Cryptographic Primitive - Rationale

Rationale

When creating cryptographic systems, designers use cryptographic primitives as their most basic building blocks. Because of this, cryptographic primitives are designed to do one very specific task in a highly reliable fashion. They include encryption schemes, hash functions and digital signatures schemes.

Since cryptographic primitives are used as building blocks, they must be very reliable, i.e. perform according to their specification. E.g. if an encryption routine claims to be only breakable with X number of computer operations, then if it can be broken with significantly less than X operations, that cryptographic primitive is said to fail. If a cryptographic primitive is found to fail, almost every protocol that uses it becomes vulnerable. Since creating cryptographic routines is very hard, and testing them to be reliable takes a long time, it is essentially never sensible (nor secure) to design a new cryptographic primitive to suit the needs of a new cryptographic system. The reasons include:

• The designer might not be competent in the mathematical and practical considerations involved in cryptographic primitives.
• Designing a new cryptographic primitive is very time-consuming and very error prone, even for experts in the field.
• Since algorithms in this field are not only required to be designed well, but also need to be tested well by the cryptologist community, even if a cryptographic routine looks good from a design point of view it might still contain errors. Successfully withstanding such scrutiny gives some confidence (in fact, so far, the only confidence) that the algorithm is indeed secure enough to use; security proofs for cryptographic primitives are generally not available.

Cryptographic primitives are similar in some ways to programming languages. A computer programmer rarely invents a new programming language while writing a new program; instead, he or she will use one of the already established programming languages to program in.

Cryptographic primitives are one of the building block of every crypto system, e.g., TLS, SSL, SSH, etc. Crypto system designers, not being in a position to definitively prove their security, must take the primitives they use as secure. Choosing the best primitive available for use in a protocol usually provides the best available security. However, compositional weaknesses are possible in any crypto system and it is the responsibility of the designer(s) to avoid them.