MAC. SKE in Practice. Lecture 5


 Kelly Ryan
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1 MAC. SKE in Practice. Lecture 5
2 Active Adversary
3 Active Adversary An active adversary can inject messages into the channel
4 Active Adversary An active adversary can inject messages into the channel Eve can send ciphertexts to Bob and get them decrypted
5 Active Adversary An active adversary can inject messages into the channel Eve can send ciphertexts to Bob and get them decrypted Chosen Ciphertext Attack (CCA)
6 Active Adversary An active adversary can inject messages into the channel Eve can send ciphertexts to Bob and get them decrypted Chosen Ciphertext Attack (CCA) If Bob decrypts all ciphertexts for Eve, no security possible
7 Active Adversary An active adversary can inject messages into the channel Eve can send ciphertexts to Bob and get them decrypted Chosen Ciphertext Attack (CCA) If Bob decrypts all ciphertexts for Eve, no security possible What can Bob do?
8 SymmetricKey Encryption RECALL SIMCCA Security Send Recv Key/Enc Key/Dec SIMCCA secure if: s.t. Replay Filter REAL IDEAL IDEAL Env Env REAL
9 SymmetricKey Encryption RECALL SIMCCA Security Invalid ciphertexts are silently ignored Send Recv Key/Enc Key/Dec SIMCCA secure if: s.t. Replay Filter REAL IDEAL IDEAL Env Env REAL
10 SymmetricKey Encryption RECALL SIMCCA Security Send Recv Key/Enc Key/Dec SIMCCA secure if: s.t. Replay Filter REAL IDEAL IDEAL Env Env REAL
11 SymmetricKey Encryption RECALL Alternately (slightly weaker form): Adv can send its own messages SIMCCA Security Send Recv Key/Enc Key/Dec SIMCCA secure if: s.t. Replay Filter REAL IDEAL IDEAL Env Env REAL
12 SymmetricKey Encryption RECALL INDCCA Security Experiment picks b {0,1} and K KeyGen Adv gets (guarded) access to Dec K oracle For as long as Adversary wants Key/Enc Enc(m b,k) INDCCA + ~correctness equivalent to SIMCCA Key/Dec Adv sends two messages m 0, m 1 to the experiment Expt returns Enc(m b,k) to the adversary m b m 0,m 1 No challenge ciphertext answered Adversary returns a guess b Experiments outputs 1 iff b =b INDCCA secure if for all feasible adversaries Pr[b =b] 1/2 b b b {0,1} b =b? Yes/No
13 SymmetricKey Encryption RECALL INDCCA Security Experiment picks b {0,1} and K KeyGen Adv gets (guarded) access to Dec K oracle For as long as Adversary wants Key/Enc Weaker Enc(m b,k) INDCCA + ~correctness equivalent to SIMCCA Key/Dec Adv sends two messages m 0, m 1 to the experiment Expt returns Enc(m b,k) to the adversary m b m 0,m 1 No challenge ciphertext answered Adversary returns a guess b Experiments outputs 1 iff b =b INDCCA secure if for all feasible adversaries Pr[b =b] 1/2 b b b {0,1} b =b? Yes/No
