FIPS.md

FIPS 140-2

BoringSSL as a whole is not FIPS validated. However, there is a core library (called BoringCrypto) that has been FIPS validated. This document contains some notes about the design of the FIPS module and some documentation on performing FIPS-related tasks. This is not a substitute for reading the offical Security Policy.

Please note that we cannot answer questions about FIPS, nor about using BoringSSL in a FIPS-compliant manner. Please consult with an accredited CMVP lab on these subjects.

Validations

BoringCrypto has undergone the following validations:

1. 2017-06-15: certificate #2964, security policy (in docx format).
2. 2018-07-30: certificate #3318, security policy (in docx format).
3. 2019-08-08: certificate #3678, security policy (in docx format).
4. 2019-10-20: certificate #3753, security policy (in docx format).
5. 2021-01-28: certificate #4156, security policy (in docx format).
6. 2021-04-29: certificate #4407.

Running ACVP tests

See util/fipstools/acvp/ACVP.md for details of how ACVP testing is done.

Breaking known-answer and continuous tests

Each known-answer test (KAT) uses a unique, random input value. util/fipstools/break-kat.go contains a listing of those values and can be used to corrupt a given test in a binary. Since changes to the KAT input values will invalidate the integrity test, BORINGSSL_FIPS_BREAK_TESTS can be defined in fips_break_tests.h to disable it for the purposes of testing.

Some FIPS tests cannot be broken by replacing a known string in the binary. For those, when BORINGSSL_FIPS_BREAK_TESTS is defined, the environment variable BORINGSSL_FIPS_BREAK_TEST can be set to one of a number of values in order to break the corresponding test:

1. RSA_PWCT
2. ECDSA_PWCT

Breaking the integrity test

The utility in util/fipstools/break-hash.go can be used to corrupt the FIPS module inside a binary and thus trigger a failure of the integrity test. Note that the binary must not be stripped, otherwise the utility will not be able to find the FIPS module.

RNG design

FIPS 140-2 requires that one of its PRNGs be used (which they call DRBGs). In BoringCrypto, we use CTR-DRBG with AES-256 exclusively and RAND_bytes (the primary interface for the rest of the system to get random data) takes its output from there.

The DRBG state is kept in a thread-local structure and is seeded using the configured entropy source.

The CTR-DRBG is reseeded every 4096 calls to RAND_bytes. Thus the process will randomly crash about every 2¹³⁵ calls.

The FIPS PRNGs allow “additional input” to be fed into a given call. We use this feature to be as robust as possible to state duplication from process forks and VM copies: for every call we read 32 bytes of “additional data” from the entropy source (without overread) which means that cloned states will diverge at the next call to RAND_bytes. This is called “prediction resistance” by FIPS, but we do not claim this property in a FIPS context because we don't implement it the way they want.

There is a second interface to the RNG which allows the caller to supply bytes that will be XORed into the generated additional data (RAND_bytes_with_additional_data). This is used in the ECDSA code to include the message and private key in the generation of k, the ECDSA nonce. This allows ECDSA to be robust to entropy failures while still following the FIPS rules.

FIPS requires that RNG state be zeroed when the process exits. In order to implement this, all per-thread RNG states are tracked in a linked list and a destructor function is included which clears them. In order for this to be safe in the presence of threads, a lock is used to stop all other threads from using the RNG once this process has begun. Thus the main thread exiting may cause other threads to deadlock, and drawing on entropy in a destructor function may also deadlock.

Entropy sources

By default, entropy is sourced using a "Passive" method where the specific entropy source depends on the OE. Seeding and reseeding material for the DRBG is sourced from the specific entropy source.

In FIPS mode, each of the entropy sources is subject to a 10× overread. That is, when n bytes of entropy are needed, 10n bytes will be read from the entropy source and XORed down to n bytes.

Modify entropy source - not recommended

Modifying the entropy source can invalidate your validation status. Changing the entropy is not recommended.

It is possible to modify the entropy method at build-time. Using ENABLE_FIPS_ENTROPY_CPU_JITTER=ON, the entropy source is switched to CPU Jitter.

CPU Jitter is less performant than the default method and can incur up to a 2-digit millisecond latency when queried. You can test CPU Jitter entropy on your system using bssl speed -filter Jitter.

Integrity Test

FIPS-140 mandates that a module calculate an HMAC of its own code in a constructor function and compare the result to a known-good value. Typical code produced by a C compiler includes large numbers of relocations: places in the machine code where the linker needs to resolve and inject the final value of a symbolic expression. These relocations mean that the bytes that make up any specific bit of code generally aren't known until the final link has completed.

