1. CS691 – Chapter 9 of Matt Bishopz
Basic Cryptography
C. Edward Chow
CS691 – Chapter 9 of Matt Bishopz
Chapter 8 of Tanenbaum

2. Topics to be Covered Introduction to Cryptography Substitution Ciphers
Transposition Ciphers
One-Time Pads: Quantum Cryptography
Two Fundamental Cryptographic Principles
Symmetric-Key Algorithms
Public-Key Algorithms
Key Management: Digital Signatures

3. Cryptography Greek words for “Secret Writing”. Cipher vs. Code
Cipher: a character-to-character or bit-to-bit transformation without regard to linguistic sturcture of the msg.
Code: replace one word woth another word or symbol. e.g., Navajo code WWII (chay-da-gahi-nail-tsaidi(“tortoise killer).
The art of devising ciphers is called cryptography
The art of breaking ciphers is called cryptanalysis
Cryptography and cryptanalysis is collectively known as Cryptology.

4. Cryptosystem
A cryptosystem is a 5-tuple (E,D,M,K,C), where M —a set of plaintexts (some use P as symbol); K —the set of keys; C —the set of ciphertexts; E: M x K  C —the set of enciphering functions; D: C x K  M —the set of deciphering functions;
Caesar cipher example:
M = { all sequences of Roman letters } K = { i | i an integer such that 0  i  25 } E = { Ek | k  K and  m  M, Ek(m)=(m+k) mod 26 } D = { Dk | k  K and  c  C, Dk(c)=(26+c-k) mod 26 }
E.g., the word m=“HELLO”  c=“KHOOR” what is k?

5. Encryption Model (Symmetric-Key Cipher)
Adapted from Tanenbaum Computer Networks Figure 8.2 Here P=M, and key is moved as subscript of functions.

6. Basic Encryption Methods
Two categories:
Substitution Ciphers: each letter or group of letters is replaced by another letter or another group of letters. It preserves the order of the plaintext symbol but disguise them.
E.g., Caesar cipher; mono-alphabetic substitution (each letter map onto another letter; 26!=4x10^26 possible keys)
Transposition Ciphers: Reorder the letters but do not disguise them.
E.g., Rail Fence cipher m=“HELLOWORLD” distribute the letter up and down between two rows from left to right; then output row-wise.  HLOOL  c=“HLOOLELWRD” ELWRD

7. Kerckhof’s Principle
“All crypto algorithms must be public; only the keys are secret”
--- “La Cryptographie Militaire,” J. des Sciences Militaires, vol. 9, pp.5-38, Jan and pp , Feb
Trying to keep the algorithm secret (security by obscurity principle?) never works. Reasons:
Logistic issue: Too much effort to invent, test, and install new algorithm
“Publish the algorithm and let academic cryptologists to break the system. If no one succeeded in 5 years, it must be pretty solid.”
Real secrecy in the key, its length a major design issue

8. Key Length and Work Factor
2 digit key  100 combinations.
6 digit key  1 million combinations.
64 bit keys to prevent kid brother from reading your .
128 bit keys for routine commercial use
>256 bits keep major governments at bay.

9. 3 Variations of Cryptoanalysis or Cipher Attacks
Ciphertext-only: cryptoanalyst has some ciphertext; no (does not know the corresponding) plaintext; no key; no E or D functions.
Known plaintext: some matched ciphertext and plaintext. (no key; may/may not know the E or D functions)
Chosen plaintext: has the ability to encrypt pieces of plaintext of his own choosing. (know the E or D function; does not know the keys used).

10. Basic Cipher Attacks for Mono-alphetic Substitution
Use statistical properties of natural languages.
Rank of Frequency of appearance of
Unigrams (single letter): e, t, o, a, n, i etc
Digrams (two-letter): th, in, er, re, an, etc
Trigrams (three-letter): the, ing, and , ion, etc
Counting the frequencies of letters in ciphertext.
Tentatively assign the most common one to e; next to t...
Then look at the trigrams of decipher text, if “tXe” appears, it should be “the”! Original deciphering function should be changed to map from X to h.
Similarly, what should we do if we see “thYt”?
How about “aZW”?

