TLS/SSL Birthday attacks on 64-bit block ciphers (SWEET32)

Description

Legacy block ciphers having a block size of 64 bits are vulnerable to a practical collision attack when used in CBC mode. All versions of the SSL/TLS protocols that support cipher suites which use 3DES as the symmetric encryption cipher are affected. The security of a block cipher is often reduced to the key size k: the best attack should be the exhaustive search of the key, with complexity 2 to the power of k. However, the block size n is also an important security parameter, defining the amount of data that can be encrypted under the same key. This is particularly important when using common modes of operation: we require block ciphers to be secure with up to 2 to the power of n queries, but most modes of operation (e.g. CBC, CTR, GCM, OCB, etc.) are unsafe with more than 2 to the power of half n blocks of message (the birthday bound). With a modern block cipher with 128-bit blocks such as AES, the birthday bound corresponds to 256 exabytes. However, for a block cipher with 64-bit blocks, the birthday bound corresponds to only 32 GB, which is easily reached in practice. Once a collision between two cipher blocks occurs it is possible to use the collision to extract the plain text data.