14 CCA Security
15 CCA Security How to obtain CCA security?
16 CCA Security How to obtain CCA security? Use a CPAsecure encryption scheme, but make sure Bob accepts and decrypts only ciphertexts produced by Alice
17 CCA Security How to obtain CCA security? Use a CPAsecure encryption scheme, but make sure Bob accepts and decrypts only ciphertexts produced by Alice i.e., Eve can t create new ciphertexts that will be accepted by Bob
18 CCA Security How to obtain CCA security? Use a CPAsecure encryption scheme, but make sure Bob accepts and decrypts only ciphertexts produced by Alice i.e., Eve can t create new ciphertexts that will be accepted by Bob Achieves the stronger guarantee: in IDEAL, Eve can t send its own messages to Bob
19 CCA Security How to obtain CCA security? Use a CPAsecure encryption scheme, but make sure Bob accepts and decrypts only ciphertexts produced by Alice i.e., Eve can t create new ciphertexts that will be accepted by Bob Achieves the stronger guarantee: in IDEAL, Eve can t send its own messages to Bob CCA secure SKE reduces to the problem of CPA secure SKE and (shared key) message authentication
20 CCA Security How to obtain CCA security? Use a CPAsecure encryption scheme, but make sure Bob accepts and decrypts only ciphertexts produced by Alice i.e., Eve can t create new ciphertexts that will be accepted by Bob Achieves the stronger guarantee: in IDEAL, Eve can t send its own messages to Bob CCA secure SKE reduces to the problem of CPA secure SKE and (shared key) message authentication MAC: Message Authentication Code
21 Message Authentication Codes
22 Message Authentication Codes A single short key shared by Alice and Bob
23 Message Authentication Codes A single short key shared by Alice and Bob Can sign any (polynomial) number of messages
24 Message Authentication Codes A single short key shared by Alice and Bob Can sign any (polynomial) number of messages A triple (KeyGen, MAC, Verify) MAC K Ver K
25 Message Authentication Codes A single short key shared by Alice and Bob Can sign any (polynomial) number of messages A triple (KeyGen, MAC, Verify) MAC K Ver K Correctness: For all K from KeyGen, and all messages M, Verify K (M,MAC K (M))=1
26 Message Authentication Codes A single short key shared by Alice and Bob Can sign any (polynomial) number of messages A triple (KeyGen, MAC, Verify) Correctness: For all K from KeyGen, and all messages M, Verify K (M,MAC K (M))=1 Security: probability that an adversary can produce (M,s) s.t. Verify K (M,s)=1 is negligible unless Alice produced an output s=mac K (M) MAC K s i = MAC K (M i ) M i (M,s) Ver K Ver K (M,s) Advantage = Pr[ Ver K (M,s)=1 and (M,s) {(M i,s i )} ]
27 CCA Secure SKE
28 CCA Secure SKE CCAEnc K1,K2 (m) = ( c:= CPAEnc K1 (m), t:= MAC K2 (c) )
29 CCA Secure SKE CCAEnc K1,K2 (m) = ( c:= CPAEnc K1 (m), t:= MAC K2 (c) ) CPA secure encryption: Blockcipher/CTR mode construction
30 CCA Secure SKE CCAEnc K1,K2 (m) = ( c:= CPAEnc K1 (m), t:= MAC K2 (c) ) CPA secure encryption: Blockcipher/CTR mode construction MAC: from a PRF or BlockCipher (coming up)
31 CCA Secure SKE CCAEnc K1,K2 (m) = ( c:= CPAEnc K1 (m), t:= MAC K2 (c) ) CPA secure encryption: Blockcipher/CTR mode construction MAC: from a PRF or BlockCipher (coming up) SKE in practice can just use BlockCiphers (coming up)
32 CCA Secure SKE CCAEnc K1,K2 (m) = ( c:= CPAEnc K1 (m), t:= MAC K2 (c) ) CPA secure encryption: Blockcipher/CTR mode construction MAC: from a PRF or BlockCipher (coming up) SKE in practice can just use BlockCiphers (coming up) In principle, constructions (less efficient) possible based on any OneWay Permutation or even any OneWay Function
33 Making a MAC
34 Onetime MAC MAC Ver
35 Onetime MAC To sign a single n bit message MAC Ver
36 Onetime MAC To sign a single n bit message A simple (but inefficient) scheme MAC Ver
37 Onetime MAC To sign a single n bit message A simple (but inefficient) scheme Shared secret key: 2n random strings (each kbit long) (r i 0,r i 1) i=1..n MAC Ver r r r r r r
38 Onetime MAC To sign a single n bit message A simple (but inefficient) scheme Shared secret key: 2n random strings (each kbit long) (r i 0,r i 1) i=1..n MAC 010 Ver r r r r r r
39 Onetime MAC To sign a single n bit message A simple (but inefficient) scheme 010 Shared secret key: 2n random strings (each kbit long) (r i 0,r i 1) i=1..n MAC r 1 0 r 2 1 r 3 0 Ver Signature for m 1...m n be (r i mi) i=1..n r r r r r r
40 Onetime MAC To sign a single n bit message A simple (but inefficient) scheme 010 Shared secret key: 2n random strings (each kbit long) (r i 0,r i 1) i=1..n MAC r 1 0 r 2 1 r 3 0 Ver Signature for m 1...m n be (r i mi) i=1..n Negligible probability that Eve can produce a signature on m m r r r r r r
41 Onetime MAC To sign a single n bit message A simple (but inefficient) scheme 010 Shared secret key: 2n random strings (each kbit long) (r i 0,r i 1) i=1..n MAC r 1 0 r 2 1 r 3 0 Ver Signature for m 1...m n be (r i mi) i=1..n Negligible probability that Eve can produce a signature on m m r r r r r r Doesn t require any computational restrictions on adversary!