11. Cipher Attack: Guess a Probable Word
Ciphertext from accounting firm CTBMN BYCTC BTJDS QXBNS GSTJC BTSWX CTQTZ CQVUJ QJSGS TJQZZ MNQJS VLNSX VSZJU JDSTS JQUUS JUBXJ DSKSU JSNTK BGAQJ ZBGYQ TLCTZ BNYBN QJSW
A likely word is financial.
Based on two i are separated by four letters, we look for same pattern in ciphertext.
There are 12 hits, position 6, 15, 27, 31, 42, 48, , 70, 71, 76, and 82.
Only 31 and 42 has the next letter n (ana) separated by one letter. i C and J; nT and S respectively.

12. Columnar Transposition Ciphers
A transposition cipher. Keyed by a phrase such as “MEGABUCK”. The letter in the key indicated the order of columns to be output. Plaintext horizontally read in, ciphertext read out column by column.

13. Breaking Transposition Cipher
Check if the frequency distribution of unigrams are the same in the ciphertext. If it is, then this is a transposition cipher.
Guess the number of columns.
Assume we suspect “milliondollars” appear in ciphertext
Analyze keylength: Observe digrams MO, IL, LL, LA, IR, and OS in ciphertext as wrapping around effect  keylength = 8.
Analyze order of columns. If keylength is small, column pairs (k(k-1) of them) of ciphertext can be examined and see if the patterns of digrams in the deciphered text match those of the language frequency distribution.

14. One-Time Pads
Unbreakable Cipher. Choose long random bit string as key (same length of the text?) Use Bit XOR as E and D. Problem: How to distribute and protect the key.
The use of a one-time pad for encryption and the possibility of getting any possible plaintext from the ciphertext by the use of some other pad.

15. Quantum Cryptography
Can be used to transfer one-time pad over networks.
Here Fiber channel is assumed.
Light comes as little packets called Photons
Photons can be polarized using filter such as sunglass.
The photons after the polarized filters will be polarized in the direction of the filter’s axis.
If the beam goes through the 2nd filter and
the two filters are perpendicular,  no photons get through.
The light intensity after the 2n filter is proportion to the square of the cosine of the angle between the two filter axes.

16. How one-time pad is sent using Quantum cryptography
Alice and Bob each has two set of filters. One with vertical and horizontal filters called rectilinear basis. The other set rotate 45 degree called diagonal basis.
Alice assigns vertical as 0 and horizontal as 1; lower left to upper right as 0 and upper left to lower right as 1. Send this assignment in plain text to Bob.
Alice pick a one-time pad
Transfer them bit by bit to Bob using one of the two bases at random. See Figure 8-5(a)
Bob does not know which bases to use, he randomly pick one. See Figure 8-5(b).
If he picks right, he gets correct bits. If not, he gets random bit.
A photon hit a filter with 45 degree to photon’s polarization, will randomly jump to the axis of the filter or perpendicular to that. Figure 8-5(c).
Bob tells Alice the axes he used in plaintext. Alice tells him which are right/wrong in plaintext. Figure 8-5(d). Now both have a correct bit string. Figure 8-5(e).
Trudy’s choices of bases Figure 8-5(f). Trudy’s bit pattern with Unknown bits.

17. Quantum Cryptography

18. Two Fundamental Cryptographic Principles
Redundancy: Messages must contains some redundancy.
E.g., Last three bytes of encrypted packet content are product # and quantity.
Recent fired employee can capture the packet replace the last three byte quantity field with a random number.
How redundancy can help?
Freshness Some method is needed to foil replay attacks.
How to defense reply attacks? Timestamp alone?

19. Symmetric-Key Algorithms
DES – The Data Encryption Standard
AES – The Advanced Encryption Standard
Cipher Modes
Other Ciphers
Cryptanalysis

20. Product Ciphers Basic elements of product ciphers.
P-box (b) S-box (c) Product.
Hardware implementation. P: permutation (transpose). S: Substitution; in, out.
Software implementation: through iterations called rounds

21. Data Encryption Standard
Ri-1 expand to 48 bit; exclusive or with Ki
Divide into 8 groups 6bit each
Each group goes through S-Box; output is 4 bits.
The data encryption standard 1977/IBM (12856 bits key) for unclassified info. (a) General outline. (b) Detail of one iteration. The circled + means exclusive OR.

22. Triple DES
IS8732; two keys
(a) Triple encryption using DES. (b) Decryption.
Compatible with old DES. If we use the same key instead a different K2.
128 bit key2^128=3*10^38keys; evaluate one key per picoseconds it takes 10^10 years.

