Which of the following symmetric key algorithms are examples of block ciphers? (Select THREE).

RC4 is a symmetric stream cipher that encrypts data one byte at a time rather than in fixed-size blocks, so it is not a block cipher.

3DES applies the DES algorithm three times sequentially to each 64-bit data block, making it a symmetric block cipher with significantly stronger security than original DES.

AES is a symmetric block cipher standardized by NIST that operates on 128-bit blocks with key sizes of 128, 192, or 256 bits.

MD5 is a one-way cryptographic hash function used for data integrity verification, not a symmetric encryption algorithm.

PGP is an encryption program and standard that uses a combination of ciphers and hashing algorithms; it is not itself a cipher.

Blowfish is a symmetric block cipher that operates on 64-bit blocks with a variable-length key from 32 to 448 bits, designed as a free alternative to DES.