42 Onetime MAC To sign a single n bit message A simple (but inefficient) scheme 010 Shared secret key: 2n random strings (each kbit long) (r i 0,r i 1) i=1..n MAC r 1 0 r 2 1 r 3 0 Ver Signature for m 1...m n be (r i mi) i=1..n Negligible probability that Eve can produce a signature on m m r r r r r r Doesn t require any computational restrictions on adversary! More efficient onetime MACs exist (later)
43 (Multimsg) MAC from PRF When Each Message is a Single Block
44 (Multimsg) MAC from PRF When Each Message is a Single Block PRF is a MAC!
45 (Multimsg) MAC from PRF When Each Message is a Single Block PRF is a MAC! MAC K (M) := F K (M) where F is a PRF
46 (Multimsg) MAC from PRF When Each Message is a Single Block PRF is a MAC! MAC K (M) := F K (M) where F is a PRF M F K F K (M)
47 (Multimsg) MAC from PRF When Each Message is a Single Block PRF is a MAC! MAC K (M) := F K (M) where F is a PRF Ver K (M,S) := 1 iff S=F K (M) M F K F K (M)
48 (Multimsg) MAC from PRF When Each Message is a Single Block PRF is a MAC! MAC K (M) := F K (M) where F is a PRF Ver K (M,S) := 1 iff S=F K (M) Output length of F K should be big enough M F K F K (M)
49 (Multimsg) MAC from PRF When Each Message is a Single Block PRF is a MAC! MAC K (M) := F K (M) where F is a PRF Ver K (M,S) := 1 iff S=F K (M) Output length of F K should be big enough M F K F K (M) If an adversary forges MAC with probability MAC, then can break PRF with advantage O( MAC 2 m(k) ) (m(k) being the output length of the PRF) [How?]
50 (Multimsg) MAC from PRF When Each Message is a Single Block PRF is a MAC! MAC K (M) := F K (M) where F is a PRF Ver K (M,S) := 1 iff S=F K (M) Output length of F K should be big enough M F K F K (M) If an adversary forges MAC with probability MAC, then can break PRF with advantage O( MAC 2 m(k) ) (m(k) being the output length of the PRF) [How?] Advantage in breaking a PRF F: diff in prob a test has of outputting 1, when given F vs. truly random R
51 (Multimsg) MAC from PRF When Each Message is a Single Block PRF is a MAC! MAC K (M) := F K (M) where F is a PRF Ver K (M,S) := 1 iff S=F K (M) Output length of F K should be big enough M F K F K (M) If an adversary forges MAC with probability MAC, then can break PRF with advantage O( MAC 2 m(k) ) (m(k) being the output length of the PRF) [How?] If random function R used as MAC, then probability of forgery, MAC* = 2 m(k) Advantage in breaking a PRF F: diff in prob a test has of outputting 1, when given F vs. truly random R
52 MAC for MultipleBlock Messages
53 MAC for MultipleBlock Messages What if message is longer than one block?
54 MAC for MultipleBlock Messages What if message is longer than one block? MAC ing each block separately is not secure (unlike in the case of CPA secure encryption)
55 MAC for MultipleBlock Messages What if message is longer than one block? MAC ing each block separately is not secure (unlike in the case of CPA secure encryption) Eve can rearrange the blocks/drop some blocks
56 MAC for MultipleBlock Messages What if message is longer than one block? MAC ing each block separately is not secure (unlike in the case of CPA secure encryption) Eve can rearrange the blocks/drop some blocks Could use a PRF that takes longer inputs
57 MAC for MultipleBlock Messages What if message is longer than one block? MAC ing each block separately is not secure (unlike in the case of CPA secure encryption) Eve can rearrange the blocks/drop some blocks Could use a PRF that takes longer inputs Can we use a PRF with a fixed blocklength (i.e., a block cipher)?
