans - move certs.c out of the library Hi all, In this new installment of "let's discuss ideas for Mbed TLS 3.0" [1]: should we move certs.c out of the library? [1]: https://developer.trustedfirmware.org/w/mbed-tls/tech-plans-3.0/ Currently the X.509 library includes test certificates and keys, which are enabled by default in config.h - `MBEDTLS_CERTS_C`. These are used in the following places: - in library/x509.c in mbedtls_x509_self_test() - in tests/suites/test_suite_ssl.function - in programs/fuzz and programs/ssl In Mbed TLS 3.0, as part of our effort to clean up and minimize our API, we'd like to remove certs.h and the certificates it contains from the library (except perhaps one static cert for mbedtls_x509_self_test()). Tests and example programs that need built-in certificates could still get them using any mechanism, included a file certs.c similar to the current one - but this file would no longer live in library or be included when building libmbedx509. Reasons include: - Including test certificates and keys in the library provides little value, as users will want to use their own certificates and keys anyway. - Shipping private keys as part of the library is bad practice, even under the `mbedtls_test_` sub-namespace. There's a slight risk that some users could use them for prototyping and then inadvertently keep using unsafe private keys in production. If you disagree or have concerns, please speak up! Also, if want to suggest ideas for where the replacement to library/certs.c should live or how it could be maintained, now's a good time as well! Regards, Manuel. -- mbed-tls mailing list mbed-tls(a)lists.trustedfirmware.org https://lists.trustedfirmware.org/mailman/listinfo/mbed-tls IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.

5 years, 5 months

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Re: [mbed-tls] 3.0 plans - move timing.c out of the library

by Manuel Pegourie-Gonnard

Hi all, So personally I'm quite strongly inclined to remove timing.c from the library, and move most of its content elsewhere, with one possible exception: > - mbedtls_timing_set_delay() and mbedtls_timing_get_delay() are an example > implementation (only for Unix and Windows) of timer callbacks for DTLS, only > used in programs/ssl/*.c Since timer callbacks are a hard requirement for using DTLS, and it seems quite desirable to be able to support DTLS out of the box at least on some platforms, I was thinking this pair of function (and the associated context type) could be kept in the library, in a new module that would be called something like ssl_dtls_timer.c. This would be somewhat similar to ssl_cookies.c, ssl_tickets.c and ssl_cache.c: they all provide implementations of callbacks that can be used with the main SSL/(D)TLS module, but users are obviously free to compile them out and use their own implementation if the one we provide does not meet their needs. As it happens, all three of these support modules work best if MBEDTLS_HAVE_TIME is defined, but can work without it. For the new ssl_dtls_timer.c I'm suggesting, the situation would be different: the module could have a hard dependency on MBEDTLS_HAVE_TIME, but work better on selected platforms (say, C11, POSIX and Windows) where we know how to access sub-second timing information. (Or alternatively, have a hard dependency on C11-or-Posix-or-Windows.) For the record, mbedtls_ssl_conf_handshake_timeout() accepts timeout values in milliseconds, but recommended values for use over the general internet start at 1 second: https://tools.ietf.org/html/rfc6347#section-4.2.4.1 So it might make sense to provide this module even when we only have second resolution - we'd just have to work out how the timer function would behave when passed sub-second values. (My first thought it rounding up to the next second would be quite OK.) What do you'all think? Personnally, I don't have a strong opinion between the three following options, though I have a slight preference for the first one: 1. Provide ssl_dtls_timer.c in the library with hard dep on HAVE_TIME and enhanced features with C11/Posix/Windows. 2. Provide ssl_dtls_timer.c in the library with hard dep on C11/Posix/Windows. 3. Move it all out of the library and let the thing live somewhere under programs/ as it's only used by example/testing programs. Note though that we could also choose to go with option 3 for Mbed TLS 3.0, see how it goes, and later switch to option 2 or 1 if we want, as adding modules can be done at any time. (In the same vein, we could start with 2 and switch to 1 at any time as well.) Regards, Manuel.

5 years, 5 months

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Re: [mbed-tls] 3.0 plans - move timing.c out of the library

by Manuel Pegourie-Gonnard

Hi Torsten, [I'm assuming didn't mean to reply on the list and not just to me, so I'm replying on the list.] > On 20/04/2020 11:51, Manuel Pegourie-Gonnard via mbed-tls wrote: >> For reference, this module includes the following functions, which are used >> in the following places: >> >> - mbedtls_timing_hardclock() implements cycle counting (on selected >> architectures, not including M-class), used only in >> programs/test/benchmark.c > > I used mbedtls_timing_hardclock() for my own benchmarking experiments on > Linux and Windows, but couldn't get it to run on the target machine. So > I've use my own ms counter instead. So, it could be easily removed as > part of the library. > Good to know, thanks for sharing this data point. > Question: How do you want to do the benchmarking instead? With the same > code, but outside the library? > Yes, I was thinking that the functions that are only used in benchmark.c could be defined in benchmark.c, and the functions that are only used in udp_proxy.c could be defined in udp_proxy.c. (Or actually for the latter, we might want to start using them in ssl_client2.c and ssl_server2.c as well, as including millisecond timestamps in the debug output would be usefull to debug some issues. In that case, we'll probably put those functions in a new file, used by udp_proxy, ssl_client2 and ssl_server2 (just like query_config.c is currently used by ssl_client2, ssl_server2 and query_compile_time_config).) Regards, Manuel.

