The Microsoft Excel File Format - OpenOffice.org

4.19 File Protection
• BIFF2-BIFF5 Stream Encryption
To encrypt record data, a 128-bit sized key sequence is used, which has been generated from a password before (see
above). All stream data is encrypted in blocks of 16 bytes using the key sequence. In every data block, each byte is
encoded using the corresponding byte of the key sequence. The offset pointer into the key sequence is reinitialised
whenever a new record is started. The initial value of this offset is dependent on the stream position and size of the new
record (it does not simply restart at 0). For details see the following algorithm.
The following pseudo-code algorithm shows how to encrypt the data of an entire record (assuming that stream points to
the start of the record data):
ALGORITHM Write_Record_XOR( stream, record_data[], record_size, key_seq[]
)
1) key_index ← ((position of stream) + record_size) AND 0F H
2) record_index ← 0
3) IF record_index = record_size THEN RETURN
4) data_byte = record_data[record_index] XOR key_seq[key_index]
5) rotate all 8 bits of data_byte right by 3 bits
6) write data_byte to stream
7) key_index ← (key_index + 1) AND 0F H {rotate inside key_seq[]}
8) record_index ← record_index + 1
9) JUMP 3)
• BIFF2-BIFF5 Stream Decryption
To decrypt record data, a 128-bit sized key sequence is used, which has been generated from a password before (see
above). Decryption works similar to encryption (see above), but in reversed order of operations.
The following pseudo-code algorithm shows how to decrypt the data of an entire record (assuming that stream points to
the start of the record data):
ALGORITHM Read_Record_XOR( stream, record_size, key_seq[] )
1) key_index ← ((position of stream) + record_size) AND 0F H
2) create a sequence record_data[] containing record_size bytes
3) record_index ← 0
4) IF record_index = record_size THEN RETURN record_data[]
5) data_byte ← read 1 byte from stream
6) rotate all 8 bits of data_byte left by 3 bits
7) record_data[record_index] ← data_byte XOR key_seq[key_index]
8) key_index ← (key_index + 1) AND 0F H {rotate inside key_seq[]}
9) record_index ← record_index + 1
10) JUMP 4)
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