58 MAC for MultipleBlock Messages
59 MAC for MultipleBlock Messages A simple solution: tie the blocks together
60 MAC for MultipleBlock Messages A simple solution: tie the blocks together Add to each block a random string r (same r for all blocks), total number of blocks, and a sequence number
61 MAC for MultipleBlock Messages A simple solution: tie the blocks together Add to each block a random string r (same r for all blocks), total number of blocks, and a sequence number B i = (r, t, i, M i )
62 MAC for MultipleBlock Messages A simple solution: tie the blocks together Add to each block a random string r (same r for all blocks), total number of blocks, and a sequence number B i = (r, t, i, M i ) MAC(M) = (r, (MAC(B i )) i=1..t )
63 MAC for MultipleBlock Messages A simple solution: tie the blocks together Add to each block a random string r (same r for all blocks), total number of blocks, and a sequence number B i = (r, t, i, M i ) MAC(M) = (r, (MAC(B i )) i=1..t ) r prevents mixing blocks from two messages, t prevents dropping blocks and i prevents rearranging
64 MAC for MultipleBlock Messages A simple solution: tie the blocks together Add to each block a random string r (same r for all blocks), total number of blocks, and a sequence number B i = (r, t, i, M i ) MAC(M) = (r, (MAC(B i )) i=1..t ) r prevents mixing blocks from two messages, t prevents dropping blocks and i prevents rearranging Inefficient! Tag length increases with message length
65 CBCMAC
66 CBCMAC PRF domain extension: Chaining the blocks
67 CBCMAC PRF domain extension: Chaining the blocks m 1 m 2 m t... F K F K F K T
68 CBCMAC PRF domain extension: Chaining the blocks cf. CBC mode for encryption (which is not a MAC!) m 1 m 2 m t... F K F K F K T
69 CBCMAC PRF domain extension: Chaining the blocks cf. CBC mode for encryption (which is not a MAC!) tblock messages, a single block tag m 1 m 2 m t... F K F K F K T
70 CBCMAC PRF domain extension: Chaining the blocks cf. CBC mode for encryption (which is not a MAC!) tblock messages, a single block tag Can be shown to be secure m 1 m 2 m t... F K F K F K T
71 CBCMAC PRF domain extension: Chaining the blocks cf. CBC mode for encryption (which is not a MAC!) tblock messages, a single block tag Can be shown to be secure m 1 m 2 m t... F K F K F K T If restricted to tblock messages (i.e., same length)
72 CBCMAC PRF domain extension: Chaining the blocks cf. CBC mode for encryption (which is not a MAC!) tblock messages, a single block tag Can be shown to be secure m 1 m 2 m t... F K F K F K T If restricted to tblock messages (i.e., same length) Else attacks possible (by extending a previously signed message)
73 Patching CBCMAC
74 Patching CBCMAC Patching CBC MAC to handle message of any (polynomial) length but still producing a single block tag (secure if blockcipher is):
75 Patching CBCMAC Patching CBC MAC to handle message of any (polynomial) length but still producing a single block tag (secure if blockcipher is): Derive K as F K (t), where t is the number of blocks
76 Patching CBCMAC Patching CBC MAC to handle message of any (polynomial) length but still producing a single block tag (secure if blockcipher is): Derive K as F K (t), where t is the number of blocks Use first block to specify number of blocks
77 Patching CBCMAC Patching CBC MAC to handle message of any (polynomial) length but still producing a single block tag (secure if blockcipher is): Derive K as F K (t), where t is the number of blocks Use first block to specify number of blocks Important that first block is used: if last block, message extension attacks still possible
78 Patching CBCMAC Patching CBC MAC to handle message of any (polynomial) length but still producing a single block tag (secure if blockcipher is): Derive K as F K (t), where t is the number of blocks Use first block to specify number of blocks Important that first block is used: if last block, message extension attacks still possible EMAC: Output not the last tag T, but F K (T), where K is an independent key (after padding the message to an integral number of blocks). No need to know message length a priori.
