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PV181 Laboratory of security and applied cryptography
HW 01 Solution
•Marek Sýs
•syso@mail.muni.cz, A405

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Task 01
key generation and bitmanipulation
•bits_used – number of generated bits
•Which bit is the first not set:
–byte_offset - index of byte in which is the bit)
–bit_offset – position of bit in the given byte (byte_offset)
–Example for (bit_offset =2 and rnd=01011… ):
–
–
–
•
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I      PA193 - Introductory info
01234567
00000000
01234567
01000000
01234567
00000000
01234567
00000000
01234567
00000000
--010111    10001100    11000111 11001001    10
10001100    11000111 11 001001   10000000
01011110   00110011   00011111  00100110
Shift by +2
Shift by -6

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Task 02
Key reconstruction
•Encryption:
–K = rand() | rand () | … but seeded with = time
–E(K,pt) = ct
•
•Attack:
–Entropy of the seed is 32 bits ! - easy to guess
–For all seeds:
•Generate key K’, encrypt E(K’,pt) = ct’
• If ct’ == ct return K’
–
–
–
–
•
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I      PA193 - Introductory info

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Task 03
Secure encryption
•Entire key  has to be generated by dev/(u)random
–If only seed is generated – same problem
•
•while(obtained < requested){
• bytes  = read(file, buffer, requested - obtained);
•    if (bytes < 0){ …unable to read from file…}
• else {obtained += bytes}
•}
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I      PA193 - Introductory info

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Consultations
•You can discuss your points 6.10 after the Seminar
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I      PA193 - Introductory info