5 years, 5 months

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Re: [mbed-tls] Some thoughts towards mbed TLS 3.0

by Manuel Pegourie-Gonnard

Hi Torsten, > this will be a long mail. Sorry for that. > On the contrary, thank you so much for this extensive and well though-out feedback, that's very helpful! I'll try to complement Gilles's reply, and skip the points he already answered unless I have something to add, as I'm generally in full agreement with what he wrote. > 3. For certification and evaluation purposes I need some test vectors > for each crypto function on target. While I know about the > comprehensive self-test program I'm now talking about built-in > functions like mbedtls_sha512_self_test(), etc to be enabled with > #define MBEDTLS_SELF_TEST. > > These self-tests are very different in coverage. For SHA-384 and > SHA-512 they are fine, for HMAC-SHA-384 and HMAC-SHA-512 I couldn't > find any as well as for HKDF-HMAC-SHA-256 (in RFC 5869) or > HKDF-HMAC-SHA-384/512 (official test vectors difficult to find). > AES-CTR and AES-XTS are only tested with key length 128 bit, not with > 256 bit. AES-CCM is not tested with 256 bit and even for 128 bit, > the test vector from the standard NIST SP 800-38C with long > additional data is not used. > The builtin self-test for GCM is the best I've seen with mbedtls: > all three key lengths are tested as well as the IUF-interface and > the comfort function. Bravo! > Indeed so far we don't have clear guidelines on self-test functions, and we should try to have more consistency here. Since this is incremental, backwards-compatible improvements, it can be done at any time (unlike some API improvements that can only be done when preparing major versions). Contributions in this area are certainly welcome! > 7. While at Elliptic Curve Cryptography: I assume that some of you > know that projectives coordinates as outer interface to ECC are > dangerous, see David Naccache, Nigel P. Smart, Jacques Stern: > Projective Coordinates Leak, Eurocrypt 2004, pp. 257–267. Yes, and as it happens this was revisited recently by researchers, and brought back to our attention: https://tls.mbed.org/tech-updates/security-advisories/mbedtls-security-advi… > you have Jacobian coordinates, i.e. projective coordinates, as outer > interface. In the comment, its is noted that only the affine part is > used, but can this be assured? In all circumstances? > In practice, I'm pretty confident this is the case, because both ecp_normalize_jac() and ecp_normalize_mxz() set Z to 1, these functions are always called before returning data, and if we forgot to call those functions, we'd get incorrect X and Y coordinates and fail unit test and interop tests. But as a matter of principle, I agree that exposing Jacobian coordinates in the API was a poor decision. Fortunately, we already intended to start phasing out the existing ECP interface in 3.0 (to which extend exactly is still to be discussed), and in the future is should be fully removed in favour of the PSA Crypto API, which doesn't have this problem. > 8. In my personal opinion the definition > [...] > mbedtls_ecp_keypair; > > is dangerous. Why not differentiate between private and public key > and domain parameters? I agree, and I think it's a generally accepted opinion in the crypto implementation community that private and public keys should be clearly distinguished from one another, for example by using distinct types (in typed languages). This is by the way a long-standing problem that existed in the RSA module from the start, and when ECC was added it followed a similar pattern. Again, I think the transition of the PSA Crypto API is going to be the answer here, as it will provide much cleaner key management. > 9. Regarding ECC examples: I found it very difficult that there isn't > a single example with known test vectors as in the relevant crypto > standards, i.e. FIPS 186-4 and ANSI X9.62-2005, with raw public > keys. What I mean are (defined) curves, public key value Q=(Qx,Qy) > and known signature values r and s. In the example ecdsa.c you > generate your own key pair and read/write the signature in > serialized form. In the example programs/pkey/pk_sign.c and > pk_verify.c you use a higher interface pk.h and keys in PEM format. > > So, it took me a while for a program to verify (all) known answer > tests in the standards (old standards as ANSI X9.62 1998 have more > detailed known answer tests). One needs this interface with raw > public keys for example for CAVP tests, see The FIPS 186-4 Elliptic > Curve Digital Signature Algorithm Validation System (ECDSA2VS). > > 11. In the moment, there is no single known answer tests for ECDSA > (which could be activated with #define MBEDTLS_SELF_TEST). I > wouldn't say that you need an example for every curve and hash > combination, as it is done in ECDSA2VS CAVP, but one example for > one of the NIST curves and one for Curve25519 and - if I have a > wish free - one for Brainpool would be fine. And this would solve > #9 above. > I might be misunderstanding, but isn't the function ecdsa_prim_test_vectors() in tests/suites/test_suite_ecdsa.function close to what you're looking for? I mean, except for the fact that it's not in the self-test function, and that it's using data from RFCs rather than from NIST or ANSI? But I agree this is a bit messy and could be made cleaner. Also, if you ever feel like contributing your dream self-test function for ECDSA as PR, this will be a very welcome contribution! > 13. Feature request: Since it was irrelevant for my task (only > verification, no generation) I didn't have a detailed look a your > ECC side-channel countermeasures. But obviously you use the same > protected code for scalar multiplication in verify and sign, > right? Wouldn't it be possible to use Shamir's trick in > verification with fast unprotected multi-scalar multiplication. In > the moment, mbedtls_ecdsa_verify is a factor 4-5 slower than > mbedtls_ecdsa_sign, while OpenSSLs verify is faster than sign. > Yes, this was done to save both on code size and developer time, but I agree it's suboptimal for performance. As Gilles wrote, as a general rule we tend to favour code size and security/maintainability over performance, but in that instance if we provide an option for a verify-only ECDSA build (which I agree we should), then we're likely to reach smaller code size by having a standalone unprotected implementation of multi-scalar multiplication, so that would probably be a great option: better performance _and_ smaller code. Unfortunately we're already quite busy with other things, so this kind of optimisation will probably have to wait for a bit more, but we're taking good note of the suggestion and will try to implement it in the future. Again, thanks for this comprehensive and useful feedback. Regards, Manuel.