79 Patching CBCMAC Patching CBC MAC to handle message of any (polynomial) length but still producing a single block tag (secure if blockcipher is): Derive K as F K (t), where t is the number of blocks Use first block to specify number of blocks Important that first block is used: if last block, message extension attacks still possible EMAC: Output not the last tag T, but F K (T), where K is an independent key (after padding the message to an integral number of blocks). No need to know message length a priori. CMAC: XOR last block with another key (derived from the original key using the blockcipher). Avoids padding when message is integral number of blocks.
80 Patching CBCMAC Patching CBC MAC to handle message of any (polynomial) length but still producing a single block tag (secure if blockcipher is): Derive K as F K (t), where t is the number of blocks Use first block to specify number of blocks Important that first block is used: if last block, message extension attacks still possible EMAC: Output not the last tag T, but F K (T), where K is an independent key (after padding the message to an integral number of blocks). No need to know message length a priori. CMAC: XOR last block with another key (derived from the original key using the blockcipher). Avoids padding when message is integral number of blocks. NIST Recommendation. 2005
81 Patching CBCMAC Patching CBC MAC to handle message of any (polynomial) length but still producing a single block tag (secure if blockcipher is): Derive K as F K (t), where t is the number of blocks Use first block to specify number of blocks Important that first block is used: if last block, message extension attacks still possible EMAC: Output not the last tag T, but F K (T), where K is an independent key (after padding the message to an integral number of blocks). No need to know message length a priori. CMAC: XOR last block with another key (derived from the original key using the blockcipher). Avoids padding when message is integral number of blocks. NIST Recommendation Later: Hashbased HMAC used in TLS and IPSec IETF Standard. 1997
82 SKE in Practice
83 Stream Ciphers
84 Stream Ciphers Used for onetime encryption
85 Stream Ciphers Used for onetime encryption RC4, estream portfolio,...
86 Stream Ciphers Used for onetime encryption RC4, estream portfolio,... In practice, stream ciphers take a key and an IV (for initialization vector) as inputs
87 Stream Ciphers Used for onetime encryption RC4, estream portfolio,... Also used to denote the random nonce chosen for encryption using a blockcipher In practice, stream ciphers take a key and an IV (for initialization vector) as inputs
88 Stream Ciphers Used for onetime encryption RC4, estream portfolio,... Also used to denote the random nonce chosen for encryption using a blockcipher In practice, stream ciphers take a key and an IV (for initialization vector) as inputs Heuristic goal: behave somewhat like a PRF (instead of a PRG) so that it can be used for multimessage encryption
89 Stream Ciphers Used for onetime encryption RC4, estream portfolio,... Also used to denote the random nonce chosen for encryption using a blockcipher In practice, stream ciphers take a key and an IV (for initialization vector) as inputs Heuristic goal: behave somewhat like a PRF (instead of a PRG) so that it can be used for multimessage encryption But often breaks if used this way
90 Stream Ciphers Used for onetime encryption RC4, estream portfolio,... Also used to denote the random nonce chosen for encryption using a blockcipher In practice, stream ciphers take a key and an IV (for initialization vector) as inputs Heuristic goal: behave somewhat like a PRF (instead of a PRG) so that it can be used for multimessage encryption But often breaks if used this way NIST Standard: For multimessage encryption, use a blockcipher in CTR mode
