Inside Windows Product Activation

Decoding

Removing the check digits results in a 41-digit decimal number. A decimal number of this length roughly corresponds to a 136-bit binary number. In fact, the 41-digit number is just the decimal encoding of such a 136-bit multi-precision integer, which is stored in little endian byte order as a byte array. Hence, the above Installation ID can also be represented as a sequence of 17 bytes as in

0xXX 0xXX 0xXX 0xXX 0xXX 0xXX 0xXX 0xXX 0x94 0xAA 0x46 0xD6 0x0F 0xBD 0x2C 0xC8 0x00

In this representation of the above Installation ID 'X' characters again substitute the digits that we prefer not to disclose. The '0x' prefix denotes hex notation throughout this paper.

Decryption

When decoding arbitrary Installation IDs it can be noticed that the most significant byte always seems to be 0x00 or 0x01, whereas the other bytes look random. The reason for this is that the lower 16 bytes of the Installation ID are encrypted, whereas the most significant byte is kept in plaintext.

The cryptographic algorithm employed to encrypt the Installation ID is a proprietary four-round Feistel cipher. Since the block of input bytes passed to a Feistel cipher is divided into two blocks of equal size, this class of ciphers is typically applied to input blocks consisting of an even number of bytes - in this case the lower 16 of the 17 input bytes. The round function of the cipher is the SHA-1 message digest algorithm keyed with a four-byte sequence.

Let + denote the concatenation of two byte sequences, ^ the XOR operation, L and R the left and right eight-byte input half for one round, L' and R' the output halves of said round, and First-8() a function that returns the first eight bytes of an SHA-1 message digest. Then one round of decryption looks as follows.

L' = R ^ First-8(SHA-1(L + Key))

R' = L

The result of the decryption is 16 bytes of plaintext, which are -

together with the 17th unencrypted byte - from now on interpreted as

four double words in little endian byte order followed by a single

byte as in

name	size	offset
H1	double word	0
H2	double word	4
P1	double word	8
P2	double word	12
P3	byte	16

H1 and H2 specify the hardware configuration that the Installation ID is linked to. P1 and P2 as well as the remaining byte P3 contain the Product ID associated with the Installation ID.