# SHA-2

The SHA-256 and SHA-512 functions are provided for interoperability with other applications. If you are looking for a generic hash function and not specifically SHA-2, using [`crypto_generichash()`](https://libsodium.gitbook.io/doc/hashing/generic_hashing) (BLAKE2b) might be a better choice.

These functions are also not suitable for hashing passwords or deriving keys from passwords. Use one of the [password hashing](https://libsodium.gitbook.io/doc/password_hashing) APIs instead.

These functions are not keyed and are thus deterministic. In addition, the untruncated versions are vulnerable to length extension attacks.

A message can be hashed in a single pass, but a streaming API is also available to process a message as a sequence of multiple chunks.

## Single-part SHA-256 example

```c
#define MESSAGE ((const unsigned char *) "test")
#define MESSAGE_LEN 4

unsigned char out[crypto_hash_sha256_BYTES];

crypto_hash_sha256(out, MESSAGE, MESSAGE_LEN);
```

## Multi-part SHA-256 example

```c
#define MESSAGE_PART1 \
    ((const unsigned char *) "Arbitrary data to hash")
#define MESSAGE_PART1_LEN 22

#define MESSAGE_PART2 \
    ((const unsigned char *) "is longer than expected")
#define MESSAGE_PART2_LEN 23

unsigned char out[crypto_hash_sha256_BYTES];
crypto_hash_sha256_state state;

crypto_hash_sha256_init(&state);

crypto_hash_sha256_update(&state, MESSAGE_PART1, MESSAGE_PART1_LEN);
crypto_hash_sha256_update(&state, MESSAGE_PART2, MESSAGE_PART2_LEN);

crypto_hash_sha256_final(&state, out);
```

## Usage

### SHA-256

Single-part:

```c
int crypto_hash_sha256(unsigned char *out, const unsigned char *in,
                       unsigned long long inlen);
```

Multi-part:

```c
int crypto_hash_sha256_init(crypto_hash_sha256_state *state);

int crypto_hash_sha256_update(crypto_hash_sha256_state *state,
                              const unsigned char *in,
                              unsigned long long inlen);

int crypto_hash_sha256_final(crypto_hash_sha256_state *state,
                             unsigned char *out);
```

### SHA-512

Single-part:

```c
int crypto_hash_sha512(unsigned char *out, const unsigned char *in,
                       unsigned long long inlen);
```

Multi-part:

```c
int crypto_hash_sha512_init(crypto_hash_sha512_state *state);

int crypto_hash_sha512_update(crypto_hash_sha512_state *state,
                              const unsigned char *in,
                              unsigned long long inlen);

int crypto_hash_sha512_final(crypto_hash_sha512_state *state,
                             unsigned char *out);
```

## Notes

The state must be initialized with `crypto_hash_sha*_init()` before updating or finalizing it.

After `crypto_hash_sha*_final()`, the state should not be used any more, unless it is reinitialized using `crypto_hash_sha*_init()`.

SHA-512-256 is also available via the higher-level interface `crypto_hash()`.

As an alternative to `crypto_auth_hmac*()` or `crypto_generichash()`, the SHA-256 and SHA-512 hash functions [can be keyed](https://tosc.iacr.org/index.php/ToSC/article/view/12072/11913), but the key must be placed after the message, after having padded the message to the block size (64 bytes for SHA-256, 128 bytes for SHA-512). Using the key as a prefix, rather than a suffix would allow for length extension attacks.

## Constants

* `crypto_hash_sha256_BYTES`
* `crypto_hash_sha512_BYTES`

## Data types

* `crypto_hash_sha256_state`
* `crypto_hash_sha512_state`


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