in port/espressif/src/base64_port.c [5:13]
int base64_port_encode( char * dst, size_t dlen, size_t * olen, char * src, size_t slen )
{
int rc = mbedtls_base64_encode( (unsigned char *)dst,
dlen,
(size_t *)olen,
(unsigned char *)src,
slen );
return rc == 0 ? 0 : -1;
}