23. AES – The Advanced Encryption Standard
Rules for AES proposals
The algorithm must be a symmetric block cipher.
The full design must be public.
Key lengths of 128, 192, and 256 bits supported.
Both software and hardware implementations required
The algorithm must be public or licensed on nondiscriminatory terms.

24. AES (2)
An outline of Rijndael, by Joan Daemen and Vincent Rijmen 86 votes

25. AES (3)
Creating of the state and rk arrays.

26. Electronic Code Book Mode
The plaintext of a file encrypted as 16 DES blocks.
AES/DES are monoalphabetic substitution cipher. Same plaintext with same key in, same ciphertext out.
Cipher Attack: Just substitute 12th block with 4th block and Leslie has a Merry Christmas.

27. Cipher Block Chaining Mode
Use the chaining mode to defeat the above attack.
C1 depends on C0.
Cipher block chaining. (a) Encryption. (b) Decryption.

28. Cipher Feedback Mode
For terminal type application, wait for 8 characters before sending the ciphertext is not an option.
Use Character Chaining mode with a shift register.
(a) Encryption. (b) Decryption.

29. Stream Cipher Mode
In previous modes, one bit Tx error messing whole block.
In Stream cipher mode, early ciphertext does not involve with later encryption and one bit Tx error one bit plaintext error
A stream cipher. (a) Encryption. (b) Decryption.

30. Counter Mode
Except electronic code book mode, previous modes requires first decrypting all the blocks ahead the current block. Make it difficult to do random access of encrypted files. (problem is reuse attack, when same key is used.
Encryption using counter mode.

31. Cryptanalysis Rijndael got adopted as AES . Serpent(cambridge) 2nd.
Some common symmetric-key cryptographic algorithms.

32. Public-Key Algorithms
1976 Dillfie and Hellman proposed crypto scheme with two keys; public key and private key. Requirement:
Must be computationally easy to encipher/decipher msg using these keys.
Must be computationally infeasible to derive the private key from public key.
Mulst be computationally infeasible to determine the private key from a chosen plaintext attack.
Symmetric key exchange protocol
RSA
Other Public-Key Algorithms

33. Diffe-Hellman’s Symmetric Key Exchange Protocol
It is based on discrete logarithm problem.
Alice and Bob chooses a prime p=53 and g=17 which is not 0, 1, or p-1=52.
Alice chooses private key=5, public key=175 mod 53 =40.
Bob choose private key=7, public key=177 mod 53=6.
Bob would like to send msg to alice.
Bob compute the shared secret key by enciphering Alice’s public key using his private key: 407 mod 53 = 38
Encipher the msg with key=38.
Alice computes the shared secret key as 65 mod 53=38.
Then decipher the msg with key=38.

34. RSA An exponentiation cipher. Choose two prime numbers p and q.
Let n = pq. The totient (n) of n is the number of numbers less than n with no factors in common with n.
(n)=(p-1)(q-1)?
E.g., (10) =4; since 1,3, 7, 9 are relative prime of 10.
Choose e <n; e be relative prime to (n).
Find d such that ed mod (n) = 1.
The public key is (e,n), private key is d.
C=me mod n
M=cd mod n

35. RSA: Confidentiality Example Encrypted using Alice’s Public key
Let p=7 and q = 11. n=77 and (n) =60.
Alice choose e=17, a relative prime to 60  private key is d=53 where e*d mod (n) =1; 17*53 mod 60 = 1
If we represent 07 as and 25 as Z, 26 as blank, then
HELLO WORLD will be Using Alice public key the cipher text is:
07^17 mod 77 = 28
04^17 mod 77 = 16
11^17 mod 77 = 44 …
03^17 mod 77 = 75.
Only Alice can decipher with private key 53.

36. RSA: Origin Authentication Example Encrypted using Alice’s Private key
Let p=7 and q = 11. n=77 and (n) =60.
Alice choose e=17,  private key is d=53.
Public key is (17, 77).
07^53 mod 77 = 35
04^53 mod 77 = 09
11^53 mod 77 = 44 …
03^53 mod 77 = 05

37. RSA: Confidentiality and Authentication Example
Encrypted using Sender’s Private key and the recipient’s private key
Bob’s public key 37, private key 13.
Alice’s public key 17, private key 53.
Encipherment:
(07^53 mod 77)^37 mod 77 = 07
(04^53 mod 77)^37 mod 77 = 37
(11^53 mod 77)^37 mod 77 = 44 …
(03^53 mod 77)^37 mod 77 = 47
Decipherment: decipher with recipient’s private key and autenticate with sender’s public key.
(07^13 mod 77)^17 mod 77 = 07
(37^13 mod 77)^17 mod 77 = 04
(44^13 mod 77)^17 mod 77 = 11
(47^13 mod 77)^17 mod 77 = 03

