IDA base64 F5

Base64 解码特征

signed int __cdecl base64_decode(BYTE *p_str_base64, int str_len, int p_out_bytes, int p_bytes_get)
{
  signed int result; // eax@3
  int v5; // [sp+0h] [bp-1Ch]@11
  int sum; // [sp+4h] [bp-18h]@7
  int suma; // [sp+4h] [bp-18h]@17
  signed int i; // [sp+8h] [bp-14h]@7
  int j; // [sp+8h] [bp-14h]@17
  signed int bits_count; // [sp+Ch] [bp-10h]@7
  int n_output_bytes; // [sp+10h] [bp-Ch]@4
  unsigned int v12; // [sp+14h] [bp-8h]@4
  signed int f_dontoutput; // [sp+18h] [bp-4h]@4

  if ( p_str_base64 && p_bytes_get )
  {
    v12 = (unsigned int)&p_str_base64[str_len];
    n_output_bytes = 0;
    f_dontoutput = p_out_bytes == 0;
    while ( (unsigned int)p_str_base64 < v12 && *p_str_base64 )
    {
      sum = 0;
      bits_count = 0;
      for ( i = 0; i < 4 && (unsigned int)p_str_base64 < v12; ++i )// 每4个字符换3个字节
      {
        v5 = sub_15E88A0((char)*p_str_base64++); //v5等于base64字符对应的数
        if ( v5 == -1 )
        {
          --i;
        }
        else
        {                                       // 应该是base64算法
          sum = v5 | (sum << 6);                // 2^6=64    高位编码在前
          bits_count += 6;                      //
        }
      }
      if ( !f_dontoutput && n_output_bytes + bits_count / 8 > *(_DWORD *)p_bytes_get )
        f_dontoutput = 1;
      suma = sum << (24 - bits_count);
      for ( j = 0; j < bits_count / 8; ++j )
      {
        if ( !f_dontoutput )
          *(_BYTE *)p_out_bytes++ = ((unsigned int)byte_FF0000 & suma) >> 16;
        suma <<= 8;
        ++n_output_bytes;
      }
    }                                           // while
    *(_DWORD *)p_bytes_get = n_output_bytes;
    if ( f_dontoutput )
      result = 0;
    else
      result = 1;
  }
  else
  {
    result = 0;
  }
  return result;
}

附上base64 table：

索引	对应字符	索引	对应字符	索引	对应字符	索引	对应字符
0	A	17	R	34	i	51	z
1	B	18	S	35	j	52	0
2	C	19	T	36	k	53	1
3	D	20	U	37	l	54	2
4	E	21	V	38	m	55	3
5	F	22	W	39	n	56	4
6	G	23	X	40	o	57	5
7	H	24	Y	41	p	58	6
8	I	25	Z	42	q	59	7
9	J	26	a	43	r	60	8
10	K	27	b	44	s	61	9
11	L	28	c	45	t	62	+
12	M	29	d	46	u	63	/
13	N	30	e	47	v
14	O	31	f	48	w
15	P	32	g	49	x
16	Q	33	h	50	y