The quality of a cipher can be described by checking its resistance to strictly technical attacks (that is bypassing the human factor). The highest defined level of security is referred to as the perfect security. In fact, to describe the practical security of ciphers, the semantic security property is usually used.
Perfect Security
The one-time pad is the only cipher known to have perfect secrecy.
Semantic Security
Any (PPTA) which receives the ciphertext created by a semantically secure cipher of any certain message and its length cannot determine any partial information on the message with probability higher than all other PPTAs that only have access to the message length and don't have access to the ciphertext.
One can prove that the OTP cipher is semantically secure if it uses a random encryption key. Similarly, each stream cipher can have the property, if a pseudorandom generator used in the cipher is secure.
Negligible value - it is assumed that e is:
- non-negligible, if e >= 1/230 (in practice likely to happen when there is over 1GB of data)
- negligible, if e <= 1/280 (in practice will not happen over life of key)
Probabilistic, polynomial-time algorithm (PPTA) is a randomized algorithm with polynomial time complexity, with an error probability of less than 0.5 for all instances.