Additionally, because of shared libraries and ASLR, some relocations can only be resolved at run-time, and thus targets of those relocations vary even after the final link.

BoringCrypto is linked (often statically) into a large number of binaries. It would be a significant cost if each of these binaries had to be post-processed in order to calculate the known-good HMAC value. We would much prefer if the value could be calculated, once, when BoringCrypto itself is compiled.

In order for the value to be calculated before the final link, there can be no relocations in the hashed code and data. This document describes how we build C and assembly code in order to produce a binary file containing all the code and data for the FIPS module without that code having any relocations.

There are two build configurations supported: static and shared. The shared build produces libcrypto.so, which includes the FIPS module and is significantly more straightforward and so is described first:

Linux Shared build

First, all the C source files for the module are compiled as a single unit by compiling a single source file that #includes them all (this is bcm.c). This, along with some assembly sources, comprise the FIPS module.

The object files resulting from compiling (or assembling) those files is linked in partial-linking mode with a linker script that causes the linker to insert symbols marking the beginning and end of the text and rodata sections. The linker script also discards other types of data sections to ensure that no unhashed data is used by the module.

One source of such data are rel.ro sections, which contain data that includes function pointers. Since these function pointers are absolute, they are written by the dynamic linker at run-time and so we must eliminate them. The pattern that causes them is when we have a static EVP_MD or EVP_CIPHER object thus, inside the module, this pattern is changed to instead reserve space in the BSS for the object, and to add a CRYPTO_once_t to protect its initialisation.

Once the partially-linked result is linked again, with other parts of libcrypto, to produce libcrypto.so, the contents of the module are fixed, as required. The module code uses the linker-added symbols to find the its code and data at run-time and hashes them upon initialisation. The result is compared against a value stored inside libcrypto.so, but outside of the module. That value will, initially, be incorrect, but inject-hash.go can inject the correct value.

Windows Shared build

The Shared Windows FIPS integrity test differs in two key ways:

1. How the start and end of the module are marked
2. How the correct integrity hash is calculated

Microsoft Visual C compiler (MSVC) does not support linker scripts which add symbols to mark the start and end of the text and rodata sections on Linux. Instead, fips_shared_library_marker.c is compiled twice to generate two object files that contain start/end functions and variables. MSVC pragma segment definitions are used to place the markers in specific sections (e.g. .fipstx$a). This particular name format uses Portable Executable Grouped Sections to control what section the code is placed in and the order within the section. With the start and end markers placed at $a and $z respectively, BCM puts everything in the $b section. When the final crypto.dll is built all the code is in the .fipstx section, all data is in .fipsda, all constants are in .fipsco, all unitialized items in .fipsbs, and everything is in the correct order.

The process to generate the expected integrity fingerprint is also different from Linux:

1. Build the required object files once: bcm.obj from bcm.c and the start/end object files
   1. bcm.obj places the power-on self tests in the .CRT$XCU section which is run automatically by the Windows Common Runtime library (CRT) startup code
2. Use MSVC's lib.exe to combine the start/end object files with bcm.obj to create the static library bcm.lib.
   1. MSVC does not support combining multiple object files into another object file like the Apple build.
3. Build fipsmodule which contains the placeholder integrity hash
4. Build precrypto.dll with bcm.obj and fipsmodule
5. Build the small application fips_empty_main.exe and link it with precrypto.dll
6. capture-hash.go runs fips_empty_main.exe
   1. The CRT runs all functions in the .CRT$XC* sections in order starting with .CRT$XCA
   2. The BCM power-on tests are in .CRT$XCU and are run after all other Windows initialization is complete
   3. BCM calculates the correct integrity value which will not match the placeholder value. Before aborting the process the correct value is printed
   4. capture-hash.go reads the correct integrity value and writes it to generated_fips_shared_support.c
7. generated_fipsmodule is built with generated_fips_shared_support.c
8. crypto.dll is built with the same original bcm.lib and generated_fipsmodule

Static build

The static build cannot depend on the shared-object link to resolve relocations and thus must take another path.

As with the shared build, all the C sources are build in a single compilation unit. The -fPIC flag is used to cause the compiler to use IP-relative addressing in many (but not all) cases. Also the -S flag is used to instruct the compiler to produce a textual assembly file rather than a binary object file.

The textual assembly file is then processed by a script to merge in assembly implementations of some primitives and to eliminate the remaining sources of relocations.

