Good cryptographic systems should always be designed so that they are as difficult to break as possible. It is possible to build systems that cannot be broken in practice (though this cannot usually be proved). This does not significantly increase system implementation effort; however, some care and expertise is required. There is no excuse for a system designer to leave the system breakable. Any mechanisms that can be used to circumvent security must be made explicit, documented, and brought into the attention of the end users.
In theory, any cryptographic method with a key can be broken by trying all possible keys in sequence. If using brute force to try all keys is the only option, the required computing power increases exponentially with the length of the key. A 32-bit key takes 232 (about 109) steps. This is something anyone can do on his/her home computer. A system with 56-bit keys, such as DES, requires a substantial effort, but using massive distributed systems requires only hours of computing. In 1999, a brute-force search using a specially designed supercomputer and a worldwide network of nearly 100,000 PCs on the Internet, found a DES key in 22 hours and 15 minutes. It is currently believed that keys with at least 128 bits (as in AES, for example) will be sufficient against brute-force attacks into the foreseeable future.
However, key length is not the only relevant issue. Many ciphers can be broken without trying all possible keys. In general, it is very difficult to design ciphers that could not be broken more effectively using other methods.
Unpublished or secret algorithms should generally be regarded with suspicion. Quite often the designer is not sure of the security of the algorithm, or its security depends on the secrecy of the algorithm. Generally, no algorithm that depends on the secrecy of the algorithm is secure. For professionals, it is easy to disassemble and reverse-engineer the algorithm. Experience has shown that the vast majority of secret algorithms that have become public knowledge later have been pitifully weak in reality.
The keys used in public-key algorithms are usually much longer than those used in symmetric algorithms. This is caused by the extra structure that is available to the cryptanalyst. There the problem is not that of guessing the right key, but deriving the matching private key from the public key. In the case of RSA, this could be done by factoring a large integer that has two large prime factors. In the case of some other cryptosystems, it is equivalent to computing the discrete logarithm modulo a large integer (which is believed to be roughly comparable to factoring when the moduli is a large prime number). There are public-key cryptosystems based on yet other problems.
To give some idea of the complexity for the RSA cryptosystem, a 256-bit modulus is easily factored at home, and 512-bit keys can be broken by university research groups within a few months. Keys with 768 bits are probably not secure in the long term. Keys with 1024 bits and more should be safe for now unless major cryptographical advances are made against RSA. RSA Security claims that 1024-bit keys are equivalent in strength to 80-bit symmetric keys and recommends their usage until 2010. 2048-bit RSA keys are claimed to be equivalent to 112-bit symmetric keys and can be used at least up to 2030.
It should be emphasized that the strength of a cryptographic system is usually equal to its weakest link. No aspect of the system design should be overlooked, from the choice of algorithms to the key distribution and usage policies.