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diff --git a/src/libs/mynewt-nimble/ext/tinycrypt/documentation/tinycrypt.rst b/src/libs/mynewt-nimble/ext/tinycrypt/documentation/tinycrypt.rst new file mode 100644 index 00000000..356c099a --- /dev/null +++ b/src/libs/mynewt-nimble/ext/tinycrypt/documentation/tinycrypt.rst @@ -0,0 +1,352 @@ + +TinyCrypt Cryptographic Library +############################### +Copyright (C) 2017 by Intel Corporation, All Rights Reserved. + +Overview +******** +The TinyCrypt Library provides an implementation for targeting constrained devices +with a minimal set of standard cryptography primitives, as listed below. To better +serve applications targeting constrained devices, TinyCrypt implementations differ +from the standard specifications (see the Important Remarks section for some +important differences). Certain cryptographic primitives depend on other +primitives, as mentioned in the list below. + +Aside from the Important Remarks section below, valuable information on the usage, +security and technicalities of each cryptographic primitive are found in the +corresponding header file. + +* SHA-256: + + * Type of primitive: Hash function. + * Standard Specification: NIST FIPS PUB 180-4. + * Requires: -- + +* HMAC-SHA256: + + * Type of primitive: Message authentication code. + * Standard Specification: RFC 2104. + * Requires: SHA-256 + +* HMAC-PRNG: + + * Type of primitive: Pseudo-random number generator (256-bit strength). + * Standard Specification: NIST SP 800-90A. + * Requires: SHA-256 and HMAC-SHA256. + +* AES-128: + + * Type of primitive: Block cipher. + * Standard Specification: NIST FIPS PUB 197. + * Requires: -- + +* AES-CBC mode: + + * Type of primitive: Encryption mode of operation. + * Standard Specification: NIST SP 800-38A. + * Requires: AES-128. + +* AES-CTR mode: + + * Type of primitive: Encryption mode of operation. + * Standard Specification: NIST SP 800-38A. + * Requires: AES-128. + +* AES-CMAC mode: + + * Type of primitive: Message authentication code. + * Standard Specification: NIST SP 800-38B. + * Requires: AES-128. + +* AES-CCM mode: + + * Type of primitive: Authenticated encryption. + * Standard Specification: NIST SP 800-38C. + * Requires: AES-128. + +* CTR-PRNG: + + * Type of primitive: Pseudo-random number generator (128-bit strength). + * Standard Specification: NIST SP 800-90A. + * Requires: AES-128. + +* ECC-DH: + + * Type of primitive: Key exchange based on curve NIST p-256. + * Standard Specification: RFC 6090. + * Requires: ECC auxiliary functions (ecc.h/c). + +* ECC-DSA: + + * Type of primitive: Digital signature based on curve NIST p-256. + * Standard Specification: RFC 6090. + * Requires: ECC auxiliary functions (ecc.h/c). + +Design Goals +************ + +* Minimize the code size of each cryptographic primitive. This means minimize + the size of a platform-independent implementation, as presented in TinyCrypt. + Note that various applications may require further features, optimizations with + respect to other metrics and countermeasures for particular threats. These + peculiarities would increase the code size and thus are not considered here. + +* Minimize the dependencies among the cryptographic primitives. This means + that it is unnecessary to build and allocate object code for more primitives + than the ones strictly required by the intended application. In other words, + one can select and compile only the primitives required by the application. + + +Important Remarks +***************** + +The cryptographic implementations in TinyCrypt library have some limitations. +Some of these limitations are inherent to the cryptographic primitives +themselves, while others are specific to TinyCrypt. These limitations were accepted +in order to meet its design goals (in special, minimal code size) and to better +serve applications targeting constrained devices in general. Some of these +limitations are discussed in-depth below. + +General Remarks +*************** + +* TinyCrypt does **not** intend to be fully side-channel resistant. Due to the + variety of side-channel attacks, many of them only relevant to certain + platforms. In this sense, instead of penalizing all library users with + side-channel countermeasures such as increasing the overall code size, + TinyCrypt only implements certain generic timing-attack countermeasures. + +Specific Remarks +**************** + +* SHA-256: + + * The number of bits_hashed in the state is not checked for overflow. Note + however that this will only be a