Cryptography Tutorials - Herong's Tutorial Examples - v5.42, by Herong Yang
DES Decryption Algorithm
This section describes DES decryption algorithm - identical to the encryption algorithm step by step in the same order, only with the subkeys applied in the reverse order.
The decryption algorithm of a block cipher should be identical to encryption algorithm step by step in reverse order. But for DES cipher, the encryption algorithm is so well designed, that the decryption algorithm is identical to the encryption algorithm step by step in the same order, only with the subkeys applied in the reverse order.
DES decryption algorithm:
Input: CC: 64 bits of cipher text k16, k15, ..., k1: 16 round keys IP: Initial permutation FP: Final permutation f(): Round function Output: TT: 64 bits of clear text Algorithm: CC' = IP(CC), applying initial permutation (LL0, RR0) = CC', dividing CC' into two 32-bit parts (LL1, RR1) = (RR0, LL0 ^ f(RR0, k16)) (LL2, RR2) = (RR1, LL1 ^ f(RR1, k15)) ...... TT' = (RR16, LL16), swapping the two parts TT = FP(TT'), applying final permutation
Here is how to approve the decryption algorithm:
Let: T: 64 bits of clear text C: 64 bits of cipher text encrypted from T CC: 64 bits of cipher text TT: 64 bits of clear text decrypted from CC If: CC = C Then: TT = T Prove: CC' = IP(CC) First step of decryption = IP(C) Assumption of CC = C = IP(FP(C')) Last step of encryption = C' IP is the inverse permutation of FP (LL0, RR0) = CC' Initializing step in decryption = C' CC' = C' = (R16, L16) Swapping step in encryption (LL1, RR1) = (RR0, LL0 ^ f(RR0, k16)) First round of decryption = (L16, R16 ^ f(L16, k16)) Previous result = (R15, (L15 ^ f(R15,k16)) ^ f(R15, k16)) (L16, R16) = (R15, L15 ^ f(R15, k16)) = (R15, L15) ^ reverse itself ...... (LL16, RR16) = (R0, L0) TT' = (RR16, LL16) Swapping in decryption = (L0, R0) Previous result = T' Initializing step in encryption TT = FP(TT') Last step in decryption = FP(T') Previous result = FP(IP(T)) First step in encryption = T FP is the inverse permutation of IP
Table of Contents
Introduction to AES (Advanced Encryption Standard)
►Introduction to DES Algorithm
DES (Data Encryption Standard) Cipher Algorithm
DES Key Schedule (Round Keys Generation) Algorithm
DES Algorithm - Illustrated with Java Programs
DES Algorithm Java Implementation
DES Algorithm - Java Implementation in JDK JCE
DES Encryption Operation Modes
PHP Implementation of DES - mcrypt
Blowfish - 8-Byte Block Cipher
Secret Key Generation and Management
Cipher - Secret Key Encryption and Decryption
RSA Implementation using java.math.BigInteger Class
Introduction of DSA (Digital Signature Algorithm)
Java Default Implementation of DSA
Private key and Public Key Pair Generation
PKCS#8/X.509 Private/Public Encoding Standards
Cipher - Public Key Encryption and Decryption
OpenSSL Introduction and Installation
OpenSSL Generating and Managing RSA Keys
OpenSSL Generating and Signing CSR
OpenSSL Validating Certificate Path
"keytool" and "keystore" from JDK
"OpenSSL" Signing CSR Generated by "keytool"
Migrating Keys from "keystore" to "OpenSSL" Key Files
Certificate X.509 Standard and DER/PEM Formats
Migrating Keys from "OpenSSL" Key Files to "keystore"