Redirector functions

The most obvious cause of relocations are out-calls from the module to non-cryptographic functions outside of the module. Most obviously these include malloc, memcpy and other libc functions, but also include calls to support code in BoringSSL such as functions for managing the error queue.

Offsets to these functions cannot be known until the final link because only the linker sees the object files containing them. Thus calls to these functions are rewritten into an IP-relative jump to a redirector function. The redirector functions contain a single jump instruction to the real function and are placed outside of the module and are thus not hashed (see diagram).

In this diagram, the integrity check hashes from module_start to module_end. Since this does not cover the jump to memcpy, it's fine that the linker will poke the final offset into that instruction.

Read-only data

Normally read-only data is placed in an .rodata segment that doesn't get mapped into memory with execute permissions. However, the offset of the data segment from the text segment is another thing that isn't determined until the final link. In order to fix data offsets before the link, read-only data is simply placed in the module's .text segment. This might make building ROP chains easier for an attacker, but so it goes.

Data containing function pointers remains an issue. The source-code changes described above for the shared build apply here too, but no direct references to a BSS section are possible because the offset to that section is not known at compile time. Instead, the script generates functions outside of the module that return pointers to these areas of memory—they effectively act like special-purpose malloc calls that cannot fail.

Read-write data

Mutable data is a problem. It cannot be in the text segment because the text segment is mapped read-only. If it's in a different segment then the code cannot reference it with a known, IP-relative offset because the segment layout is only fixed during the final link.

In order to allow this we use a similar design to the redirector functions: the code references a symbol that's in the text segment, but out of the module and thus not hashed. A relocation record is emitted to instruct the linker to poke the final offset to the variable in that location. Thus the only change needed is an extra indirection when loading the value.

Other transforms

The script performs a number of other transformations which are worth noting but do not warrant their own discussions:

1. It duplicates each global symbol with a local symbol that has _local_target appended to the name. References to the global symbols are rewritten to use these duplicates instead. Otherwise, although the generated code uses IP-relative references, relocations are emitted for global symbols in case they are overridden by a different object file during the link.
2. Various sections, notably .rodata, are moved to the .text section, inside the module, so module code may reference it without relocations.
3. For each BSS symbol, it generates a function named after that symbol but with _bss_get appended, which returns its address.
4. It inserts the labels that delimit the module's code and data (called module_start and module_end in the diagram above).
5. It adds a 64-byte, read-only array outside of the module to contain the known-good HMAC value.

Integrity testing

In order to actually implement the integrity test, a constructor function within the module calculates an HMAC from module_start to module_end using a fixed, all-zero key. It compares the result with the known-good value added (by the script) to the unhashed portion of the text segment. If they don't match, it calls exit in an infinite loop.

Initially the known-good value will be incorrect. Another script (inject_hash.go) calculates the correct value from the assembled object and injects it back into the object.

Comparison with OpenSSL's method

(This is based on reading OpenSSL's user guide and inspecting the code of OpenSSL FIPS 2.0.12.)

OpenSSL's solution to this problem is very similar to our shared build, with just a few differences:

1. OpenSSL deals with run-time relocations by not hashing parts of the module's data.
2. OpenSSL uses ld -r (the partial linking mode) to merge a number of object files into their fipscanister.o. For BoringCrypto's static build, we merge all the C source files by building a single C file that #includes all the others, and we merge the assembly sources by appending them to the assembly output from the C compiler.
3. OpenSSL depends on the link order and inserts two object files, fips_start.o and fips_end.o, in order to establish the module_start and module_end values. BoringCrypto adds labels at the correct places in the assembly for the static build, or uses a linker script for the shared build.
4. OpenSSL calculates the hash after the final link and either injects it into the binary or recompiles with the value of the hash passed in as a #define. BoringCrypto calculates it prior to the final link and injects it into the object file.
5. OpenSSL references read-write data directly, since it can know the offsets to it. BoringCrypto indirects these loads and stores.
6. OpenSSL doesn't run the power-on test until FIPS_module_mode_set is called. BoringCrypto does it in a constructor function. Failure of the test is non-fatal in OpenSSL, BoringCrypto will crash.
7. Since the contents of OpenSSL's module change between compilation and use, OpenSSL generates fipscanister.o.sha1 to check that the compiled object doesn't change before linking. Since BoringCrypto's module is fixed after compilation (in the static case), the final integrity check is unbroken through the linking process.

Some of the similarities are worth noting:

1. OpenSSL has all out-calls from the module indirecting via the PLT, which is equivalent to the redirector functions described above.