Question

I need some assistance finding a second preimage attack on a modified CBC Mac. Its a modified CBC mac where the last 20 bytes is taken instead of the usual 16. The message is also padded with PCKS7 before encryption, with this padding done in 64 byte blocks. I am given the key and IV, so Just need to find another message which gives the same MAC.

Answer 1

MAC functions based on block ciphers are of great practical significance. CBCMAC is a well-known method to generate a MAC based on a block cipher in CBC mode. you can say in general MAC functions based on block cipher in CBC encryption mode the CBC-like MAC functions. CBC-MAC is used to compress the message M of a fixed length mn with a secret key K, where n is the length of a block and m is the number of blocks. Its a modified CBC mac where the last 20 bytes is taken instead of the usual 16. The message is also padded with PCKS7 before encryption