标题: Skype登录过程中对SHA1的一种使用

创建: 2006-11-14 18:43
链接: https://scz.617.cn/misc/200611141843.txt

参看[1]的"Analysis of the login phase"部分，其中提到经SHA1处理得到256-bits
的AES Encrypt Key。我们知道SHA1本身输出是160-bits，显然Skype做了组合变换。
[1]的作者没有给出细节，我们逆向分析了这个地方，下面是其C风格的伪代码:

--------------------------------------------------------------------------
void SkypeSHA1
(
    in,
    insize,
    out,
    outsize
)
{
    hash[20];
    buf[buflen];
    i;
    j;

    buflen  = 4 + insize;
    memcpy( buf + 4, in, insize );
    j       = 0;
    while ( 1 )
    {
        for ( i = 0; i < 32; i += 8 )
        {
            buf[i/8]    = j >> ( 24 - i );
        }  /* end of for */
        SHA1( buf, buflen, hash );
        if ( outsize <= sizeof( hash ) )
        {
            memcpy( out, hash, outsize );
            break;
        }
        memcpy( out, hash, sizeof( hash ) );
        out        += sizeof( hash );
        outsize    -= sizeof( hash );
        j++;
    }  /* end of while */
    return;
}  /* end of SkypeSHA1 */
--------------------------------------------------------------------------

验证集如下:

--------------------------------------------------------------------------
[ 192 bytes ] -> 16 bytes per line
00000000  01 60 B2 14 51 35 8C DC D6 66 F0 83 D3 E1 97 99
00000010  9B C6 25 39 3C 02 2B 57 84 60 B2 14 51 35 8C DC
00000020  D6 66 F0 83 D3 E1 97 99 9B C6 25 39 3C 02 2B 57
00000030  84 60 B2 14 51 35 8C DC D6 66 F0 83 D3 E1 97 99
00000040  9B C6 25 39 3C 02 2B 57 84 60 B2 14 51 35 8C DC
00000050  D6 66 F0 83 D3 E1 97 99 9B C6 25 39 3C 02 2B 57
00000060  84 60 B2 14 51 35 8C DC D6 66 F0 83 D3 E1 97 99
00000070  9B C6 25 39 3C 02 2B 57 84 60 B2 14 51 35 8C DC
00000080  D6 66 F0 83 D3 E1 97 99 9B C6 25 39 3C 02 2B 57
00000090  84 60 B2 14 51 35 8C DC D6 66 F0 83 D3 E1 97 99
000000A0  9B C6 25 39 3C 02 2B 57 84 60 B2 14 51 35 8C DC
000000B0  D6 66 F0 83 D3 E1 97 99 9B C6 25 39 3C 02 2B 57
[ 32 bytes ] -> 16 bytes per line
00000000  21 A3 B0 73 A3 23 44 24 2E 64 FB EE 0A 68 5B 70
00000010  55 7C 20 7D 7C 7A C0 BE 38 CC E7 9E BE 71 48 2A
--------------------------------------------------------------------------

参考资源:

[ 1] Silver Needle in the Skype - [2006-03-02]
     http://www.blackhat.com/presentations/bh-europe-06/bh-eu-06-biondi/bh-eu-06-biondi-up.pdf