PGP (Pretty Good Privacy) is a widely used encryption technology that ensures the confidentiality and integrity of data exchanged over the internet. It uses a combination of asymmetric and symmetric encryption algorithms to provide secure communication between parties.

Block ciphers are a type of symmetric encryption algorithm that operates on fixed-size blocks of data, encrypting or decrypting one block at a time. PGP uses a block cipher called the International Data Encryption Algorithm (IDEA) to encrypt data.

The IDEA algorithm was developed in the early 1990s by James Massey and Xuejia Lai. It uses a 128-bit key and operates on 64-bit blocks of data. IDEA employs a combination of modular arithmetic and bitwise operations to perform the encryption process. It uses a series of rounds, each consisting of a mix of substitution and permutation operations, to scramble the data and make it unreadable to unauthorized parties.

IDEA is considered to be highly secure and provides a good level of protection against attacks. It has been extensively analyzed and no significant vulnerabilities have been found. However, due to the increasing computational power of modern computers, IDEA is now considered to have a relatively short key size compared to more modern encryption algorithms.

Despite this, IDEA is still widely used in PGP because it provides a good balance between security and performance. PGP also supports other block ciphers, such as AES (Advanced Encryption Standard), which is currently considered to be the most secure symmetric encryption algorithm.

In conclusion, PGP encrypts data using the International Data Encryption Algorithm (IDEA), a block cipher that provides secure and efficient encryption. However, it is important to note that PGP also supports other block ciphers, such as AES, which can be used for encryption depending on the user's preference and security requirements.