Much of the theoretical work in cryptography concerns cryptographic primitives algorithms with basic cryptographic properties - and their relationship to other cryptographic problems. More complicated cryptographic tools are then built from these basic primitives. Complex functionality in an application must be built in using combinations of these algorithms and assorted protocols. Such combinations are called（  1  ） and it is they which users actually encounter. Examples include PGP and its variants, ssh, SSL/TLS, all PKIs, （ 2 ）, etc For example, a （ 3 ） is function intended to be easy to compute but hard to invert.  
But note that, in a very general sense, for any cryptographic application to be secure (if based on computational feasibility assumptions) one-way functions must exist. However, if one-way functions exist, this implies that （ 4 ）≠NP. Since the P versus NP problem is currently unsolved, it is not known if one-way functions really do exist. For instance, if one way functions exist, then secure pseudorandom generators and secure pseudorandom functions exist.  Other （  5  ）include the encryption algorithms themselves, one-way permutations, trapdoor permutations, etc. 
（1）A. Password base   
B. cryptosystems    
C. Principles of cryptography   
D. Equipment system   
（2）A. Certification  
B. Digest Summary  
C. digital signatures  
D. Identification   
（3）A. Hash function  
B. one-way function   
C. Bidirectional function  
D. Power function  
（4）A. NPC  
B. N  
C. NP  
D. P   
（5）A. cryptographic primitives   
B. Principles of cryptography   
C. cryptosystems           
D. Password base