Hashing and Certificates - Hashing and Certicates We already knew that MD5 is a broken hash function Now researchers have successfully forged MD5-signed

Hashing and Certificates - Hashing and Certicates We...

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We already knew that MD5 is a broken hash function. Now researchers have successfully forged MD5-signed certificates. … I'm not losing a whole lot of sleep because of these attacks. But—come on, people—no one should be using MD5 anymore.— Bruce Schneier, January 2009Hashing and Certificates
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What is hashing?2
Alice wants to send Bob an “I owe you” messageBob wants to be able to show the message to a judge to compel Alice to pay upAlice wants to prevent Bob from changing the contents of the message or making up his own IOU from AliceUsing cryptographic hashes3“I, Alice, owe Bob $50, and willpay on or before 7/1/2013”AliceBob
Alice wants to send Bob an “I owe you” messageBob wants to be able to show the message to a judge to compel Alice to pay upAlice wants to prevent Bob from changing the contents of the message or making up his own IOU from AliceUsing cryptographic hashes3“I, Alice, owe Bob $50, and willpay on or before 7/1/2013”AliceBob
IOU protocol, first try4AliceBobJudge
IOU protocol, first try4AliceBobJudgeMH(M)
IOU protocol, first try4AliceBobJudgeMH(M)MH(M)
IOU protocol, first try4AliceBobJudgeMH(M)MH(M)Bob could have justmade up M and H(M)!
IOU protocol, second try5AliceBobJudge
IOU protocol, second try5AliceBobJudgeUse Diffie-Hellman to establish shared secret KASecret key KA
IOU protocol, second try5AliceBobJudgeMEKA[H(M)]Use Diffie-Hellman to establish shared secret KASecret key KA
IOU protocol, second try5AliceBobJudgeMEKA[H(M)]Use Diffie-Hellman to establish shared secret KASecret key KAMEKA[H(M)]
IOU protocol, second try5AliceBobJudgeMEKA[H(M)]Use Diffie-Hellman to establish shared secret KASecret key KAMEKA[H(M)]Can Bob cheat?
IOU protocol, second try5AliceBobJudgeMEKA[H(M)]Use Diffie-Hellman to establish shared secret KASecret key KAMEKA[H(M)]Can Bob cheat?NO
IOU protocol, second try5AliceBobJudgeMEKA[H(M)]Use Diffie-Hellman to establish shared secret KASecret key KAMEKA[H(M)]Can Bob cheat?NOCan Alice cheat?
IOU protocol, second try5AliceBobJudgeMEKA[H(M)]Use Diffie-Hellman to establish shared secret KASecret key KAMEKA[H(M)]Can Bob cheat?NOCan Alice cheat?

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