In what cases might PRESENT be implemented as encryption-only? In the paper, under section 4.1 Goals and environment of use we read:

In applications that demand the most efficient use of space, the block cipher will often only be implemented as encryption-only. 

I presume this is NOT the same as one-way encryption (perhaps I should say hash?) as used for passwords.


So in what cases might we need only encryption but not decryption? It seems strange we would encrypt something that does not need decrypting at some point.


Furthermore, the quote above continues with ...

In this way it can be used in challenge-response authentication protocols and, with some careful management, it could be used for both encryption and decryption ...

This just added to my confusion. 


It seems PRESENT can decrypt a ciphertext by running it in reverse. So why the need for some careful management for PRESENT to be used for both encryption and decryption?
So in what cases might we need only encryption but not decryption? It seems strange we would encrypt something that does not need decrypting at some point.
  


For example, the CTR mode uses only Encryption, and CFB, OFB. 


  
  It seems PRESENT can decrypt a ciphertext by running it in reverse. So why the need for some careful management for PRESENT to be used for both encryption and decryption?
  


The correct sentence is 


  In this way it can be used within challenge-response authentication protocols and, with some careful state management.


All about the hardware implementation of PRESENT.

This paragraph from the article;


  Encryption and decryption with present have roughly the same physical requirements. Opting to support both encryption and decryption will result in a lightweight block cipher implementation that is still smaller than an encryption-only AES. Opting to implement an encryption only present will give an ultra-lightweight solution. The encryption subkeys can be computed on-the-fly.


So they compare their encryption and decryption supported hardware implementation to encryption-only AES claiming that PRESENT has smaller requirements.