Cifra

Cifra is a collection of cryptographic primitives targeted at embedded use.

Aims

In order of descending emphasis, cifra aims for:

• Clarity and simplicity.
• Countermeasures for side channel leaks inherent in some algorithms.
• Suitability for embedded use. Particularly: cifra uses an absolute minimum of the standard C library and is reasonably efficient with respect to code and data space.

Features

• AES in the GCM, CCM, EAX and OCB3 authenticated encryption modes.
• NORX authenticated encryption system.
• SHA224, SHA256, SHA384 and SHA512 hash functions (including HMAC and PBKDF2).
• SHA3-224, SHA3-256, SHA3-384, SHA3-512 hash functions (FIPS 202 compatible).
• ChaCha20 and Salsa20 stream ciphers.
• Poly1305 one time MAC.
• 100% code coverage by line, zero static analysis defects, valgrind-clean.

Additionally cifra imports curve25519 from elsewhere (μNaCl, NaCl, tweetNaCl, Adam Langley's curve25519-donna) for comparison between various implementations on embedded targets.

Documentation

Available at Read the Docs.

Testing

There is quite a lot of testing available:

• Host builds: run make test in the src directory. This builds and runs assorted test programs.
• Emulated embedded builds: run make test in the src/arm directory. This expects to find qemu-system-gnuarmeclipse on the path. These tests assume a Cortex-M3 target.
• Cortex-M0 on-target tests: run make test.stm32f0 in the src/arm directory. This expects to find openocd on the path, with an STM32F0xx attached via stlinkv2. It uses ARM semihosting to report results.
• Cortex-M3/4 on-target tests: run make test.stm32f1 or make test.stm32f3 as above.

Additionally all embedded targets expect to find the arm-none-eabi toolchain to be on the path.

Measurements

All measurements performed at -Os (optimise for space), on the following MCUs:

Core | Part number | Price (1s) | Max clock | Flash | SRAM ---------- | ------------- | ------------ | ---------- | ----- | ----- Cortex-M0 | STM32F030F4P6 | 1.17EUR | 48MHz | 16KB | 4KB Cortex-M3 | STM32F103C8T6 | 2.87EUR | 72MHz | 64KB | 20KB Cortex-M4F | STM32F303K6T6 | 4.53EUR | 72MHz | 32KB | 12KB

More measurements are available for AEAD modes on my blog post: Benchmarking Modern Authenticated Encryption on €1 devices.

AES

This test does a key schedule, then encrypts one block.

128-bit key

Core | Cycles (key schedule + block) | Cycles (key schedule) | Cycles (block) | Stack | Code size ---------- | ----------------------------- | --------------------- | -------------- | ----- | --------- Cortex-M0 | 7156 | 2147 | 5009 | 312B | 1020B
Cortex-M3 | 4692 | 1591 | 3101 | 300B | 960B
Cortex-M4F | 4591 | 1571 | 3020 | 300B | 960B

256-bit key

Core | Cycles (key schedule + block) | Cycles (key schedule) | Cycles (block) | Stack | Code size ---------- | ----------------------------- | --------------------- | -------------- | ----- | --------- Cortex-M0 | 10611 | 3650 | 6961 | 396B | 1100B
Cortex-M3 | 6735 | 2450 | 4285 | 380B | 1048B
Cortex-M4F | 6588 | 2416 | 4172 | 380B | 1048B

AES128-GCM

This test encrypts and authenticates a 16 byte message, with 16 bytes additionally authenticated data. It includes the initial key schedule.

Core | Cycles | Stack | Code size ---------- | ------ | ----- | --------- Cortex-M0 | 57022 | 796B | 2600B Cortex-M3 | 44306 | 812B | 2644B Cortex-M4F | 43657 | 812B | 2644B

AES128-EAX

This test encrypts and authenticates a 16 byte message, with 16 bytes additionally authenticated data. It includes the initial key schedule.

Core | Cycles | Stack | Code size ---------- | ------ | ----- | --------- Cortex-M0 | 50564 | 932B | 2836B Cortex-M3 | 32855 | 932B | 2780B Cortex-M4F | 32159 | 932B | 2780B

AES128-CCM

This test encrypts and authenticates a 16 byte message, with 16 bytes additionally authenticated data. It includes the initial key schedule.

Core | Cycles | Stack | Code size ---------- | ------ | ----- | --------- Cortex-M0 | 37677 | 772B | 2280B Cortex-M3 | 24462 | 780B | 2256B Cortex-M4F | 23949 | 780B | 2256B

NORX32

This test encrypts and authenticates a 16 byte message, with 16 bytes additionally authenticated data.