5 years, 5 months

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Re: [mbed-tls] 3.0 plans - move timing.c out of the library

by Torsten Schuetze

On 20/04/2020 11:51, Manuel Pegourie-Gonnard via mbed-tls wrote: > [ Context: https://developer.trustedfirmware.org/w/mbed-tls/tech-plans-3.0/ ] > > Currently the crypto library includes a module called timing.c, enabled by > default in config.h (but disabled by config.py baremetal), containing various > functions related to timing (not to be confused with the time abstration > present in the platorm module). > > In Mbed TLS 3.0, as part of our effort to clean up and minimize our API, > we're considering removing (parts of) this module from the library, and > moving its parts to a variety of other places, such as example programs (or > a library/file shared by them, like the current query_config mechanism). > > For reference, this module includes the following functions, which are used in > the following places: > > - mbedtls_timing_hardclock() implements cycle counting (on selected > architectures, not including M-class), used only in > programs/test/benchmark.c I used mbedtls_timing_hardclock() for my own benchmarking experiments on Linux and Windows, but couldn't get it to run on the target machine. So I've use my own ms counter instead. So, it could be easily removed as part of the library. Question: How do you want to do the benchmarking instead? With the same code, but outside the library? Torsten

5 years, 5 months

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3.0 plans - move net.c out of the library

by Manuel Pegourie-Gonnard

Hi all, [ Context: https://developer.trustedfirmware.org/w/mbed-tls/tech-plans-3.0/ ] Currently the SSL/TLS library includes a module called net_sockets.c (formerly net.c), enabled by default in config.h (but disabled by config.py baremetal), that contains some networking functions based on POSIX or Windows sockets, including functions suitable for use as I/O callbacks with our SSL/TLS modules. Those functions are used only in example/testing programs, but nowhere in the library itself. In Mbed TLS 3.0, as part of our effort to clean up and minimize our API, we're considering removing (parts of) this module from the library, and moving its parts to a variety of other places, such as example programs (or a library/file shared by them, like the current query_config mechanism). Reasons for removing this module from the library include: - overall the module is less portable and perhaps of lesser quality than the rest of the library - it's currently entirely untested on Windows (despite being very platform-specific) - there are often confusions about whether the module is meant to be a general-purpose networking library, or just provide basic support for our example and test, as well a simple prototypes - it's not our core area of expertise, let's do one thing and to it well Reasons for keeping (parts of) it in the library include: - having mbedtls_net_recv{,_timeout}() and mbedtls_net_send() available right in the library makes it easier to test and develop prototype - this module is listed as a component in our high-level design document [2], so perhaps removing it from the library can be seen as a bigger change? [2]: https://tls.mbed.org/high-level-design What do you think? Should we remove the entire module from the library, keep it all, or just keep some parts? In that case, which parts and where? Regards, Manuel.

