PKI is a framework which enables the integration of various services that are related to cryptography.
The aim of PKI is to provide confidentiality, integrity, access control, authentication, and most importantly non-repudiation.
Note
Non-repudiation is a concept or a way to ensure that the sender of a message cannot deny in future the sending of such a message. This is the confirmation of the authenticity of the sender's message. Because it is encrypted with a private key, and only one person has the private key, it has to be this person who sent the message/e-mail. One of the important audit checks for non-repudiation is the time stamp. The time stamp is an audit trail that provides the information on the time the message was sent.
Encryption/decryption, digital signature, and key exchange are the three primary functions of a PKI.
RSS and Elliptic curve algorithms provide all three primary functions; that is, encryption/decryption, digital signature, and key exchange. The Diffie-Hellman algorithm supports key exchange while the Digital Signature Standard (DSS) is used in the digital signature.
Public Key Encryption is the encryption methodology used in PKI and was initially proposed by Diffie and Hellman in 1976. The algorithm is based on mathematical functions and it uses asymmetric cryptography; that is, it uses a pair of keys:
The preceding image represents a simple document-signing function. In PKI, every user will have two keys called a pair of keys. One key is called a private key and the other key is called a public key. The private key is never revealed and it is kept with the owner. The public key is accessible by everyone and is stored in a key repository.
A key can be used to encrypt or decrypt a message. The key pairs work together and, based on their function, they can either encrypt or decrypt (not both for the same info/function). Most importantly, a message that is encrypted with a private key can only be decrypted with a corresponding public key and, similarly, a message that is encrypted with a public key can only be decrypted with the corresponding private key.
In the preceding example image, Bob wants to send a confidential document electronically to Alice. Bob has four issues to address before this electronic transmission:
- To ensure that the contents of the document are encrypted such that the document is kept confidential.
- To ensure that the document is not altered during transmission and maintain integrity.
- Since Alice does not know Bob, he has to somehow prove that the document is indeed sent by him; that is, source authenticity.
- To ensure that Bob cannot deny sending it in the future.
PKI supports all four requirements by way of methods such as secure messaging, message digest, digital signature, and non-repudiation services.
To ensure that the document is protected from eavesdropping and not altered during the transmission, Bob will first encrypt the document using Alice's public key. This ensures two things: one, that the document is encrypted, and two, only Alice can open it as the document requires the private key of Alice to open it. In summary, encryption is done using the public key of the receiver, and the receiver decrypts with his/her private key. In this method, Bob could ensure that the document is encrypted and the intended receiver (Alice) only can open it. However, Bob cannot ensure whether the contents are altered (Integrity) during transmission by just document encryption.
In summary, when confidentiality is required, the sender will use the receiver's public key to encrypt the message body.
In order to ensure that the document is not altered during transmission, Bob performs a hash function on the document. The hash value is a computational value based on the contents of the document. This hash value is called message digest. By performing the same hash function on the decrypted document, the message digest can be obtained by Alice and she can compare it with the one sent by Bob to ensure that the contents are not altered.
This process will ensure the integrity requirement. However, the hash (message digest) will be encrypted using the public key of the receiver. Otherwise, without encrypting the hash (message digest), a hacker could simply alter the information and recompute a hash for the manipulated data. Because a digital signature is protected by the sender's private key, the encrypted hash could not be recreated.
In order to prove that the document is sent by him to Alice, Bob needs to use a digital signature. Digital signature means applying a sender's private key to the message, document, or to the message digest. This process is called signing. The message can only be decrypted using the sender's public key:
Bob will encrypt the message digest with his private key to create a digital signature. In this scenario, Bob will encrypt the document using Alice's public key and he will digitally sign it using his private key. This ensures that Alice can verify that the document was sent by Bob. She can do so by verifying the digital signature (Bob's private key) using Bob's public key (remember a private key and the corresponding public key are linked, albeit mathematically). She can also verify that the document is not altered by validating the message digest. She can also open the encrypted document using her private key.
By digitally signing the document, Bob has assured that the document was sent by him to Alice. However, he has not yet proved that he is Bob. To prove this, Bob needs to use a digital certificate. That is, digitally signing with the private key should prove Bob is genuine.
A digital certificate is an electronic identity issued to a person, system, or an organization by a competent authority after verifying the credentials of the entity. A digital certificate contains a public key that is unique for each entity. A certification authority issues digital certificates.
In PKI, digital certificates are used for authentic verification of an entity. An entity can be an individual, system, or an organization.
An organization that is involved in issuing, distributing, and revoking digital certificates is known as a certification authority (CA). A CA acts as a notary by verifying an entity's identity. A Certification Authority is a trusted third party. A CA digitally signs and publishes the public key of the user. This is done using the CA's private key. Hence, the trust of the user relies on the trust of the CA.
One of the important PKI standards pertaining to digital certificates is X.509. This is an International Telecommunication Union (ITU) published standard. It specifies, among other things, the standard format for digital certificates.
PKI also provides the key exchange functionality that facilitates a secure exchange of public keys so that authenticity of the parties can be verified. Hence, the significant function is for PKI to be used for the secure exchange of session (symmetric) keys.