Core | Cycles | Stack | Code size ---------- | ------ | ----- | --------- Cortex-M0 | 10692 | 320B | 1636B Cortex-M3 | 6909 | 320B | 1820B Cortex-M4F | 6855 | 320B | 1820B

ChaCha20

This test encrypts a 64 byte message.

Core | Cycles | Stack | Code size ---------- | ------ | ----- | --------- Cortex-M0 | 5981 | 564B | 1416B Cortex-M3 | 3487 | 544B | 1328B Cortex-M4F | 3468 | 544B | 1328B

(For comparison with AES, add an AES256 key schedule plus 4 blocks. That's about 33K cycles.)

Salsa20

This test encrypts a 64 byte message.

Core | Cycles | Stack | Code size ---------- | ------ | ----- | --------- Cortex-M0 | 6173 | 560B | 1412B Cortex-M3 | 3320 | 552B | 1272B Cortex-M4F | 3311 | 552B | 1272B

SHA256

This test hashes the empty string (one compression function invocation).

Core | Cycles | Stack | Code size ---------- | ------ | ----- | --------- Cortex-M0 | 11561 | 312B | 1760B Cortex-M3 | 6530 | 300B | 1708B Cortex-M4F | 6278 | 300B | 1708B

SHA512

This test hashes the empty string (one compression function invocation).

Core | Cycles | Stack | Code size ---------- | ------ | ----- | --------- Cortex-M0 | 38447 | 796B | 2888B Cortex-M3 | 28771 | 836B | 2988B Cortex-M4F | 28777 | 836B | 2980B

SHA3-256

This test hashes the empty string (one sponge permutation).

Core | Cycles | Stack | Code size ---------- | ------ | ----- | --------- Cortex-M0 | 93648 | 848B | 2212B Cortex-M3 | 74321 | 856B | 2164B Cortex-M4F | 72215 | 856B | 2140B

SHA3-512

This test hashes the empty string (one sponge permutation).

Core | Cycles | Stack | Code size ---------- | ------ | ----- | --------- Cortex-M0 | 92565 | 880B | 2212B Cortex-M3 | 73509 | 888B | 2164B Cortex-M4F | 71419 | 888B | 2140B

HMAC-SHA256

This test computes a MAC with a 32 byte key over the message "hello world".

Core | Cycles | Stack | Code size ---------- | ------ | ----- | --------- Cortex-M0 | 48924 | 1328B | 2200B Cortex-M3 | 28333 | 1324B | 2132B Cortex-M4F | 27337 | 1324B | 2132B

Poly1305-AES

This test computes a MAC with a 32 byte key over the message "hello world". It includes the AES nonce processing.

Core | Cycles | Stack | Code size ---------- | ------ | ----- | --------- Cortex-M0 | 15719 | 704B | 1920B Cortex-M3 | 11328 | 696B | 1964B Cortex-M4F | 10706 | 696B | 1932B

Curve25519

This test is one point multiplication.

This uses the implementation from μNaCl by Düll, Haase, Hinterwälder, Hutter, Paar, Sánchez and Schwabe.

Core | Cycles | Stack | Code size ---------- | ------- | ----- | --------- Cortex-M0 | 4070271 | 464B | 5596B Cortex-M3 | 3720363 | 448B | 5536B Cortex-M4F | 3720105 | 448B | 5536B

See curve25519-shootout for comparitive measurements for other curve25519 implementations.

C library requirements

Cifra requires memcpy, memset, and abort.

Future

• ~~Keccak hash function (aka SHA3)~~.
• ~~Poly1305 one-time MAC~~.
• Constant time curve25519 for Cortex-M4F using the FPU.
• Constant time curve25519 for Cortex-M3 (avoiding the variable-time multiplier).

Notable past bugs/advisories

• Issue #2: in all versions before commit d62aa26e (September 16th 2015) too much padding was added when hashing messages of certain lengths. Specifically, messages whose length satisfied len % 64 = 55 for SHA1/SHA224/SHA256 or len % 128 = 119 for SHA384/SHA512. SHA3 was not affected. Better testing is now in place.
• Issue #3: in all versions before commit 82d77cd8 (April 16th 2016) EAX would produce wrong tags for empty AADs or messages. The underlying CMAC is now more resistant to this case.
• Issue #6: in all versions before commit b6cdf9f3 the chacha20poly1305 construction would produce incorrect tags for some message or AAD lengths. These case weren't covered by the standard test vectors, so extra tests have been added computed with libsodium.
• Issue #11: in all versions before commit df1e4130 our salsa20 used inc