38. RSA Cipher Attack/Defense Example
Simplified example, character as block. NO can be swapped and interpreted as ON. (Attack is on or not?)
Defense: 6 bits for block # followed by 8 bit character. The rest 1010 bits contains random data. Rearrange of blocks can be detected by checking the block # field.
This is an application of the Redundancy principle.

39. RSA
Actual RSA primes should be at least 512 bits  modulus at least 1024 bits.
RSA combined with hash to prevent reordering attack.
An example of the RSA algorithm. Here p=3, q=11, n=33, z=(p-1)*(q-1)=20, choose d=7, which is relative primve of z. choose e=3 where e*d mod 20 = 1.
Here (3, 20) is public key. (7,20) is private key.
C=Pe mod n; P=Cd mod n;

40. Digital Signatures
Symmetric-Key Signatures
Public-Key Signatures
Message Digests
The Birthday Attack

41. Symmetric-Key Signatures
Digital signatures with Big Brother (BB: a central authority trust by everyone; knows everything!).
Everyone (i) carries secret key Ki to BB’s office.
Alice enciphers the following data with her secret key KA (B:Bob’s ID, RA: a random number chosen by Alice, t: timestamp, P: plaintext msg.); send it to BB.
BB deciphers with KA; encrypted the data together with a signed msg KBB(A, t, P) using B’s secret key.

42. Symmetric-Key Signatures: Nonrepudiation and Against Relay Attack
BB will not accept a msg from Alice unless it is encrypted with KA.
KBB(A, t, P) contains the proof that A sent P at time t.
Against replay attack:
Against instant replay: Within some time period, Bob can check if RA has been reuse; drop those packet.
Against old replay: Based on the timestamp, Bob can discard old messages.
Disadvantage:
BB needs to be trusted by everyone.
BB gets to read all signed msgs.

43. Public-Key Signatures
Digital signatures using public-key cryptography.
1991, NIST proposed Digital Signature Standard (DSS) using variant of the EL Gamal public key algorithm (discrete logarithm).
But it is too secret (NSA designed); too slow (10-40 times slower than RSA for checking signature); to new (not yet thoroughly analyzed); to insecure (fixed 512bits; later changed to 1024 bits)

44. Message Digests Authentication without encrypting the entire msg.
4 properties of Message Digest (MD hash function: arbitrarily long plaintext fixed-length bit string.
Given P, it is easy to compute MD(P).
Given MD(P), it is effectively impossible to find P.
Given P no one can find P’ such that MD(P’)=MD(P).
A change to the input of even 1 bit produces a very different output.
For example, Instead of KBB(A, t, P) , we have KBB(A, t, MD(P))
Digital signatures using message digests.

45. MD5 Designed by Ronald Rivest 1992. 5th of a series of MD.
Pre-computation Step:
First pad the msg a length of 448 bits (mod 512).
Original msg is then appended with 64 bit integer; make it a multiple of 512 bits.
Initialize 128-bit buffer to a fixed value.
Computation step:
Take 512 bit block of input, perform 4 round of mixing, thoroughly with 128 bit buffer (with sine function and table look up)
Process continues until all input consume. The content of 128-bit buffer form the message digest.
You can access it on our CS Unix machines using OpenSSL pacckage “openssl md5”
For example, the following yield a pretty good random key:
bash-2.05a# (date; ps auxg) | openssl md5
ddf1371aefc921d504fa51e6a
7c56744bb2440abcc2de7a492ae32d06

46. SHA-1 Developed by NSA and blessed by NIST in FIP 180-1
It generates 160 bit message digest.
Use of SHA-1 and RSA for signing nonsecret messages.
bash-2.05a# (date; ps auxg) | openssl sha1 44c702745bdeced27d8c01b8bcda28bb311e51f4

