Editing Product key (section)

===== Decryption =====
The lower 16 bytes of the Installation ID are encrypted, whereas the most significant byte is kept in plaintext. The cryptographic algorithm used 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|message digest algorithm]] keyed with a four-byte sequence. 

Let + denote the concatenation of two byte sequences, ^ the [[XOR cipher|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 which will be interpreted as four [[Double word|double words]] in little endian byte order followed by a single byte. 
{| class="wikitable"
|+
!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.