problem if you intend to hash more than + 2^64 bits, which is an extremely large window. + +* HMAC: + + * The HMAC verification process is assumed to be performed by the application. + This compares the computed tag with some given tag. + Note that conventional memory-comparison methods (such as memcmp function) + might be vulnerable to timing attacks; thus be sure to use a constant-time + memory comparison function (such as compare_constant_time + function provided in lib/utils.c). + + * The tc_hmac_final function, responsible for computing the message tag, + cleans the state context before exiting. Thus, applications do not need to + clean the TCHmacState_t ctx after calling tc_hmac_final. This should not + be changed in future versions of the library as there are applications + currently relying on this good-practice/feature of TinyCrypt. + +* HMAC-PRNG: + + * Before using HMAC-PRNG, you *must* find an entropy source to produce a seed. + PRNGs only stretch the seed into a seemingly random output of arbitrary + length. The security of the output is exactly equal to the + unpredictability of the seed. + + * NIST SP 800-90A requires three items as seed material in the initialization + step: entropy seed, personalization and a nonce (which is not implemented). + TinyCrypt requires the personalization byte array and automatically creates + the entropy seed using a mandatory call to the re-seed function. + +* AES-128: + + * The current implementation does not support other key-lengths (such as 256 + bits). Note that if you need AES-256, it doesn't sound as though your + application is running in a constrained environment. AES-256 requires keys + twice the size as for AES-128, and the key schedule is 40% larger. + +* CTR mode: + + * The AES-CTR mode limits the size of a data message they encrypt to 2^32 + blocks. If you need to encrypt larger data sets, your application would + need to replace the key after 2^32 block encryptions. + +* CTR-PRNG: + + * Before using CTR-PRNG, you *must* find an entropy source to produce a seed. + PRNGs only stretch the seed into a seemingly random output of arbitrary + length. The security of the output is exactly equal to the + unpredictability of the seed. + +* CBC mode: + + * TinyCrypt CBC decryption assumes that the iv and the ciphertext are + contiguous (as produced by TinyCrypt CBC encryption). This allows for a + very efficient decryption algorithm that would not otherwise be possible. + +* CMAC mode: + + * AES128-CMAC mode of operation offers 64 bits of security against collision + attacks. Note however that an external attacker cannot generate the tags + him/herself without knowing the MAC key. In this sense, to attack the + collision property of AES128-CMAC, an external attacker would need the + cooperation of the legal user to produce an exponentially high number of + tags (e.g. 2^64) to finally be able to look for collisions and benefit + from them. As an extra precaution, the current implementation allows to at + most 2^48 calls to tc_cmac_update function before re-calling tc_cmac_setup + (allowing a new key to be set), as suggested in Appendix B of SP 800-38B. + +* CCM mode: + + * There are a few tradeoffs for the selection of the parameters of CCM mode. + In special, there is a tradeoff between the maximum number of invocations + of CCM under a given key and the maximum payload length for those + invocations. Both things are related to the parameter 'q' of CCM mode. The + maximum number of invocations of CCM under a given key is determined by + the nonce size, which is: 15-q bytes. The maximum payload length for those + invocations is defined as 2^(8q) bytes. + + To achieve minimal code size, TinyCrypt CCM implementation fixes q = 2, + which is a quite reasonable choice for constrained applications. The + implications of this choice are: + + The nonce size is: 13 bytes. + + The maximum payload length is: 2^16 bytes = 65 KB. + + The mac size parameter is an important parameter to estimate the security + against collision attacks (that aim at finding different messages that + produce the same authentication tag). TinyCrypt CCM implementation + accepts any even integer between 4 and 16, as suggested in SP 800-38C. + + * TinyCrypt CCM implementation accepts associated data of any length between + 0 and (2^16 - 2^8) = 65280 bytes. + + * TinyCrypt CCM implementation accepts: + + * Both non-empty payload and associated data (it encrypts and + authenticates the payload and only authenticates the associated data); + + * Non-empty