Creating an encrypted log file

Don't encrypt individual log entries separately and write them to a file as suggested by other posters, because an attacker would easily be able to identify patterns in the log file. See the block cipher modes Wikipedia entry to learn more about this problem.

Instead, make sure that the encryption of a log entry depends on the previous log entries. Although this has some drawbacks (you cannot decrypt individual log entries as you always need to decrypt the entire file), it makes the encryption a lot stronger. For our own logging library, SmartInspect, we use AES encryption and the CBC mode to avoid the pattern problem. Feel free to give SmartInspect a try if a commercial solution would be suitable.

This is not really my thing, I'll admit that readily, but can't you encrypt each entry individually and then append it to the logfile? If you that refrain from encrypting the timestamp, you can easily find entries your are looking for and decrypt those when needed.

My point being mainly that appending individual encrypted entries to a file does not necessarily need to be binary entries appended to a binary file. Encryption with (for example) gpg will yield ascii garble that can be appended to an ascii file. Would that solve you problem?

FWIW, the one time I needed an encrypted logger I used a symmetric key (for performance reasons) to encrypt the actual log entries.

The symmetric 'log file key' was then encrypted under a public key and stored at the beginning of the log file and a separate log reader used the private key to decrypt the 'log file key' and read the entries.

The whole thing was implemented using log4j and an XML log file format (to make it easier for the reader to parse) and each time the log files were rolled over a new 'log file key' was generated.