/usr/bin/avos:AOS compare SIGx checksum()

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Description

Decrypt the SIGx (x = 0 to 4) block, calculate the md5 checksum of the file on disk and then compare this checksum to the checksum contained in the SIGx block after decryption. If they don't match, the file is deemed invalid.

Notice that AOS_decipher_SIGx_block() can fail silently.

References To

• AOS_decipher_SIGx_block()
• Keyring_GetRelMPK()
• Keyring_GetDevMPK()
• Keyring_GetPlugMPK()
• Keyring_GetHDDMPK()
• Keyring_GetGAMESMPK()

The Code

int AOS_SIGx_key_id;
 
int AOS_compare_SIGx_checksum(int fd)
{
	uint32_t sign;			// -0x424C
	char bigbuf[0x4000];		// -0x4240
	char md5sum[16];		// -0x240
	char SIGx[136];			// -0x230
	int read;			// -0x1C
	uint32_t pos;			// -0x18
	int remaining;			// -0x14
	int i;				// -0x10
 
	read = fread(fd, &SIGx, 136);
	if(read != 136)
		return 205; // failure
 
	pos = (uint32_t)ftell64(fd);
	sign = *(uint32_t *)&SIGx[0];
 
	switch(sign) {
		case 0x53494730: // SIG0
		{
			AOS_decipher_SIGx_block(&SIGx[8], &Keyring_GetRelMPK);
			AOS_SIGx_key_id = 1;
			break;
		}
		case 0x53494731: // SIG1
		{
			AOS_decipher_SIGx_block(&SIGx[8], &Keyring_GetDevMPK);
			AOS_SIGx_key_id = 2;
			break;
		}
		case 0x53494732: // SIG2
		{
			AOS_decipher_SIGx_block(&SIGx[8], &Keyring_GetPlugMPK);
			AOS_SIGx_key_id = 3;
			break;
		}
		case 0x53494733: // SIG3
		{
			AOS_decipher_SIGx_block(&SIGx[8], &Keyring_GetHDDMPK);
			AOS_SIGx_key_id = 4;
			break;
		}
		case 0x53494734: // SIG4
		{
			AOS_decipher_SIGx_block(&SIGx[8], &Keyring_GetGAMESMPK);
			AOS_SIGx_key_id = 5;
			break;
		}
		case 0x5349474E: // SIGN
		default: // not recognized
		{
			return 204; // failure
		}
	}
 
	MD5Digest(1, 0, 0, NULL);
 
	i = (AOS_file_length - pos);
 
	while(i != 0) {
		remaining = min(i, 0x4000); // returns the minimum between i and 0x4000
 
		read = fread(fd, &bigbuf, remaining);
		if(remaining != read)
			return 206; // failure
 
		MD5Digest(2, &bigbuf, read, NULL);
 
		i = i-remaining;
	}
 
	MD5Digest(3, 0, 0, &md5sum);
	if(memcmp(&md5sum, &SIGx[8], 16) != 0)
		return 207; // failure
 
	fseek64(fd, SEEK_SET, pos);
 
	return 0;
}