47. SHA-1 (2) (a) A message padded out to a multiple of 512 bits.
(b) The output 32 bit variables. (c) The word array.

48. SHA1 Copy 16 work block input to w0 to w15.
Scramble them to w16 to w79 with Wi= S1(Wi-3 XOR Wi-8 XOR Wi-14 XOR Wi-16) (16 <=i<= 79)
Sb(W) represent the left circular rotation of 32-bit word W by b bits.
Actual step in pseudo-C code: for (i = 0; i < 80; i++) { temp = S5(A) + fi (B, C, D) + E + Wi + Ki; E=D; D=C; C = S30 (B); B=A; A =temp;}
f (B, C, D) = (B AND C) OR (NOT B AND D) ( 0<= i <=19) f (B, C, D) = B XOR C XOR D (20 <= i <= 39) f (B, C, D) = (B AND C) OR (B AND D) OR (C AND D) (40 <= i <= 59) f (B, C, D) = B XOR C XOR D (60<= i <= 79)
At the end of 80 iterations, A-E added to H0-H4 respectively.
Continue the rest of the input blocks.
Work ongoing for 256, 384, 512bit hashes.

49. The Birth Day Attack It takes 2m operations to attack m-bit MD.
But it takes 2m/2 operations using birthday attack. Yuval 1979 paper on “how to swindle Rabin”
Example: two tenure faculty up for promotion: Tom and Dick. Tom earlier by two years. Tom asks Dept. Chair, Marilyn to write recommendation letter:
Secretary Ellen Loves Dick. She prepares two letters.

50. Official Letter 1 Dear Dean Smith,
This [letter I message] is to give my [honest I frank] opinion of Prof. Tom Wilson, who is [a candidate I up] for tenure [now I this year]. I have [known I worked with] Prof. Wilson for [about I almost] six years. He is an [outstanding I excellent] researcher of great [talent I ability] known [worldwide I internationally] for his [brilliant I creative] insights into [many I a wide variety of] [difficult I chal­lenging] problems.
He is also a [highly I greatly] [respected I admired] [teacher I educator]. His students give his [classes I courses] [rave I spectacular] reviews. He is [our I the Department's] [most popular I best-loved] [teacher I instructor].
[In addition I Additionally] Prof. Wilson is a [gifted I effective] fund raiser. His [grants I contracts] have brought a [large I substantial] amount of money into [the I our] Department. [This money has I These funds have] [enabled I permitted] us to [pursue I carry out] many [special I important] programs, [such as I for ex­ample] your State 2000 program. Without these funds we would [be unable I not be able] to continue this program. which is so [important I essential] to both of us. I strongly urge you to grant him tenure.

51. Fake Letter Dear Dean Smith.
This [letter I message] is to give my [honest I frank] opinion of Prof. Tom Wilson, who is [a candidate I up] for tenure [now I this year]. I have [known I worked with] Tom for [about I almost] six years. He is a [poor I weak] researcher not well known in his [field I area]. His research [hardly ever I rarely] shows [insight in I understanding of] the [key I major] problems of [the I our] day.
Furthermore, he is not a [respected I admired] [teacher I educator]. His stu­dents give his [classes I courses] [poor I bad ] reviews. He is [our I the Department's] least popular [teacher I instructor], known [mostly I primarily] within [the I our] Department for his [tendency I propensity] to [ridicule I embar­rass] students [foolish I imprudent] enough to ask questions in his classes.
[In addition I Additionally] Tom is a [poor I marginal] fund raiser. His [grants I contracts] have brought only a [meager I insignificant] amount of money into [the I our] Department. Unless new [money is I funds are] quickly located, we may have to cancel some essential programs, such as your State 2000 program. Unfortunately, under these [conditions I circumstances] I cannot in good [consci­ence I faith] recommend him to you for [tenure I a permanent position].

52. The Rest of Story
Now Ellen programs her computer to compute the 232 message digests of each letter overnight. Chances are. one digest of the first letter will match one digest of the second letter. If not. she can add a few more options and try again during the weekend. Suppose that she finds a match. Call the "good" letter A and the "bad" one B.
Ellen now s letter A to Marilyn for her approval. Letter B she keeps completely secret, showing it to no one. Marilyn, of course. approves, computes her 64-bit message digest. signs the digest. and s the signed digest off to Dean Smith. Independently, Ellen s letter B to the Dean (not letter A, as she is supposed to).
After getting the letter and signed message digest. the Dean runs the message digest algorithm on letter B, sees that it agrees with what Marilyn sent him, and fires Tom. The Dean does not realize that Ellen managed to generate two letters with the same message digest and sent her a different one than Marilyn saw and approved. (Optional ending: Ellen tells Dick what she did. Dick is appalled and breaks off with her. Ellen is furious and confesses to Marilyn. Marilyn calls the Dean. Tom gets tenure after all.) With MD5 the birthday attack is difficult be­cause even at 1 billion digests per second, it would take over 500 years to com­pute all 264 digests of two letters with 64 variants each, and even then a match is not guaranteed. Of course, with 5000 computers working in parallel, 500 years becomes 5 weeks. SHA-1 is better (because it is longer).