91 Block Ciphers
92 Block Ciphers DES, 3DES, Blowfish, AES,...
93 Block Ciphers DES, 3DES, Blowfish, AES,... Heuristic constructions
94 Block Ciphers DES, 3DES, Blowfish, AES,... Heuristic constructions Permutations that can be inverted with the key
95 Block Ciphers DES, 3DES, Blowfish, AES,... Heuristic constructions Permutations that can be inverted with the key Speed (hardware/software) is of the essence
96 Block Ciphers DES, 3DES, Blowfish, AES,... Heuristic constructions Permutations that can be inverted with the key Speed (hardware/software) is of the essence But should withstand known attacks
97 Block Ciphers DES, 3DES, Blowfish, AES,... Heuristic constructions Permutations that can be inverted with the key Speed (hardware/software) is of the essence But should withstand known attacks As a PRP (or at least, against key recovery)
98 Feistel Network
99 Feistel Network Building a permutation from a (block) function
100 Feistel Network Building a permutation from a (block) function Let f: {0,1} m {0,1} m be an arbitrary function
101 Feistel Network Building a permutation from a (block) function Let f: {0,1} m {0,1} m be an arbitrary function F f : {0,1} 2m {0,1} 2m defined as F f (x,y) = ( y, x f(y) ) f 1
102 Feistel Network Building a permutation from a (block) function Let f: {0,1} m {0,1} m be an arbitrary function F f : {0,1} 2m {0,1} 2m defined as F f (x,y) = ( y, x f(y) ) F f is a permutation (Why?) f 1
103 Feistel Network Building a permutation from a (block) function Let f: {0,1} m {0,1} m be an arbitrary function F f : {0,1} 2m {0,1} 2m defined as F f (x,y) = ( y, x f(y) ) F f is a permutation (Why?) Can invert (How?) f 1
104 Feistel Network Building a permutation from a (block) function Let f: {0,1} m {0,1} m be an arbitrary function F f : {0,1} 2m {0,1} 2m defined as F f (x,y) = ( y, x f(y) ) F f is a permutation (Why?) Can invert (How?) Given functions f 1,...,f t can build a tlayer Feistel network F f1...ft f 1
105 Feistel Network Building a permutation from a (block) function Let f: {0,1} m {0,1} m be an arbitrary function F f : {0,1} 2m {0,1} 2m defined as F f (x,y) = ( y, x f(y) ) F f is a permutation (Why?) Can invert (How?) Given functions f 1,...,f t can build a tlayer Feistel network F f1...ft f1 f 2
106 Feistel Network Building a permutation from a (block) function Let f: {0,1} m {0,1} m be an arbitrary function F f : {0,1} 2m {0,1} 2m defined as F f (x,y) = ( y, x f(y) ) F f is a permutation (Why?) Can invert (How?) Given functions f 1,...,f t can build a tlayer Feistel network F f1...ft Still a permutation from {0,1} 2m to {0,1} 2m f1 f 2
107 Feistel Network Building a permutation from a (block) function Let f: {0,1} m {0,1} m be an arbitrary function F f : {0,1} 2m {0,1} 2m defined as F f (x,y) = ( y, x f(y) ) F f is a permutation (Why?) Can invert (How?) Given functions f 1,...,f t can build a tlayer Feistel network F f1...ft Still a permutation from {0,1} 2m to {0,1} 2m LubyRackoff: A 3layer Feistel network, in which 3 PRFs with independent seeds are the 3 round functions, is a PRP. A 4layer Feistel gives a strong PRP f1 f 2
108 Feistel Network Building a permutation from a (block) function Let f: {0,1} m {0,1} m be an arbitrary function F f : {0,1} 2m {0,1} 2m defined as F f (x,y) = ( y, x f(y) ) F f is a permutation (Why?) Can invert (How?) Given functions f 1,...,f t can build a tlayer Feistel network F f1...ft Still a permutation from {0,1} 2m to {0,1} 2m LubyRackoff: A 3layer Feistel network, in which 3 PRFs with independent seeds are the 3 round functions, is a PRP. A 4layer Feistel gives a strong PRP Fewer layers do not suffice! [Exercise] f1 f 2
109 DES Block Cipher
110 NIST Standard DES Block Cipher Data Encryption Standard (DES), TripleDES, DESX
111 NIST Standard DES Block Cipher Data Encryption Standard (DES), TripleDES, DESX DES uses a 16layer Feistel network (and a few other steps)
112 NIST Standard DES Block Cipher Data Encryption Standard (DES), TripleDES, DESX DES uses a 16layer Feistel network (and a few other steps) The round functions are not PRFs, but ad hoc
