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3DES or Triple DES is a block cipher formed by using the Data Encryption Standard (DES) cipher three times.
3DES is typically implemented as ENCRYPT(block,K1), DECRYPT(block,K2), ENCRYPT(block,K3). This way, if you set K1==K2 it's backwards compatible with single-key DES with K3. It's pretty cool, but perhaps less useful today than it was when 3DES was first brought out.