Source: sha256.h

sha256¶

SHA-256 as FIPS 180-4 defines it, carried in the tree rather than linked from a library.

Functions¶

Return	Name	Description
`void`	`sha256`	Hash `len` bytes at `data` into `out`, which must hold 32 bytes.
`void`	`sha256Hex`	Hash `len` bytes and write the first `outBytes` of the digest as lowercase hex into `out`, which must hold `outBytes * 2 + 1` characters.

sha256¶

void sha256(const void * data, size_t len, uint8_t out)

Hash len bytes at data into out, which must hold 32 bytes.

sha256Hex¶

void sha256Hex(const void * data, size_t len, char * out, size_t outBytes = 16)

Hash len bytes and write the first outBytes of the digest as lowercase hex into out, which must hold outBytes * 2 + 1 characters.

Variables¶

Return	Name	Description
`constexpr size_t`	`kSha256DigestSize` `constexpr`	Digest length in bytes.

kSha256DigestSize¶

constexpr

constexpr size_t kSha256DigestSize = 32

Digest length in bytes.

More info¶

Why a copy rather than a library¶

This is the only cryptographic primitive MoonLight needs, and it needs it on all five targets. OpenSSL does not exist on ESP32, where ESP-IDF ships mbedtls, so linking it would mean an mbedtls path for devices and an OpenSSL path for the desktop. That is two implementations of one function that must agree byte for byte, and a divergence produces identifiers that silently differ between platforms. One vendored file is the smaller thing to own, and the algorithm is frozen. FIPS 180-4 has not changed since 2015 and its test vectors are published, so unit_sha256.cpp pins this against them.

What it is not¶

The one use today is the MoonStats installation id. This is no general-purpose crypto layer: no HMAC, no streaming over a socket, no constant-time comparison. Nothing here needs them, and an unused primitive is a maintenance cost with no user.

Why one shot and why truncation is safe¶

sha256 takes a whole buffer instead of the usual init, update and final trio. Every caller hashes a short buffer already in memory, so the streaming form would be three functions nobody calls with more than one update.

sha256Hex truncates, which is the standard way to derive a shorter identifier (NIST SP 800-107 section 5.1). At the default of 16 bytes it leaves 128 bits, far past any collision concern for a population of LED controllers.