5 years, 5 months

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3.0 plans - move timing.c out of the library

by Manuel Pegourie-Gonnard

[ Context: https://developer.trustedfirmware.org/w/mbed-tls/tech-plans-3.0/ ] Currently the crypto library includes a module called timing.c, enabled by default in config.h (but disabled by config.py baremetal), containing various functions related to timing (not to be confused with the time abstration present in the platorm module). In Mbed TLS 3.0, as part of our effort to clean up and minimize our API, we're considering removing (parts of) this module from the library, and moving its parts to a variety of other places, such as example programs (or a library/file shared by them, like the current query_config mechanism). For reference, this module includes the following functions, which are used in the following places: - mbedtls_timing_hardclock() implements cycle counting (on selected architectures, not including M-class), used only in programs/test/benchmark.c - mbedtls_timing_get_timer() is a millisecond counter (only for Unix and Windows), used only in programs/test/udp_proxy.c - mbedtls_set_alarm() is an abstraction of alarm() (only for Unix and Windows), used only in programs/test/benchmark.c - mbedtls_timing_set_delay() and mbedtls_timing_get_delay() are an example implementation (only for Unix and Windows) of timer callbacks for DTLS, only used in programs/ssl/*.c Reasons for removing this module from the library include: - overall the module is less portable and of lesser quality than the rest of the library - it's also very hard to test and we've lost a lot of time and energy to that, with very limited success - it doesn't really belong in libmbedcrypto - it's not our core area of expertise, let's do one thing and to it well Reasons for keeping (parts of) it in the library include: - for mbedtls_timing_set_delay() and mbedtls_timing_get_delay(): they're convenient to have for DTLS examples and prototyping, we could keep them in some place in libmbedtls (not libmbedcrypto). What do you think? Should we remove the entire module from the library, or keep some parts? In that case, which parts and where should they live? Regards, Manuel.

5 years, 5 months

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3.0 plans - move certs.c out of the library

by Manuel Pegourie-Gonnard

Hi all, In this new installment of "let's discuss ideas for Mbed TLS 3.0" [1]: should we move certs.c out of the library? [1]: https://developer.trustedfirmware.org/w/mbed-tls/tech-plans-3.0/ Currently the X.509 library includes test certificates and keys, which are enabled by default in config.h - `MBEDTLS_CERTS_C`. These are used in the following places: - in library/x509.c in mbedtls_x509_self_test() - in tests/suites/test_suite_ssl.function - in programs/fuzz and programs/ssl In Mbed TLS 3.0, as part of our effort to clean up and minimize our API, we'd like to remove certs.h and the certificates it contains from the library (except perhaps one static cert for mbedtls_x509_self_test()). Tests and example programs that need built-in certificates could still get them using any mechanism, included a file certs.c similar to the current one - but this file would no longer live in library or be included when building libmbedx509. Reasons include: - Including test certificates and keys in the library provides little value, as users will want to use their own certificates and keys anyway. - Shipping private keys as part of the library is bad practice, even under the `mbedtls_test_` sub-namespace. There's a slight risk that some users could use them for prototyping and then inadvertently keep using unsafe private keys in production. If you disagree or have concerns, please speak up! Also, if want to suggest ideas for where the replacement to library/certs.c should live or how it could be maintained, now's a good time as well! Regards, Manuel.

5 years, 5 months

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Session resumption + PSKs in Mbed TLS 3.X