53. Management of Public Keys
Certificates
X.509
Public Key Infrastructures

54. Problems with Public-Key Encryption
A way for Trudy to subvert public-key encryption.

55. Certificates
A possible certificate and its signed hash.

56. X.509
The basic fields of an X.509 certificate.

57. Public-Key Infrastructures
(a) A hierarchical PKI. (b) A chain of certificates.

58. Communication Security
IPsec
Firewalls
Virtual Private Networks
Wireless Security

59. IPsec
The IPsec authentication header in transport mode for IPv4.

60. IPsec (2)
(a) ESP in transport mode. (b) ESP in tunnel mode.

61. Firewalls
A firewall consisting of two packet filters and an application gateway.

62. Virtual Private Networks
(a) A leased-line private network. (b) A virtual private network.

63. Security
Packet encryption using WEP.

64. Authentication Protocols
Authentication Based on a Shared Secret Key
Establishing a Shared Key: Diffie-Hellman
Authentication Using a Key Distribution Center
Authentication Using Kerberos
Authentication Using Public-Key Cryptography

65. Authentication Based on a Shared Secret Key
Two-way authentication using a challenge-response protocol.

66. Authentication Based on a Shared Secret Key (2)
A shortened two-way authentication protocol.

67. Authentication Based on a Shared Secret Key (3)
The reflection attack.

68. Authentication Based on a Shared Secret Key (4)
A reflection attack on the protocol of Fig

69. Authentication Based on a Shared Secret Key (5)
Authentication using HMACs.

70. Establishing a Shared Key: The Diffie-Hellman Key Exchange

71. Establishing a Shared Key: The Diffie-Hellman Key Exchange
The bucket brigade or man-in-the-middle attack.

72. Authentication Using a Key Distribution Center
A first attempt at an authentication protocol using a KDC.

73. Authentication Using a Key Distribution Center (2)
The Needham-Schroeder authentication protocol.

74. Authentication Using a Key Distribution Center (3)
The Otway-Rees authentication protocol (slightly simplified).

75. Authentication Using Kerberos
The operation of Kerberos V4.

76. Authentication Using Public-Key Cryptography
Mutual authentication using public-key cryptography.

77. Security
PGP – Pretty Good Privacy
PEM – Privacy Enhanced Mail
S/MIME

78. PGP – Pretty Good Privacy
PGP in operation for sending a message.

79. PGP – Pretty Good Privacy (2)
A PGP message.

80. Web Security
Threats
Secure Naming
SSL – The Secure Sockets Layer
Mobile Code Security

81. Secure Naming
(a) Normal situation. (b) An attack based on breaking into DNS and modifying Bob's record.

82. Secure Naming (2)
How Trudy spoofs Alice's ISP.

83. Secure DNS
An example RRSet for bob.com. The KEY record is Bob's public key. The SIG record is the top-level com server's signed has of the A and KEY records to verify their authenticity.

84. Self-Certifying Names
A self-certifying URL containing a hash of server's name and public key.

85. SSL—The Secure Sockets Layer
Layers (and protocols) for a home user browsing with SSL.

86. SSL (2)
A simplified version of the SSL connection establishment subprotocol.

87. SSL (3)
Data transmission using SSL.

88. Java Applet Security
Applets inserted into a Java Virtual Machine interpreter inside the browser.

89. Social Issues
Privacy
Freedom of Speech
Copyright

90. Anonymous R ers
Users who wish anonymity chain requests through multiple anonymous r ers.

91. Freedom of Speech Possibly banned material:
Material inappropriate for children or teenagers.
Hate aimed at various ethnic, religious, sexual, or other groups.
Information about democracy and democratic values.
Accounts of historical events contradicting the government's version.
Manuals for picking locks, building weapons, encrypting messages, etc.

92. Steganography
(a) Three zebras and a tree. (b) Three zebras, a tree, and the complete text of five plays by William Shakespeare.