# Ciphers

Banned Posts: 151
Hi,

If I get a question like what Cipher does symentric alrogithms use and the choice is between:

block and stream or subsusiton and transoption.

Which one is correct?

I know what they mean but am a bit confused still.

Thanks

• MSIT InfoSec, CISSP, SSCP, GSEC, EnCE, C|EH, CySA+, PenTest+, CASP+, Security+ Surf City, USAAdmin Posts: 12,170 Admin
A cipher can be either symmetric or asymmetric in its keying operation. A cipher can also be either block or stream in its data ingestion operation. For any given cipher, you should memorize both if it uses symmetric or asymmetric keying and if it is a block or stream cipher.
• Banned Posts: 151
OK Thanks!
• Member Posts: 14 ■□□□□□□□□□
The best mnemonic I've heard for remembering assymetric is this one:
D.E.E.R.

Diffie Helman (Key exchange)
Ellipcitical Curve
El Gamal
RSA

And everything else pretty much uses symmetric. I also think its important to remember that Diffie Helman is ONLY Key Exchange. Repeat that to yourself about 150 times.

I took the test this past Monday. I think its important not to go overboard with focusing on asymmetric/symmetric and which one uses which so much as its important to know the differences in the two and the pros and cons of each.
• Member Posts: 10 ■□□□□□□□□□
Hi,

Symmetric/Asymmetric is related to the key.

Block/Stream is related input/output length of the cipher/algorithm

Substitution and transposition is a common technique used in symmetric block encryption. AFAIK, if you are to use transposition it must be a symmetric block cipher.

So a cipher can be symmetric, block and performing substitutions and transpositions at the same time, IMO the question is not valid.
• Member Posts: 10 ■□□□□□□□□□
natestreet wrote: »
The best mnemonic I've heard for remembering assymetric is this one:
D.E.E.R.

Diffie Helman (Key exchange)
Ellipcitical Curve
El Gamal
RSA

And everything else pretty much uses symmetric

Nice, just have to squeeze Digital Signature Algorithm in there somehow too .