by Hanno Becker

Hi all, I would like to bring up the following: (How) Shall the APIs for PSKs and TLS session resumption be modified in Mbed TLS 3.0? The motivation for this question is the consolidation of PSKs, ticket-based resumption and session-id-based resumption in TLS 1.3. Now, while there will be no TLS 1.3 support in Mbed TLS 3.0 as it stands, I think it would be beneficial to make the API amenable to TLS 1.3 as much as possible already now. Finding the right API here is non-trivial, and I'll describe first how the current API works and how TLS 1.2 and 1.3 differ, before making suggestions on how the Mbed TLS 3 API might look like. The mail is therefore quite long, but I hope you'll bear with me. # Session Resumption in Mbed TLS 2.X Recall how resumption works in Mbed TLS 2.X on the client: 1. Establish a connection as usual 2. Store established session for later re-use `mbedtls_ssl_session_save()` saves connection parameters relevant for session resumption in an instance of `mbedtls_ssl_session`. Optional: 2.1: Serialize session for offline storage 2.2: Deserialize session 3. Loading a stored session `mbedtls_ssl_session_load()` is called on the stored session after initialization and before running the handshake via `mbedtls_ssl_handshake()`. 4. Connect using stored session Run `mbedtls_ssl_handshake()` as usual. To my knowledge, the following is transparent to the user: * The means by which resumption is achieved, which can be either id- or ticket-based. * Whether the session was resumed or not - there is a silent fallback to a full handshake when session resumption is rejected by the server. On the server, ticket- and id-based resumption are configured independently as follows: - ID-based resumption is controlled via `mbedtls_ssl_conf_session_cache()` - Ticket-based resumption is controlled via `mbedtls_ssl_conf_session_tickets_cb()`. # PSKs in Mbed TLS 2.X Coming to PSKs: First, recall that, under the hood, resumed connections establish a master secret purely on the basis of a previous master secret, without resorting to asymmetric crypto. The same is true for PSK-based connections, and this observation is embodied in TLS 1.3 which, as mentioned, consolidates resumption and PSKs. In TLS 1.2, though, PSKs and session resumption are different, and consequently Mbed TLS handles them through a separate API `mbedtls_ssl_conf_psk()`, which takes the PSK identifier and the PSK itself. Side-question: Should this apply to the SSL configuration or the SSL context? It seems to me that somebody might want to run multiple PSK-based connections with different PSKs, but same configuration otherwise. At the moment, this would require a configuration per connection. Maybe this decision of what belongs at the level of the config could be thoroughly discussed independently? # PSKs in TLS 1.3 In TLS 1.3, all kinds of resumptions are hanlded ander the umbrella of PSKs: The client offers PSK identities in its ClientHello, the server picks one or leaves it, and the master secret for the connection is derived from the PSK associated with the PSK identity. Underlying this is some form of translation `PSK-Identity -> PSK` on the server which allows to derive the PSK from the PSK identity. How this mapping is realized is entirely implementation-specific, but it can take at least the following forms, giving back all forms of session resumption: - 'PSK Identity -> PSK' via database lookup: This subsumes classical PSKs and ID-based resumption, since the data necessary to resume a session is stored on the server. - 'PSK Identity -> PSK' via AEAD: The PSK can be encoded in the PSK identity itself by using some authenticated encryption of the PSK as the PSK identity, the key for which resides on the server. This subsumes Ticket-based resumption. There are the following noteworthy differences, though: 1. In TLS 1.2, the server provides a 'PSK identity hint' and the client picks a particular PSK identity. In TLS 1.3, the client offers a _list_ of PSKs, and the server picks one or none. 2. In TLS 1.2, the server can issue only a single session ticket. In TLS 1.3, the server can issue as many session tickets as he wishes, and the client can use any of those for later session resumption. This has the following consequences for the API: 1. The PSK-configuration API must allow the configuration of multiple PSKs on the client. 2. There must be a means to export more than one session ticket for later resumption. # API considerations for Mbed TLS 3.X ## Part 1: Load/Save API In essence, keep the flow of `mbedtls_ssl_session_load/save()`, but make the following adjustments. Whether the target structure remains `mbedtls_ssl_session` and how it looks like internally isn't relevant at this point and is up to discussion. 1. It must be possible 'save' a session multiple times, which in TLS 1.3 amounts to exporting the tickets issued by the server one-by-one. This can be realized in a variety of ways, for example: - Allow calling `mbedtls_ssl_session_save()` multiple times, and modify its semantics from the idempotent nature of Mbed TLS 2.X to iterating through the available tickets, and returning an error when no more tickets are available. This has the benefit that it likely requires no code change for users migrating from Mbed TLS 2.X, since a change is needed only when an application stores a single session multiple times, which could be done by calling `mbedtls_ssl_session_save()` multiple times before, while now one would need to call it once and manually replicate the obtained session structure. On the negative side, this wastes memory for applications not interested in session resumption, or those that only need a single ticket. We don't want the server to be able to flood the client with arbitrarily many tickets. - Allow registering a callback that's triggered whenever a new ticket arrives. The drawback of this is that it is larger deviation from the Mbed TLS 2.X API. On the positive side, applications that don't need tickets, or only need a limited number of such, can simply drop further tickets and thereby prevent allocating RAM for them. 2. It must be possible to 'load' multiple sessions, meaning that the client should offer any of them to the server for resumption. ## Part 2: Consolidation with PSK Make it possible to setup an `mbedtls_ssl_session` (or whatever it will be called) structure manually from an externally PSK provisioned PSK, and allow loading this session for resumption as above, via one of potentially many calls to `mbedtls_ssl_session_load()`. There are multiple variants to this, for example: 1. Make `mbedtls_ssl_session` transparent so users can build it manually by filling the respective fields. 2. Provide a helper API to build a session from a PSK. The old API `mbedtls_ssl_conf_psk()` can then be _derived_ from this for convenience, building a session from the provided PSK and loading it, in a single API. Before going into further details and making the mail even longer: What do you think of this approach? Do you see alternatives? Best, Hanno IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.

5 years, 5 months

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Re: [mbed-tls] Some thoughts towards mbed TLS 3.0

