# Dat 9 - plain block = FP(L[0]R[0 To encrypt or decrypt more...

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This has been a description of how to use the DES algorithm to encrypt one 64-bit block. To decrypt, use the same process, but just use the keys K[i] in reverse order. That is, instead of applying K[1] for the first iteration, apply K[16], and then K[15] for the second, on down to K[1]. Summaries: Key schedule: C[0]D[0] = PC1(key) for 1 <= i <= 16 C[i] = LS[i](C[i-1]) D[i] = LS[i](D[i-1]) K[i] = PC2(C[i]D[i]) Encipherment: L[0]R[0] = IP(plain block) for 1 <= i <= 16 L[i] = R[i-1] R[i] = L[i-1] xor f(R[i-1], K[i]) cipher block = FP(R[16]L[16]) Decipherment: R[16]L[16] = IP(cipher block) for 1 <= i <= 16 R[i-1] = L[i] L[i-1] = R[i] xor f(L[i], K[i])
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Unformatted text preview: plain block = FP(L[0]R[0]) To encrypt or decrypt more than 64 bits there are four official modes (defined in FIPS PUB 81). One is to go through the above-described process for each block in succession. This is called Electronic Codebook (ECB) mode. A stronger method is to exclusive-or each plaintext block with the preceding ciphertext block prior to encryption. (The first block is exclusive-or'ed with a secret 64-bit initialization vector (IV).) This is called Cipher Block Chaining (CBC) mode. The other two modes are Output Feedback (OFB) and Cipher Feedback (CFB)....
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