113 NIST Standard DES Block Cipher Data Encryption Standard (DES), TripleDES, DESX DES uses a 16layer Feistel network (and a few other steps) The round functions are not PRFs, but ad hoc Confuse and diffuse
114 NIST Standard DES Block Cipher Data Encryption Standard (DES), TripleDES, DESX DES uses a 16layer Feistel network (and a few other steps) The round functions are not PRFs, but ad hoc Confuse and diffuse Defined for fixed key/block lengths (56 bits and 64 bits); key is used to generate subkeys for round functions
115 NIST Standard DES Block Cipher Data Encryption Standard (DES), TripleDES, DESX DES uses a 16layer Feistel network (and a few other steps) The round functions are not PRFs, but ad hoc Confuse and diffuse Defined for fixed key/block lengths (56 bits and 64 bits); key is used to generate subkeys for round functions DES s key length too short
116 NIST Standard DES Block Cipher Data Encryption Standard (DES), TripleDES, DESX DES uses a 16layer Feistel network (and a few other steps) The round functions are not PRFs, but ad hoc Confuse and diffuse Defined for fixed key/block lengths (56 bits and 64 bits); key is used to generate subkeys for round functions DES s key length too short Can now mount brute force keyrecovery attacks (e.g. using $10K hardware, running for under a week, in 2006; now, in under a day)
117 NIST Standard DES Block Cipher Data Encryption Standard (DES), TripleDES, DESX DES uses a 16layer Feistel network (and a few other steps) The round functions are not PRFs, but ad hoc Confuse and diffuse Defined for fixed key/block lengths (56 bits and 64 bits); key is used to generate subkeys for round functions DES s key length too short Can now mount brute force keyrecovery attacks (e.g. using $10K hardware, running for under a week, in 2006; now, in under a day) DESX: extra keys to pad input and output
118 NIST Standard DES Block Cipher Data Encryption Standard (DES), TripleDES, DESX DES uses a 16layer Feistel network (and a few other steps) The round functions are not PRFs, but ad hoc Confuse and diffuse Defined for fixed key/block lengths (56 bits and 64 bits); key is used to generate subkeys for round functions DES s key length too short Can now mount brute force keyrecovery attacks (e.g. using $10K hardware, running for under a week, in 2006; now, in under a day) DESX: extra keys to pad input and output Triple DES: 3 successive applications of DES (or DES 1 ) with 3 keys
119 AES Block Cipher
120 NIST Standard AES Block Cipher Advanced Encryption Standard (AES)
121 NIST Standard AES Block Cipher Advanced Encryption Standard (AES) AES128, AES192, AES256 (3 key sizes; block size = 128 bits)
122 NIST Standard AES Block Cipher Advanced Encryption Standard (AES) AES128, AES192, AES256 (3 key sizes; block size = 128 bits) Very efficient in software implementations (unlike DES)
123 NIST Standard AES Block Cipher Advanced Encryption Standard (AES) AES128, AES192, AES256 (3 key sizes; block size = 128 bits) Very efficient in software implementations (unlike DES) Uses SubstituteandPermute instead of Feistel networks
124 NIST Standard AES Block Cipher Advanced Encryption Standard (AES) AES128, AES192, AES256 (3 key sizes; block size = 128 bits) Very efficient in software implementations (unlike DES) Uses SubstituteandPermute instead of Feistel networks Has some algebraic structure
125 NIST Standard AES Block Cipher Advanced Encryption Standard (AES) AES128, AES192, AES256 (3 key sizes; block size = 128 bits) Very efficient in software implementations (unlike DES) Uses SubstituteandPermute instead of Feistel networks Has some algebraic structure Operations in a vector space over the field GF(2 8 )
126 NIST Standard AES Block Cipher Advanced Encryption Standard (AES) AES128, AES192, AES256 (3 key sizes; block size = 128 bits) Very efficient in software implementations (unlike DES) Uses SubstituteandPermute instead of Feistel networks Has some algebraic structure Operations in a vector space over the field GF(2 8 ) The algebraic structure may lead to attacks?