by Gilles Peskine

Wow! First, thanks for this extensive feedback! That's very helpful, and appreciated. I'm going to reply to a few points. On a general note, you can see headlines for the main topics we're currently planning to work on in the Mbed TLS roadmap at https://developer.trustedfirmware.org/w/mbed-tls/roadmap/ . Please note that everything I write is based on current plans, which may change through the TrustedFirmware planning process or if real life shows that a current plan is not doable. Over time, we're transitioning the API for the crypto part of the library from the current mbedtls_xxx functions to psa_xxx, which have a somewhat different philosophy: less exposure of internals, more protection against misuse, no reliance on malloc. Mbed TLS 3.0 will start the transition. On 14/04/2020 21:10, Torsten Schuetze via mbed-tls wrote 1. I really missed an Initialize, Update, Finalize (IUF) interface for CCM. For GCM, we have mbedtls_gcm_init(), mbedtls_gcm_setkey(), mbedtls_gcm_starts(), mbedtls_gcm_update() iterated, mbedtls_gcm_finish(), mbedtls_gcm_free() or the comfort functions mbedtls_gcm_crypt_and_tag() and mbedtls_gcm_auth_decrypt(). For CCM, only mbedtls_ccm_init(), mbedtls_ccm_setkey(), mbedtls_ccm_encrypt_and_tag() or mbedtls_ccm_auth_decrypt() and mbedtls_ccm_free(). With this interface it was only possible to encrypt and tag 128 kByte on my target system, while with GCM I could encrypt much larger files. see Github issue #662 and my comment there In Mbed TLS 3.0, mbedtls_ccm_xxx() will not be a public interface anymore. We are planning to add support for multipart CCM in the psa_aead_xxx() interface (the prototypes are already in psa/crypto.h but their implementation is planned for some time in the next few months). We are not currently planning to add support for multipart CCM through mbedtls_cipher_xxx(), which in Mbed TLS 3 will be legacy functions. However, we would probably accept such support if it was contributed externally. 2. The next step, of course, is to integrate this into the higher mbedtls_cipher layer. Regarding higher, abstract layers: I often didn't understand which interface I was supposed to use. In general, I like to use the lowest available interface, for example, #include "mbedtls/sha512.h" when I want to use sha512. However, if I need HMAC-SHA-512 or HKDF-HMAC-SHA-512 then I have to use the interface in md.h. For hash functions this is fine. Almost all hash functions are supported via md.h. (I missed SHA-512/256 which is sometimes preferable to SHA-256 on 64bit systems). But with cipher.h, I can only access Chacha20Poly1305 and AES-GCM, not AES-CCM. In Mbed TLS 3, there will generally be a single public layer. Exposing lower layers helps with code size on resource-constrained devices, but it also has downsides, including locking down the APIs. 4. That I couldn't configure AES-256 only, i.e. without AES-128 and AES-192, was to be expected (and the code overhead is not that much). But in modern modes of operations nobody needs AES decryption, only the forward direction. Sometimes modern publications as Schwabe/Stoffelen "All the AES you need on Cortex-M3 and M4" provide only the forward direction. So, it would be fine if one could configure an AES (ECB) encryption only without decryption. Of course, this is only possible if we don't use CBC mode, etc. This wouldn't only save the AES decryption code but also the rather large T-tables for decryption. For information, there is a branch of Mbed TLS called "baremetal", forked from Mbed TLS 2.16, which you can find on GitHub: https://github.com/ARMmbed/mbedtls/blob/baremetal . This branch is optimized for small code size, sometimes at the expense of speed and often at the expense of features. It has build options MBEDTLS_AES_ONLY_ENCRYPT and MBEDTLS_AES_ONLY_128_BIT_KEY_LENGTH. However I would not recommend using it in production because Arm (who still maintain this branch even after Mbed TLS itself has moved to TrustedFirmware) does not make any promise of stability. A feature that you rely on may be removed without notice. I mention this branch because eventually, we do plan to port the improvements that don't sacrifice features to the Mbed TLS development branch. I can't give a timeline for this however. With the current Mbed TLS, if you don't use CBC, I think you can save some code in aes.o by defining MBEDTLS_AES_DECRYPT_ALT and MBEDTLS_AES_SETKEY_DEC_ALT and providing functions mbedtls_aes_setkey_dec() and mbedtls_internal_aes_decrypt() that do nothing. 5. Regarding AES or better the AES context-type definition [snip] 6. In general, the contexts of mbedTLS are rather full of implementation specific details. Most extreme is mbedtls_ecp_group in ecp.h. Wouldn't it be clearer if one separates the standard things (domain parameters in this case) from implementation specific details? As a general design principle, context types in Mbed TLS 3 will be opaque. This will let us, for example, redesign mbedtls_aes_context and mbedtls_cipher_context. 9. Regarding ECC examples: I found it very difficult that there isn't a single example with known test vectors as in the relevant crypto standards, i.e. FIPS 186-4 and ANSI X9.62-2005, with raw public keys. What I mean are (defined) curves, public key value Q=(Qx,Qy) and known signature values r and s. In the example ecdsa.c you generate your own key pair and read/write the signature in serialized form. In the example programs/pkey/pk_sign.c and pk_verify.c you use a higher interface pk.h and keys in PEM format. So, it took me a while for a program to verify (all) known answer tests in the standards (old standards as ANSI X9.62 1998 have more detailed known answer tests). One needs this interface with raw public keys for example for CAVP tests, see The FIPS 186-4 Elliptic Curve Digital Signature Algorithm Validation System (ECDSA2VS). 