The Microsoft Excel File Format - OpenOffice.org

4 Worksheet/Workbook Stream
• BIFF2-BIFF5 Encryption Key
The 16-bit encryption key is used for the BIFF2-BIFF5 XOR stream encryption algorithm (➜4.19.1). It is calculated
from the password. The FILEPASS record stores this key and the hash value (➜4.18.4) of the password (the hash value
is not used to encrypt the data). The length of the password is restricted to 15 characters.
The following pseudo-code algorithm shows how to create the encryption key from a given byte-string password:
ALGORITHM Get_Encryption_Key_XOR( password )
1) key ← 0 ; key_base ← 8000 H ; key_final ← FFFF H ; char_index ← 0
2) char_count ← character count of password
3) char ← character from password with index char_index {right-to-left, 0 is rightmost}
4) char ← char AND 7F H {use only the lower 7 bits of each character}
5) bit_index ← 0
6) rotate the lower 16 bits of key_base left by 1 bit
7) IF (least significant bit of key_base) = 1 THEN key_base ← key_base XOR 1020 H
8) rotate the lower 16 bits of key_final left by 1 bit
9) IF (least significant bit of key_final) = 1 THEN key_final ← key_final XOR 1020 H
10) IF (bit with index bit_index in char) = 1 THEN key ← key XOR key_base
11) bit_index ← bit_index + 1
12) IF bit_index < 8 THEN JUMP 6)
13) char_index ← char_index + 1
14) IF char_index < char_count THEN JUMP 3)
15) RETURN key XOR key_final
• BIFF2-BIFF5 Encryption Key Sequence
The encryption sequence is needed to encrypt or decrypt the record contents. It is generated from the password and the
encryption key (which is generated from the password too, see above). The size of the encryption key sequence is always
128 bit.
The following pseudo-code algorithm shows how to create the encryption key from a given byte-string password (and the
encryption key “key”):
ALGORITHM Get_Key_Sequence_XOR( password, key )
1) char_count ← character count of password
2) create a sequence key_seq[] containing 16 bytes
3) fill the first char_count bytes of key_seq[] with all characters of password
4) fill the remaining (16 – char_count) bytes of key_seq[] with the first bytes of the sequence
{ BB H ,FF H ,FF H ,BA H ,FF H ,FF H ,B9 H ,80 H ,00 H ,BE H ,0F H ,00 H ,BF H ,0F H ,00 H }
5) key_lower ← lower 8 bits of key ; key_upper ← upper 8 bits of key
6) seq_index ← 0
7) key_seq[seq_index] ← key_seq[seq_index] XOR key_lower
8) key_seq[seq_index + 1] ← key_seq[seq_index + 1] XOR key_upper
9) seq_index ← seq_index + 2
10) IF seq_index < 16 THEN JUMP 7)
11) rotate all bytes of key_seq left by 2 bits
12) RETURN key_seq[]
118