int EVP_EncodeBlock(unsigned char *t, const unsigned char *f, int dlen)
{
    int i, ret = 0;
    unsigned long l;

    for (i = dlen; i > 0; i -= 3) {
        if (i >= 3) {
            l = (((unsigned long)f[0]) << 16L) |
                (((unsigned long)f[1]) << 8L) | f[2];
            *(t++) = conv_bin2ascii(l >> 18L);
            *(t++) = conv_bin2ascii(l >> 12L);
            *(t++) = conv_bin2ascii(l >> 6L);
            *(t++) = conv_bin2ascii(l);
        } else {
            l = ((unsigned long)f[0]) << 16L;
            if (i == 2)
                l |= ((unsigned long)f[1] << 8L);

            *(t++) = conv_bin2ascii(l >> 18L);
            *(t++) = conv_bin2ascii(l >> 12L);
            *(t++) = (i == 1) ? '=' : conv_bin2ascii(l >> 6L);
            *(t++) = '=';
        }
        ret += 4;
        f += 3;
    }

    *t = '\0';
    return (ret);
}

This weakness involves numeric computation errors, such as integer overflows, underflows, or precision losses, which can lead to miscalculations and exploitable behaviors in software.