11. In the moment, there is no single known answer tests for ECDSA (which could be activated with #define MBEDTLS_SELF_TEST). I wouldn't say that you need an example for every curve and hash combination, as it is done in ECDSA2VS CAVP, but one example for one of the NIST curves and one for Curve25519 and - if I have a wish free - one for Brainpool would be fine. And this would solve #9 above. I don't get the point here. ECDSA is randomized, so you can't have a known answer test. The test suite does have known answer tests for deterministic ECDSA. 10. While debugging mbedtls_ecdsa_verify() in my example program, I found out, that the ECDSA, ECC and MPI operations are very, let's say, nested. So, IMHO there is a lot of function call overhead and special cases. It would be interesting to see what's the performance impact of a clean, straight-forward mbedtls_ecdsa_verify without restartable code, etc. to the current one. As far as I remember, the refactoring done to add the restartable code had no measurable impact on performance. What does have a significant impact on performance is that the bignum module uses malloc all the time. We would like to completely rewrite bignum operations at some point during the 3.x series, not only for performance but also because its design makes it hard not to leak information through side channels. 12. Just a minor issue: I only needed ECDSA signature verification, therefore I only included MBEDTLS_ASN1_PARSE_C. But it is not possible to compile without MBEDTLS_ASN1_WRITE_C needed for ECDSA signature generation. I'm not sure if I've written it down anywhere, but I'd like to remove the dependency of ECDSA on ASN1 altogether. Parsing and writing a SEQUENCE of two INTEGERs can be done with ad hoc code. Likewise for what little ASN1 the RSA module uses. And then asn1*.o can move out of libmbedcrypto and libpsacrypto, and into libmbedx509 where it belongs. Having only signature verification would be useful, indeed. That may happen with the bignum rewrite I mentioned above, if signature verification ends up using some faster non-constant-time code (this is also relevant for #13). 14. Design question: In the moment, both GCM and CCM use their own implementation of CTR encryption which is very simple. But then we have mbedtls_aes_crypt_ctr() in aes.h which is very simple, too. Let's assume at one day we have a performance optimized CTR encryption (for example from Schwabe & Stoffelen) with all fancy stuff like counter-mode caching etc. Then this would have to be replaced at three places at minimum. While isn't the code at this point more modularized? Is this a dedicated design decision? Having a single implementation of CTR is on the PSA roadmap because if there's a hardware accelerator that does it, we want to use it everywhere it's relevant. In Mbed TLS (or more precisely in its ancestor PolarSSL, if not _its_ ancestor XySSL), there was a conscious design decision to make each .c file as independent from the others as possible, which explains why camellia_ctr is completely independent from aes_ctr. But I don't know why ccm and gcm reimplement ctr. Why do I find at so many places for( i = 0; i < 16; i++ ) y[i] ^= b[i]; instead of a fast 128-bit XOR macro with 32bit aligned data? As a programmer who doesn't write compilers, I think this is the right way to xor 16 bytes, and it's the compiler's job to optimize it to word or vector operations if possible. Admittedly this does mean the compiler has to know that the data is well-aligned, which can be hard to guarantee and easy to forget. So, that's it for the moment. I hope I could give some hints for the further development of mbedTLS. Feel free to discuss any of the above points. It's clear to me that we cannot have both: clear and simple to understand code and performance records. Right. Also maintainable code and minimal code size, because minimal code size comes from letting the application developer #ifdef out everything that they don't care about, but this is a nightmare to test. It's one of the topics we're thinking about for Mbed TLS 3 and beyond. In general, Mbed TLS is primarily targeted at embedded systems, and is likely to privilege 1. security (including side channel resistance) and 2. code size. This doesn't mean that we don't care about performance, just that it isn't our top priority. That being said, we also do have some code that's optimized for performance (without compromising security) and not code size: the library already includes X25519 from Project Everest (https://project-everest.github.io/) (turn it on with MBEDTLS_ECDH_VARIANT_EVEREST_ENABLED). This is code that's formally proven not only for functional correctness, but also for side channel resistance; the implementation has aggressive inlining which makes it very fast, but obviously also large in terms of code size. Hopefully other algorithms will follow soon. Ciao, Torsten Once again, thanks for the detailed feedback, and I hope we can improve Mbed TLS for everyone! -- Gilles Peskine Mbed TLS developer IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.