127 NIST Standard AES Block Cipher Advanced Encryption Standard (AES) AES128, AES192, AES256 (3 key sizes; block size = 128 bits) Very efficient in software implementations (unlike DES) Uses SubstituteandPermute instead of Feistel networks Has some algebraic structure Operations in a vector space over the field GF(2 8 ) The algebraic structure may lead to attacks? Some implementations may lead to sidechannel attacks (e.g. cachetiming attacks)
128 NIST Standard AES Block Cipher Advanced Encryption Standard (AES) AES128, AES192, AES256 (3 key sizes; block size = 128 bits) Very efficient in software implementations (unlike DES) Uses SubstituteandPermute instead of Feistel networks Has some algebraic structure Operations in a vector space over the field GF(2 8 ) The algebraic structure may lead to attacks? Some implementations may lead to sidechannel attacks (e.g. cachetiming attacks) No simple hardness assumption known to imply any sort of security for AES
129 By Jeff Moser (http://www.moserware.com/2009/09/stickfigureguidetoadvanced.html)
130 Cryptanalysis
131 Cryptanalysis Attacking stream ciphers and block ciphers
132 Cryptanalysis Attacking stream ciphers and block ciphers Typically for key recovery
133 Cryptanalysis Attacking stream ciphers and block ciphers Typically for key recovery Brute force cryptanalysis, using specialized hardware
134 Cryptanalysis Attacking stream ciphers and block ciphers Typically for key recovery Brute force cryptanalysis, using specialized hardware e.g. Attack on DES in 1998
135 Cryptanalysis Attacking stream ciphers and block ciphers Typically for key recovery Brute force cryptanalysis, using specialized hardware e.g. Attack on DES in 1998 Several other analytical techniques to speed up attacks
136 Cryptanalysis Attacking stream ciphers and block ciphers Typically for key recovery Brute force cryptanalysis, using specialized hardware e.g. Attack on DES in 1998 Several other analytical techniques to speed up attacks Sometimes theoretical : on weakened ( reduced round ) constructions, showing improvement over bruteforce attack
137 Cryptanalysis Attacking stream ciphers and block ciphers Typically for key recovery Brute force cryptanalysis, using specialized hardware e.g. Attack on DES in 1998 Several other analytical techniques to speed up attacks Sometimes theoretical : on weakened ( reduced round ) constructions, showing improvement over bruteforce attack Meetinthemiddle, linear cryptanalysis, differential cryptanalysis, impossible differential cryptanalysis, boomerang attack, integral cryptanalysis, cube attack,...
138 Authenticated Encryption
139 Authenticated Encryption Doing encryption + authentication better
140 Authenticated Encryption Doing encryption + authentication better Generic composition: encrypt, then MAC
141 Authenticated Encryption Doing encryption + authentication better Generic composition: encrypt, then MAC Needs two keys and two passes
142 Authenticated Encryption Doing encryption + authentication better Generic composition: encrypt, then MAC Needs two keys and two passes AE aims to do this more efficiently
143 Authenticated Encryption Doing encryption + authentication better Generic composition: encrypt, then MAC Needs two keys and two passes AE aims to do this more efficiently Several constructions based on blockciphers (modes of operation) provably secure modeling blockcipher as PRP
144 Authenticated Encryption Doing encryption + authentication better Generic composition: encrypt, then MAC Needs two keys and two passes AE aims to do this more efficiently Several constructions based on blockciphers (modes of operation) provably secure modeling blockcipher as PRP One pass: IAPM, OCB,... [patented]
145 Authenticated Encryption Doing encryption + authentication better Generic composition: encrypt, then MAC Needs two keys and two passes AE aims to do this more efficiently Several constructions based on blockciphers (modes of operation) provably secure modeling blockcipher as PRP One pass: IAPM, OCB,... [patented] Two pass: CCM, GCM, SIV,... [included in NIST standards]
146 Authenticated Encryption Doing encryption + authentication better Generic composition: encrypt, then MAC Needs two keys and two passes AE aims to do this more efficiently Several constructions based on blockciphers (modes of operation) provably secure modeling blockcipher as PRP One pass: IAPM, OCB,... [patented] Two pass: CCM, GCM, SIV,... [included in NIST standards] AE with Associated Data: Allows unencrypted (but authenticated) parts of the plaintext, for headers etc.
147 SKE today
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149 SKE today SKE in IPsec, TLS etc. mainly based on AES blockciphers AES128, AES192, AES256
150 SKE today SKE in IPsec, TLS etc. mainly based on AES blockciphers AES128, AES192, AES256 Recommended: AES Countermode + CMAC (or HMAC)
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154 SKE today SKE in IPsec, TLS etc. mainly based on AES blockciphers AES128, AES192, AES256 Recommended: AES Countermode + CMAC (or HMAC) Gives CCA security, and provides authentication Older components/modes still in use Supported by many standards for legacy purposes In many applications (sometimes with modifications)
155 SKE today SKE in IPsec, TLS etc. mainly based on AES blockciphers AES128, AES192, AES256 Recommended: AES Countermode + CMAC (or HMAC) Gives CCA security, and provides authentication Older components/modes still in use Supported by many standards for legacy purposes In many applications (sometimes with modifications) e.g. RC4 in BitTorrent, Skype, PDF
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