payload and empty associated data (it encrypts and + authenticates the payload); + + * Non-empty associated data and empty payload (it degenerates to an + authentication-only mode on the associated data). + + * RFC-3610, which also specifies CCM, presents a few relevant security + suggestions, such as: it is recommended for most applications to use a + mac size greater than 8. Besides, it is emphasized that the usage of the + same nonce for two different messages which are encrypted with the same + key obviously destroys the security properties of CCM mode. + +* ECC-DH and ECC-DSA: + + * TinyCrypt ECC implementation is based on micro-ecc (see + https://github.com/kmackay/micro-ecc). In the original micro-ecc + documentation, there is an important remark about the way integers are + represented: + + "Integer representation: To reduce code size, all large integers are + represented using little-endian words - so the least significant word is + first. You can use the 'ecc_bytes2native()' and 'ecc_native2bytes()' + functions to convert between the native integer representation and the + standardized octet representation." + + Note that the assumed bit layout is: {31, 30, ..., 0}, {63, 62, ..., 32}, + {95, 94, ..., 64}, {127, 126, ..., 96} for a very-long-integer (vli) + consisting of 4 unsigned integers (as an example). + + * A cryptographically-secure PRNG function must be set (using uECC_set_rng()) + before calling uECC_make_key() or uECC_sign(). + +Examples of Applications +************************ +It is possible to do useful cryptography with only the given small set of +primitives. With this list of primitives it becomes feasible to support a range +of cryptography usages: + + * Measurement of code, data structures, and other digital artifacts (SHA256); + + * Generate commitments (SHA256); + + * Construct keys (HMAC-SHA256); + + * Extract entropy from strings containing some randomness (HMAC-SHA256); + + * Construct random mappings (HMAC-SHA256); + + * Construct nonces and challenges (HMAC-PRNG, CTR-PRNG); + + * Authenticate using a shared secret (HMAC-SHA256); + + * Create an authenticated, replay-protected session (HMAC-SHA256 + HMAC-PRNG); + + * Authenticated encryption (AES-128 + AES-CCM); + + * Key-exchange (EC-DH); + + * Digital signature (EC-DSA); + +Test Vectors +************ + +The library provides a test program for each cryptographic primitive (see 'test' +folder). Besides illustrating how to use the primitives, these tests evaluate +the correctness of the implementations by checking the results against +well-known publicly validated test vectors. + +For the case of the HMAC-PRNG, due to the necessity of performing an extensive +battery test to produce meaningful conclusions, we suggest the user to evaluate +the unpredictability of the implementation by using the NIST Statistical Test +Suite (see References). + +For the case of the EC-DH and EC-DSA implementations, most of the test vectors +were obtained from the site of the NIST Cryptographic Algorithm Validation +Program (CAVP), see References. + +References +********** + +* `NIST FIPS PUB 180-4 (SHA-256)`_ + +.. _NIST FIPS PUB 180-4 (SHA-256): + http://csrc.nist.gov/publications/fips/fips180-4/fips-180-4.pdf + +* `NIST FIPS PUB 197 (AES-128)`_ + +.. _NIST FIPS PUB 197 (AES-128): + http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf + +* `NIST SP800-90A (HMAC-PRNG)`_ + +.. _NIST SP800-90A (HMAC-PRNG): + http://csrc.nist.gov/publications/nistpubs/800-90A/SP800-90A.pdf + +* `NIST SP 800-38A (AES-CBC and AES-CTR)`_ + +.. _NIST SP 800-38A (AES-CBC and AES-CTR): + http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf + +* `NIST SP 800-38B (AES-CMAC)`_ + +.. _NIST SP 800-38B (AES-CMAC): + http://csrc.nist.gov/publications/nistpubs/800-38B/SP_800-38B.pdf + +* `NIST SP 800-38C (AES-CCM)`_ + +.. _NIST SP 800-38C (AES-CCM): + http://csrc.nist.gov/publications/nistpubs/800-38C/SP800-38C_updated-July20_2007.pdf + +* `NIST Statistical Test Suite (useful for testing HMAC-PRNG)`_ + +.. _NIST Statistical Test Suite (useful for testing HMAC-PRNG): + http://csrc.nist.gov/groups/ST/toolkit/rng/documentation_software.html + +* `NIST Cryptographic Algorithm Validation Program (CAVP) site`_ + +.. _NIST Cryptographic Algorithm Validation Program (CAVP) site: + http://csrc.nist.gov/groups/STM/cavp/ + +* `RFC 2104 (HMAC-SHA256)`_ + +.. _RFC 2104 (HMAC-SHA256): + https://www.ietf.org/rfc/rfc2104.txt + +* `RFC 6090 (ECC-DH and ECC-DSA)`_ + +.. _RFC 6090 (ECC-DH and ECC-DSA): + https://www.ietf.org/rfc/rfc6090.txt |