The application layer is the first layer (topmost) of the Open System Interconnection (OSI) model. OSI model has seven layers which are Application layer, Presentation layer, Session layer, Transport layer, Network layer, Datalink layer and Physical layer. Being on the top, it should also be strong enough to protect all other’s layers communication. The application layer provides end-user services, application interconnection and process to process connection over the network. So, Secure Protocols of Application Layer are crucial to strengthening secure communication. The application layer provides the possibility of persuasive communication between applications and also manages communication methods. However, It is also dependant on other layers for the competition of process and its present data in visual form.   

What are the functions of the application layer?

Following are some functions of the application layer:

  • Virtual terminal
  • Transport access and management 
  • Directory services
  • Mail services
  • File transfer 

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How functions of Application layer proceed?

Application layer gives different services to the user, and these services are provided through protocols so, application layer working is basically done through various protocols. Following are some examples of application layer protocols:

  1. HTTPS (Hypertext Transfer Protocol Secure)
  2. FTP (File Transfer Protocol)
  3. DNS (Domain Name Server)
  4. TFTP (Trivial File Transfer Protocol)
  5. SMTP (Simple Mail Transfer Protocol)
  6. TELNET (TELecommuncation NETwork)
  7. NFS (Network File System)
  8. LPD (Line Printer Daemon)
  9. DHCP (Dynamic Host Configuration Protocol)
  10. SNMP (Simple Network Management Protocol)
  11. S/MIME (Secure/Multipurpose Internet Mail Extension)
  12. OpenPGP (Open Pretty Good Privacy) 
  13. MSP (Message Security Protocol)
  14. PEM (Privacy Enhanced Mail)
  15. SSL (Secure Socket Layer)
  16. OFX (Open Financial Exchange)

Secure protocols

Now we discuss in detail of some only secure protocols.


HTTPS (Hypertext Transfer Protocol Secure) is an updated version of HTTP, communication through HTTP is not secure for making information securable HTTPS introduced. Data communication through HTTPS is in encrypted form for ensuring security. Data sent and received in encrypted form. Therefore, no one can see sensitive information during transmission.

How HTTPS works?

HTTPS used two more protocols for encryption of data that are SSL (Secure Socket Layer) and TLS (Transport Layer Security) for providing more security. However, SSL and TLS protocols use an asymmetric public key framework, this framework uses two different keys for encryption of data, and these keys are:

  • Private Key: This key is managed and controlled by the owner of the website or system. Firstly, this key used to decrypt the data encrypted by the public key. Key is also available on the webserver.
  • Public Key: Firstly, the Information is encrypted using the public key and then decrypted using the private key. This key is accessible to everyone on the server which interacts with the server. 

HTTPS provides three layers of protection which are:

  • Encryption: When you deal me sensitive data, its critical to safe from attackers. So that’s why in HTTPS data is sent and receive in encrypted form so on one can see or read your data. This encryption is done using the public key and is decrypted by the private key, and that key knows the only owner.
  • Data Integrity: Encryption provides data integrity, so on one can modify your sensitive data.
  • Authentication: This confirms communication user means your communication with the right user. Authentication protects from a man-in-the-middle attack.  


PEM (Privacy Enhanced Mail) protocol which is developed by IETF (Internet Engineering Task Force). It is a security protocol of application layer which provide confidentiality and authentication to make electronic messages more securable. PEM is a very competent standard because it is compatible with existing mail standards, support individually addressed and list address. It is consistent with the key management scheme and provides all four security standards.

What are PEM messages types?

Following are the PEM message types.

  • MIC-CLEAR: This message grant integrity and authentication only.     
  • MIC-ONLY: This message gives authentication and integrity with the addition of gateway implementation.
  • ENCRYPTED: This message provides three security services such as confidentiality, integrity and authentication. 

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S/MIME (Secure/ Multipurpose Internet Mail Extension) is a secure protocol of the application layer and give four security standards to electronic messages. It is developed by RSA Data security and handled by the IETF S/MIME group. S/MIME is also suitable for all existing services of internet mail and does not support non-secure message delivery. Mostly features of S/MIME are identical with PEM.   

What are the message types of S/MIME?

There are two message types of S/MIME protocol: 

  • Signed: Signed message type provide integrity and also the sender authentication. 
  • Signed and Enveloped: Signed and Enveloped message type provides three security services like confidentiality, integrity and authentication. 


OpenPGP (Open Pretty Good Privacy), also known as PGP/MIME, is another application layer protocol. On the internet, OpenPGP is most extensively security message program; its features and also the functionality are very identical with S/MIME protocol. Two features of OpenPGP are also diverse form S/MIME protocol and that are MIEM encapsulation and encryption algorithm. 


OFX (Open Financial Exchange) Protocol, designed by Checkfree for the application layer. This protocol used in systems whose has many users and used in the small banking system over the internet. OFX is an open-source specification that is available to every financial company to utilize OFX services. Therefore, OFX also provides security services of confidentiality, integrity and authentication using SSL and a digital certificate.  

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