5 years, 5 months

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Re: [mbed-tls] 3.0 plans: remove memory_buffer_alloc.c from the code base

by Gilles Peskine

>From the perspective of an application developer whose platform doesn't have a native malloc, the advantage for Mbed TLS to include its own malloc is one less dependency to integrate (so one less source of potential incompatibilities, one less component to integrate into the build, one less set of parameters to configure, one less feed of security updates to keep up with, etc.). I expect that choosing the Mbed TLS allocator would give poorer performance and higher RAM consumption than choosing the best allocator that exists out there. But it gives better time to market, and potentially a better security posture (the highest-performance allocator may or may not turn out to be actively maintained). -- Gilles Peskine Mbed TLS developer On 16/04/2020 14:19, Janos Follath via mbed-tls wrote: > Hi, > > Thank you for sharing your view, it makes perfect sense. > > You mention that you would consider keeping the module internally for the sake of baremetal applications. How would this serve the user better than choosing a third party malloc implementation? (Assuming that there are better implementation out there than ours, which I haven't confirmed, but it is not hard to imagine that such exists.) > > Regards, > Janos > > > On 15/04/2020, 21:11, "mbed-tls on behalf of Gilles Peskine via mbed-tls" <mbed-tls-bounces(a)lists.trustedfirmware.org on behalf of mbed-tls(a)lists.trustedfirmware.org> wrote: > > On 09/04/2020 13:17, Janos Follath via mbed-tls wrote: > > Hi, > > > > Based on what I read on this thread it seems an accepted point of view that the toolchain provided C standard library implementations are less trusted than the toolchains themselves. Can somebody please help me understand the reasoning behind this distinction? > > Well, I disagree with this statement. I trust the toolchain to implement > the C standard library correctly, and with good performance for the > target platform. I do not want to provide my own implementation of > standard functions. > > I only trust the toolchain to be functionally correct. I don't know > about its security. I certainly can't rely on the toolchain to have > security characteristics that are not guaranteed by the language > definition. For example, I do trust memset_s() to zero out sensitive > memory if the toolchain provides it, but I don't trust memset() for this > task. > > On the topic at hand, my personal opinion of memory_buffer_alloc is that > it doesn't belong in Mbed TLS. I hope that when PSA crypto is a > standalone product, it won't use malloc internally, and so it certainly > won't provide a malloc implementation. I wouldn't necessarily say the > same thing of Mbed TLS 4.0: it's difficult to design an X.509 interface > that doesn't use malloc. But if we can do it, I think we should. > > I recognize that there are many bare-metal applications that don't use > malloc themselves, but use Mbed TLS. For their sake, it does make some > sense for Mbed TLS to have its own malloc implementation. But the focus > is for internal use, not on serving as a general-purpose allocator for > applications that also use malloc for non-mbedtls-related purposes. > > -- > Gilles Peskine > Mbed TLS developer > > > IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you. > -- > mbed-tls mailing list > mbed-tls(a)lists.trustedfirmware.org > https://lists.trustedfirmware.org/mailman/listinfo/mbed-tls > > IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.

5 years, 5 months

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Re: [mbed-tls] 3.0 plans: remove memory_buffer_alloc.c from the code base

by Janos Follath

Hi, Thank you for sharing your view, it makes perfect sense. You mention that you would consider keeping the module internally for the sake of baremetal applications. How would this serve the user better than choosing a third party malloc implementation? (Assuming that there are better implementation out there than ours, which I haven't confirmed, but it is not hard to imagine that such exists.) Regards, Janos On 15/04/2020, 21:11, "mbed-tls on behalf of Gilles Peskine via mbed-tls" <mbed-tls-bounces(a)lists.trustedfirmware.org on behalf of mbed-tls(a)lists.trustedfirmware.org> wrote: On 09/04/2020 13:17, Janos Follath via mbed-tls wrote: > Hi, > > Based on what I read on this thread it seems an accepted point of view that the toolchain provided C standard library implementations are less trusted than the toolchains themselves. Can somebody please help me understand the reasoning behind this distinction? Well, I disagree with this statement. I trust the toolchain to implement the C standard library correctly, and with good performance for the target platform. I do not want to provide my own implementation of standard functions. I only trust the toolchain to be functionally correct. I don't know about its security. I certainly can't rely on the toolchain to have security characteristics that are not guaranteed by the language definition. For example, I do trust memset_s() to zero out sensitive memory if the toolchain provides it, but I don't trust memset() for this task. On the topic at hand, my personal opinion of memory_buffer_alloc is that it doesn't belong in Mbed TLS. I hope that when PSA crypto is a standalone product, it won't use malloc internally, and so it certainly won't provide a malloc implementation. I wouldn't necessarily say the same thing of Mbed TLS 4.0: it's difficult to design an X.509 interface that doesn't use malloc. But if we can do it, I think we should. I recognize that there are many bare-metal applications that don't use malloc themselves, but use Mbed TLS. For their sake, it does make some sense for Mbed TLS to have its own malloc implementation. But the focus is for internal use, not on serving as a general-purpose allocator for applications that also use malloc for non-mbedtls-related purposes. -- Gilles Peskine Mbed TLS developer IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you. -- mbed-tls mailing list mbed-tls(a)lists.trustedfirmware.org https://lists.trustedfirmware.org/mailman/listinfo/mbed-tls

5 years, 5 months

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Re: [mbed-tls] 3.0 plans: remove pkcs11.c from the code base

by Manuel Pegourie-Gonnard

Hi Gilles, Thanks for this precision, as well as the reminder of the existing github issue. With no opposition here in a week, and none on github for nearly 3 months either, plus no opposition when we marked it as deprecated in 2.21 in February, I'm also moving this to the "Accepted" section on the wiki discussion tracking page: https://developer.trustedfirmware.org/w/mbed-tls/tech-plans-3.0/ - but again, it's still time to voice concerns until we've actually removed it from the code base. Regards, Manuel.

5 years, 5 months

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