int EVP_DecodeBlock(unsigned char *t, const unsigned char *f, int n)
{
    int i, ret = 0, a, b, c, d;
    unsigned long l;

    /* trim white space from the start of the line. */
    while ((conv_ascii2bin(*f) == B64_WS) && (n > 0)) {
        f++;
        n--;
    }

    /*
     * strip off stuff at the end of the line ascii2bin values B64_WS,
     * B64_EOLN, B64_EOLN and B64_EOF
     */
    while ((n > 3) && (B64_NOT_BASE64(conv_ascii2bin(f[n - 1]))))
        n--;

    if (n % 4 != 0)
        return (-1);

    for (i = 0; i < n; i += 4) {
        a = conv_ascii2bin(*(f++));
        b = conv_ascii2bin(*(f++));
        c = conv_ascii2bin(*(f++));
        d = conv_ascii2bin(*(f++));
        if ((a & 0x80) || (b & 0x80) || (c & 0x80) || (d & 0x80))
            return (-1);
        l = ((((unsigned long)a) << 18L) |
             (((unsigned long)b) << 12L) |
             (((unsigned long)c) << 6L) | (((unsigned long)d)));
        *(t++) = (unsigned char)(l >> 16L) & 0xff;
        *(t++) = (unsigned char)(l >> 8L) & 0xff;
        *(t++) = (unsigned char)(l) & 0xff;
        ret += 3;
    }
    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.