tailieunhanh - cryptography for developers 2006 phần 6

Trước khi chúng ta có thể giải mã văn bản với cùng một cấu trúc CTR, chúng tôi có để thiết lập lại IV. Điều này là bởi vì sau khi mã hóa thô, giá trị chuỗi được lưu trữ trong cơ cấu CTR đã thay đổi. Nếu chúng ta cố gắng để giải mã nó ngay bây giờ, nó sẽ không làm việc. | Advanced Encryption Standard Chapter 4 199 The ctr_encrypt function can be called as many times as required to encrypt the plaintext. Each time the same CTR structure is passed in it is updated so that the next call will proceed from the point the previous call left off. For example ctr_encrypt hello ciphertext 5 ctr ctr_encrypt world ciphertext 5 6 ctr and ctr_encrypt hello world ciphertext 11 ctr perform the same operation. 028 reset the IV 029 ctr_setiv IV 16 ctr 030 031 decrypt it 032 ctr_decrypt ciphertext buf 32 ctr Before we can decrypt the text with the same CTR structure we have to reset the IV. This is because after encrypting the plaintext the chaining value stored in the CTR structure has changed. If we attempted to decrypt it now it would not work. We use the ctr_decrypt function to perform the decryption from ciphertext to the buf array. For the curious ctr_decrypt is just a placeholder that eventually calls ctr_encrypt to perform the decryption. 034 print it 035 for x 0 x 32 x printf c buf x 036 printf n 037 038 return EXIT_SUCCESS 039 At this point the user should be presented with the string hello world how are you and the program should terminate normally. 200 Chapter 4 Advanced Encryption Standard Frequently Asked Questions The following Frequently Asked Questions answered by the authors of this book are designed to both measure your understanding of the concepts presented in this chapter and to assist you with real-life implementation of these concepts. To have your questions about this chapter answered by the author browse to solutions and click on the Ask the Author form. Q What is a cipher A A cipher is an algorithm that transforms an input plaintext into an output ciphertext with a secret key. Q What is the purpose of a cipher A The first and foremost purpose of a cipher is to provide privacy to the is accomplished by controlling the mapping from plaintext to ciphertext with a secret key. Q What .

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