---
name: "libkeccak_hmac_fast_digest"
section: "3"
platform: "linux"
package: "libkeccak"
version: "1.4.3-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/libkeccak_hmac_fast_digest"
---

# libkeccak\_hmac\_fast\_digest(3)

Complete the HMAC-hashing of a message without erasure

## NAME

libkeccak\_hmac\_fast\_digest - Complete the HMAC-hashing of a message without erasure

## SYNOPSIS

```
#include <libkeccak.h>
int libkeccak_hmac_fast_digest(struct libkeccak_hmac_state *state, const void *msg, size_t msglen,
                               size_t bits, const char *suffix, void *hashsum);
```

Link with *-lkeccak*.

## DESCRIPTION

The **libkeccak\_hmac\_fast\_digest**() function absorbes the last part of (or all of) a message, and returns the HMAC hash of the entire message. The last part of the message is specified by the *msg* parameter, and its byte-size is specified by the *msglen* parameter. If all of the message has already be processed by calls to the [**libkeccak\_hmac\_update**(3)](/libkeccak_hmac_update/3.md) function or the [**libkeccak\_hmac\_fast\_update**(3)](/libkeccak_hmac_fast_update/3.md) function (with the same pointer on *state*,) *msg* and *msglen* should be set to *NULL* and 0, respectively.

If the message is not comprised a whole number of bytes, the number of bits, modulus 8, in the message should be specified in the *bits* parameter.  *msglen* must only count the number of whole bytes, that is, the floor of the number of bits in the message divided by 8.

*suffix* should be a NUL-terminated string of ASCII '1':s and '0':s, representing the bits that should be appended to the message. If this string is empty, *NULL* may be used instead. This is used to select hash algorithm. For pure Keccak, *NULL* or "" is used. For the other algorithms the constants **LIBKECCAK\_SHA3\_SUFFIX** (for SHA-3), **LIBKECCAK\_RAWSHAKE\_SUFFIX** (for RawSHAKE), and **LIBKECCAK\_SHAKE\_SUFFIX** (for SHAKE) are used.

The hash of the message will be stored to *hashsum*, unless *hashsum* is *NULL* (which increases the performance of the call.) A total of ((*state-\>n* + 7) / 8) bytes will be written to the beginning of *hashsum*. Therefore, *hashsum* needs at least an allocation size of that number of bytes.

The **libkeccak\_hmac\_fast\_digest**() function may reallocate the state's message chunk buffer. When doing so, it attempts to do so as quickly as possible, rather than ensuring that the information in the old allocation is securely removed if a new allocation is required.

## RETURN VALUES

The **libkeccak\_hmac\_fast\_digest**() function returns 0 upon successful completion. On error, -1 is returned and *errno* is set to describe the error.

## ERRORS

The **libkeccak\_hmac\_fast\_digest**() function may fail for any reason specified by the function [**realloc**(3)](/realloc/3.md).

## SEE ALSO

[**libkeccak\_hmac\_create**(3)](/libkeccak_hmac_create/3.md), [**libkeccak\_hmac\_initialise**(3)](/libkeccak_hmac_initialise/3.md), [**libkeccak\_hmac\_fast\_update**(3)](/libkeccak_hmac_fast_update/3.md), [**libkeccak\_hmac\_fast\_digest**(3)](/libkeccak_hmac_fast_digest/3.md)