Cryptology I



  1. Please fill in the table below (2pts)

Action Use whose and what key
Create a signature Use  private’s _128 bit key to encrypt message
Decrypt a signature to authenticate sender Use  public ’s 128 bit key to decrypt massage
Send an encrypted message (such as a session key) with RSA Use  _132 bit ’s private___ key
Decrypt an encrypted message (such as a session key) with RSA Use  132bit ’s __public key
Send an encrypted message signature with AES Use  __64 bit  ’s private_ key
Decrypt an encrypted message signature with AES Use  _132’s public_ key












  1. Please address the learning objectives (one to two paragraphs for each objective) (2pt)

Confidentiality. The main object of encryption is to enhance confidentiality, this is obtained by sending the encrypted messages created by shifting the letters of the original message. If the scrambling technique is difficult, only the general with the key can read the message.

Security. The other objective of encryption is to secure files and data by converting them in an encrypted language which can also be converted to cipher text from plain text. Encrypting data is very important because it gives us surety that data free of damage or access by unauthorized person.

  • Understand how symmetric and asymmetric cryptograph works.

Symmetric encryption works by mixing the secret input with secret key in a way that can’t derive the input or key from the output. Mixing details differ significant, but there are blocker ciphers and steam ciphers. (Daemen & Rijmen, 2009). Block cipher functions by observing the input data in 8, 16 and 32byte block at a time, and distributing the keys and inputs within those blocks various modes of operations are required to encrypt more data that fits than fits in the block. Symmetric cipher are the best when handling bulk data encryption.

Asymmetrical encryption works by exploiting very hard mathematical problems with black doors which enhances fast helps in solving key distribution problems. Instead of obtaining  O(N^2) key pairs between every pair of individuals  wanting to encrypt to talk among themselves it requires  O(N) keys  one private /public pair per person, and all of them just needs to know each other’s public portion. Asymmetric ciphers are used to transfer session keys for symmetric ciphers. When small portion of data is going to be transferred. Cryptographers would prefer to send the actual data encrypted with a symmetric cipher, and send the key encrypted with an asymmetric cipher. The advantage of using asymmetric is that you can send a message to thousands of different recipients, and the capacity of the message will be O(size of message + 1000*2048 bits) ( Boneh & Franklin, 2007).You  can also encrypt the session key to very recipients , and only transfer the message once. Asymmetric ciphers are best for encrypting small amounts of random data, such as message digest and session keys.







  • Discuss the pros and cons of symmetric and asymmetric cryptography.

Advantages of symmetric cryptograph

They are extremely secure when it uses secure algorithms. When you use it with the most secure 256-bit key length, it can take even billions of years for a 10petaflop computer to guess the key through a brute force attack ( Boneh & Franklin, 2007).

Encrypting and decrypting symmetric key is relatively faster .solid states drives stores data and they are fast in terms encrypting and decrypting data.

  • In Symmetric Cryptosystems, encrypted data can be transferred on the link even if there is a possibility that the data will be intercepted. Since there is no key transmitted with the data, the chances of data being decrypted are null.


  • A symmetric cryptosystem uses password authentication to prove the receiver’s identity.
  • A system only which possesses the secret key can decrypt a message.




Disadvantages of symmetric

  • Sharing of the key is the main problem in symmetric encryption because you need to have a way to give the key to the other party with whom you are sharing the data with. Encryption are not like password or simple strings. They are block of gibberish and therefore you will need to have a safer way to take the key to the other party so that no one else accesses the data ( Boneh & Franklin, 2007).


  • Data damages. When someone accesses the symmetric key, they can encrypt or decrypt everything using the key. When using symmetric encryption in two way communication, the messages for both sides can be easily compromised.
  • Symmetric cryptosystem have a problem of transportation. The secret key has to be transmitted to the recipient system before the actual message is sent this would mean that communication is insecure as it does not guarantee no one will hijack communication (Daemen & Rijmen, 2009).
  • Symmetric encryption cannot provide digital signatures that cannot be repudiated



Advantages of asymmetric

  • In asymmetric or public key, cryptography there is no need for exchanging keys, thus eliminating the key distribution problem.
  • The primary advantage of public-key cryptography is increased security: the private keys do not ever need to be transmitted or revealed to anyone.
  • Can provide digital signatures that can be repudiated




Disadvantages of asymmetric


  • A disadvantage of using public-key cryptography for encryption is speed: there are popular secret-key encryption methods which are significantly faster than any currently available public-key encryption method.


4) Identify different roles of encryption key, message digest, and digital signature.

Roles of encryption keys


Procter the data from being accessed by unauthorized person. When a message is send, the two inputs are the public key of the intended recipient. Key protect the data that is send through internet. If the data is send to a wrong person, no information can be drawn from it as it is fully protected by the keys (Daemen & Rijmen, 2009).

Encryption key is used to the Manage Domain password encryption,


Role of message digest

  • Used to produce digital summaries of information
  • Provide a digital identifier for each digital file or document
  • Process information to produce a different message digest for each unique document.

Role of digital signatures

Digital signatures has the private-key that signs signatures, and the public-key that verifies signatures

  • Digital signatures enables the owner (of the private-key) can create the digital signature, hence it can be used to verify who created a message
  • Only individuals with public key can verify the signature.
  •  Digital signatures usually don’t sign the whole message but just a hash of the message
  • digital signatures can provide non-repudiation of message origin, since an asymmetric algorithm is used in their creation,














Boneh, D., & Franklin, M. (2007). Identity-based encryption from the Weil pairing. In Advances in Cryptology—CRYPTO 2001 (pp. 213-229). Springer Berlin Heidelberg.

Daemen, J., & Rijmen, V. (2009). The design of Rijndael: AES-the advanced encryption standard. Springer Science & Business Media.