Computing websocket Sec-WebSocket-Accept value using libtomcrypt

Question

RFC6455 specifies a method of computing the Sec-WebSocket-Accept response header from the value of the Sec-WebSocket-Key header. This method is based on SHA-1 hashing and Base64-encoding the result.

How can I implement this method in plain C using libtomcrypt for SHA-1 and Base64?

Note: This question intentionally does not show any effort because I immediately answered it myself. See below for my effort.

Answer 1

Here's a full compilable example that uses only libtomcrypt without any dynamic memory allocation and successfully computes the reference example from RFC6455:

//This file is licensed under CC0 1.0 Universal (public domain)
//Compile like this: gcc -o wsencodetest wsencodetest.c -ltomcrypt
#include <stdio.h>
#include <string.h>
#include <stdint.h>
#include <ctype.h>
#include <tomcrypt.h>

#define SHA1_HASHSIZE 20

//Magic GUID as defined in RFC6455 section 1.3
static const char magicGUID[] = "258EAFA5-E914-47DA-95CA-C5AB0DC85B11";

/**
 * Compute the value of the Sec-WebSocket-Accept response header
 * from the value of the Sec-WebSocket-Key header.
 * @param key The whitespace or NUL terminated Sec-WebSocket-Key value
 * @param out Where to store the base64-encoded output. Must provide 29 bytes of memory.
 * The 29 bytes starting at out contain the resulting value (plus a terminating NUL byte)
 */
void computeWebsocketSecAccept(const char* key, char* dst) {
    /**
     * Determine start & length of key minus leading/trailing whitespace
     * See RFC6455 section 1.3
     */
    //Skip leading whitespace
    while(isspace(*key)) {
        key++;
    }
    //Determine key size.
    size_t keySize = 0;
    while(!isspace(key[keySize]) && key[keySize] != 0) {
        keySize++;
    }
    //Compute SHA1 hash. See RFC6455 section 1.3
    char hashOut[SHA1_HASHSIZE];
    hash_state md;

    sha1_desc.init(&md);
    sha1_desc.process(&md, key, keySize);
    sha1_desc.process(&md, magicGUID, sizeof(magicGUID));
    sha1_desc.done(&md, hashOut);
    //Encode hash to output buffer
    size_t outlen = 29; //We know the output is 28 in size
    base64_encode(hashOut, SHA1_HASHSIZE, dst, &outlen);
}

/**
 * Usage example
 */
int main(int argc, char** argv) {
    //Whitespace needs to be removed according to RFC6455
    //Example from RFC6455
    const char* key = " dGhlIHNhbXBsZSBub25jZQ== ";
    char buf[29];
    //Perform computation
    computeWebsocketSecAccept(key, buf);
    //Should print s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
    printf("%s\n", buf);
}