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.

License

CC0.

Please attribute the author. This is a request only, and not a license term.

Author

Joseph Birr-Pixton [email protected]