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Homework 1 ICS 247 Security Algorithms
Please answer the following questions, each of which is worth 10 points.
1. Show how a man-in-the-middle attack can defeat the Diffie-Hellman method for establishing a shared secret key between two parties (Alice and Bob).
2. Decrypt the following cipher text, which was generated from English text using a substitution cipher:
ZLFIF ADVZ RF W BWH PDZ, YPF ZLPDOLZ. PJ UPDIVF, SJ LF LWG IFAFARFIFG ZP RISNO LSV IDRRFI ULSUMFN, SZ BPDQG RF FWVH. RDZ SN ZLSV IPPA WQQ YPF UPDQG VFF BWV W ZWRQF, W ZPH XWNGW, WNG W LWAAFI. PDZ PJ JIDVZIWZSPN, YPF LSZ ZLF XWNGW BSZL ZLF LWAAFI. ZP LSV OIFWZ VDIXISVF, ZLF XWNGW RIPMF PXFN WNG IFCFWQFG LSV BWH PJ FVUWXF--W HFQQPB, IDRRFI ULSUMFN!
3. In a public-key system using RSA, you intercept the ciphertex C = 10, sent to a user whose public key is e = 5, n = 35. What is the plaintext M?
4. In a public-key system using RSA, the public key of a certain user is e = 31, n = 3599. What is the private key d? Hint: you may use the Unix program factor.
5. In a public-key system using RSA, the public key of a certain user with public key e, n leaks his private key d. Being lazy, he recomputes a new e and d using the same n. Is this safe? Why or why not?