Product key
A product key, also known as a software key, serial key or activation key, is a specific software-based key for a computer program. It certifies that the copy of the program is original.
Product keys consist of a series of numbers and/or letters. This sequence is typically entered by the user during the installation of computer software, and is then passed to a verification function in the program. This function manipulates the key sequence according to an algorithm or mathematical formula and attempts to match the results to a set of valid solutions. If they match, the program is activated, permitting its use or unlocking features. With knowledge about the algorithm used, such as that obtained via reverse engineering of the program, it is possible to create programs called keygens that generate these keys for a particular program.
Effectiveness
Standard key generation, where product keys are generated mathematically, is not completely effective in stopping copyright infringement of software, as these keys can be distributed. In addition, with improved communication from the rise of the Internet, more sophisticated attacks on keys such as cracks and product key generators have become common.Because of this, software publishers use additional product activation methods to verify that keys are both valid and uncompromised. One method assigns a product key based on a unique feature of the purchaser's computer hardware, which cannot be as easily duplicated since it depends on the user's hardware. Another method involves requiring one-time or periodical validation of the product key with an internet server. The server can deactivate unmodified client software presenting invalid, duplicated, missing or otherwise compromised keys. Modified clients may bypass these checks, but the server can still deny those clients information or communication.
Examples
Windows 95 retail key
Windows 95 retail product keys take the form XXX-XXXXXXX. To determine whether the key is valid, Windows 95 performs the following checks:- The first 3 characters must not be equal to 333, 444, 555, 666, 777, 888 or 999.
- The last 7 characters must all be numbers from 0-8.
- The sum of the last 7 numbers must be divisible by 7 with no remainder.
- The fourth character is unchecked.
Windows 95 OEM key
Windows 95 OEM keys take the form XXXXX-OEM-XXXXXXX-XXXXX.- The first 3 characters must be a number between 0-366.
- The next 2 characters must be a number between 04-93.
- The next 3 characters must be OEM.
- The sum of the next 7 numbers must be divisible by 7 with no remainder.
- The rest of the characters are unchecked.
Windows XP retail key
Installation ID
The installation ID is a 50 digit decimal string that is divided into 8 groups of six digits each with 2 digits at the end, which takes the form of XXXXXX-XXXXXX-XXXXXX-XXXXXX-XXXXXX-XXXXXX-XXXXXX-XXXXXX-XX. The installation ID is regenerated every time msoobe.exe is ran.Check digits
The right most digit in each group of the installation ID is a check digit.- Each check digit is calculated by adding the other five digits in its group.
- Then adding the digits in that group in the even positions a second time.
- Then dividing the sum of them by 7.
- The remainder is the value of the check digit in its group.
Decoding
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 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
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 words in little endian byte order followed by a single byte.
| 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.
Product ID
The Product ID consists of five groups of decimal digits, as in AAAAA-BBB-CCCCCCC-DDEEE.Decoding
The mapping between the Product ID in decimal representation and its binary encoding in the double words P1 and P2 and the byte P3 is summarized in the following table.| Digits | Length | Encoding | Meaning |
| AAAAA | 17 Bits | Bit 0 to bit 16 of P1 | Always 55034 |
| BBB | 10 Bits | Bit 17 to bit 26 of P1 | Most significant three digits of Raw Product Key |
| CCCCCCC | 28 Bits | Bit 27 to bit 31 of P1 and Bit 0 to bit 22 of P2 | Least significant six digits of Raw Product Key and check digit |
| DDEEE | 17 Bits | Bit 23 to bit 31 of P2 and Bit 0 to bit 7 of P3 | DD = Index of the public key used to verify the Product key EEE = Random value |
Product key
The Product key takes the form as XXXXX-XXXXX-XXXXX-XXXXX-XXXXX. Each character is one of the following 24 letters and digits: B C D F G H J K M P Q R T V W X Y 2 3 4 6 7 8 9The 25 characters of the Product Key form a base-24 encoding of the binary representation of the Product Key. The Product Key is a multi-precision integer of roughly 115 bits, which is stored in little endian byte order in an array of 15 bytes. Of these 15 bytes the least significant four bytes contain the Raw Product Key in little endian byte order. The least significant bit is removed by shifting this 32-bit value to the left by one bit position. The eleven remaining bytes form a digital signature, allowing verification of the authenticity of the Product Key by means of a hard-coded public key.
To obtain the CCCCCCC component, a check digit is appended, and the check digit that is chosen such that the sum of all digits, including the check digit, is divisible by seven. For verifying a Product Key, more than one public key is available. If verification with the first public key fails, the second is tried, etc. The DD component of the Product ID specifies which of the public keys in this sequence was successfully used to verify the Product Key.
Hardware information bit-fields
The hardware configuration linked to the Installation ID is represented by the two double words H1 and H2. For this purpose, the double words are divided into twelve bit-fields.| Double Word | Offset | Length | Bit-Field-value based on |
| H1 | 0 | 10 | Volume serial number string of system volume |
| H1 | 10 | 10 | Network adapter MAC address string |
| H1 | 20 | 7 | CD-ROM drive hardware identification string |
| H1 | 27 | 5 | Graphics adapter hardware identification string |
| H2 | 0 | 3 | Unused, set to 001 |
| H2 | 3 | 6 | CPU serial number string |
| H2 | 9 | 7 | Hard drive hardware identification string |
| H2 | 16 | 5 | SCSI host adapter hardware identification string |
| H2 | 21 | 4 | IDE controller hardware identification string |
| H2 | 25 | 3 | Processor model string |
| H2 | 28 | 3 | RAM size |
| H2 | 31 | 1 | 1 = dockable, 0 = not dockable |
If docking is possible, the activation mechanism will be more tolerant with respect to future hardware modifications. If the hardware component corresponding to one of the remaining ten bit-fields is present, the respective bit-field contains a non-zero value describing the component. A value of zero marks the hardware component as not present. All hardware components are identified by a hardware identification string obtained from the registry. Hashing this string provides the value for the corresponding bit-field.