Public and Private Keys in PKI

In Public Key Infrastructure (PKI), public and private keys are fundamental components used for various cryptographic functions, including encryption, decryption, and digital signatures. Here’s a detailed look at how these keys work and their roles in PKI:

Public and Private Key Basics

1. Key Pair:

   • Public Key: One half of a cryptographic key pair that is shared openly. It is used for encrypting data or verifying digital signatures.
   • Private Key: The other half of the key pair that is kept secret and secure. It is used for decrypting data or creating digital signatures.

2. Key Generation:

   • Keys are generated as a pair. Each public key is paired with a corresponding private key. These keys are mathematically related but are not easily derived from one another.

Roles of Public and Private Keys

1. Encryption and Decryption:

   • Encryption: When data is encrypted using a public key, it can only be decrypted by the corresponding private key. This ensures that only the holder of the private key can access the encrypted data.
   • Decryption: The private key is used to decrypt data that was encrypted with the corresponding public key. This process ensures that only the intended recipient can read the data.

2. Digital Signatures:

   • Signing: A digital signature is created using a private key. This involves generating a hash of the data and then encrypting the hash with the private key. The signature proves that the data was created by the holder of the private key and has not been altered.
   • Verification: To verify a digital signature, the recipient uses the corresponding public key. The recipient decrypts the hash with the public key and compares it to a newly computed hash of the data. If they match, the signature is valid and the data is confirmed as authentic.

Detailed Use Cases in PKI

1. Secure Communication:

   • Public Key Encryption: When communicating securely, a sender can use the recipient’s public key to encrypt a message. Only the recipient’s private key can decrypt this message, ensuring confidentiality.
   • Private Key Decryption: The recipient uses their private key to decrypt the message, which ensures that only the intended recipient can read it.

2. Authentication:

   • Digital Certificates: Public keys are included in digital certificates issued by a Certificate Authority (CA). These certificates confirm the identity of the certificate holder and their associated public key.
   • Certificate Validation: When a certificate is presented, the public key in the certificate can be used to verify signatures made by the private key, confirming the authenticity of the certificate.

3. Data Integrity:

   • Digital Signatures: A digital signature ensures that the data has not been tampered with. The signature is created using the private key and can be verified by anyone with the corresponding public key, ensuring data integrity and authenticity.

4. Non-Repudiation:

   • Proof of Origin: Digital signatures provide non-repudiation, meaning that the sender cannot deny having sent the message. This is because the signature can only be created with the sender’s private key, and the recipient can verify it with the sender’s public key.

Key Management in PKI

1. Key Storage:

   • Private Key Security: Private keys must be stored securely to prevent unauthorized access. This often involves hardware security modules (HSMs) or encrypted storage solutions.
   • Public Key Distribution: Public keys are shared openly, often via digital certificates. They are distributed through directories or included in certificates to facilitate secure communication and verification.

2. Key Lifecycle:

   • Key Generation: Keys are generated using cryptographic algorithms.
   • Key Usage: Keys are used for encryption, decryption, signing, and verification.
   • Key Expiry and Renewal: Keys have expiration dates. When they expire, new keys must be generated and certificates renewed.
   • Key Revocation: If a private key is compromised, it must be revoked. The CA updates the Certificate Revocation List (CRL) or OCSP to reflect the revocation status.

Example Scenario

1. Secure Email:
   • Encryption: Alice wants to send a secure email to Bob. She uses Bob’s public key to encrypt the email. Only Bob’s private key can decrypt it.
   • Signing: Alice also signs the email with her private key. Bob uses Alice’s public key to verify the signature and ensure the email’s authenticity and integrity.

In summary, public and private keys are essential components of PKI, enabling secure communication, authentication, and data integrity. The public key is used for encryption and verification, while the private key is used for decryption and signing. Proper management and security of these keys are crucial to maintaining the effectiveness and trustworthiness of a PKI system.