Hi All I have to encrypt my data using Blowfish algorithm in C++...can you guys tell me if the other end knows what algorithm I am using can they not extract the key and decrypt it ? How safe is the data by this method?
can you guys tell me if the other end knows what algorithm I am using can they not extract the key and decrypt it ?
No. The whole point of standardized encryption algorithms (as opposed to those that rely on obscurity) is that even though everyone knows all details of it, one cannot decrypt it unless one has the key.
This approach works because the only way to crack the encryption is to try all possible keys, of which there are too many. As computation power increases, formerly "secure" algorithms do become "unsecure". With some algorithms there may also be flaws that allow other forms of deductions to take place that significantly reduce the possible key-space and hence speed up these brute-force attacks. But (as far as we know) Blowfish is safe here.
Keeping the key secret is essential of course. If your program also includes the key (as opposed to asking the user or some device for it, or using random session keys that are themselves encrypted using public-key crypto), then a reverse-engineer can probably find it and break your scheme.
An important part of keeping the key secret is to keep it "non-guessable" and to not reuse it for different purposes. Randomly generated keys are the best.
Also, by "the other end" you mean an attacker, right? Usually, "the other end" refers to Bob The Intended Recipient, and he of course needs to be able to decrypt the message.