CIPHER or CAESARIAN SHIFT 9 Message: C O D E Ciphertext (key=1): D P E F Ciphertext (key=2): E Q F G http://upload.wikimedia.org/wikipedia/commons/thumb/2/2b/Caesar3.svg

CIPHER or CAESARIAN SHIFT 9 Message: C O D E Ciphertext (key=1): D P E F Ciphertext (key=2): E Q F G Ciphertext (key=-1): B M C D http://upload.wikimedia.org/wikipedia/commons/thumb/2/2b/Caesar3.svg

CIPHER or CAESARIAN SHIFT 9 Message: C O D E Ciphertext (key=1): D P E F Ciphertext (key=2): E Q F G Ciphertext (key=-1): B M C D Ciphertext (key=0): C O D E http://upload.wikimedia.org/wikipedia/commons/thumb/2/2b/Caesar3.svg

CIPHER or CAESARIAN SHIFT 9 Message: C O D E Ciphertext (key=1): D P E F Ciphertext (key=2): E Q F G Ciphertext (key=-1): B M C D Ciphertext (key=0): C O D E Ciphertext (key=26): C O D E http://upload.wikimedia.org/wikipedia/commons/thumb/2/2b/Caesar3.svg

CIPHER or CAESARIAN SHIFT 9 Message: C O D E Ciphertext (key=1): D P E F Ciphertext (key=2): E Q F G Ciphertext (key=-1): B M C D Ciphertext (key=0): C O D E Ciphertext (key=26): C O D E Ciphertext (key=52): C O D E http://upload.wikimedia.org/wikipedia/commons/thumb/2/2b/Caesar3.svg

and weary, Over many a quaint and curious volume of forgotten lore— While I nodded, nearly napping, suddenly there came a tapping, As of some one gently rapping—rapping at my chamber door. "'Tis some visitor," I muttered, "tapping at my chamber door— Only this and nothing more." 12

can decrypt. - message is only for Alice = encryption 22 Encrypt with private key: - only public key can decrypt. - message is guaranteed coming for Alice = signing

small and large messages. ✗ need to send over the key to the other side. Asymmetrical ✓ no need to send over the (whole) key. ✓ can be used for encryption and validation (signing). ✗ very resource intensive. ✗ only useful for small messages. 23

practically impossible to refactor a large number back into 2 separate prime numbers Prime number is only divisible by 1 and itself: 2, 3, 5, 7, 11, 13, 17, 19 etc... 28

(large) prime number (but not too close to p) ➡ n = p . q (bit length of the RSA key) ➡ φ = (p-1) . (q-1) (the φ thingie is called phi) ➡ e = gcd(e, φ) = 1

(large) prime number (but not too close to p) ➡ n = p . q (bit length of the RSA key) ➡ φ = (p-1) . (q-1) (the φ thingie is called phi) ➡ e = gcd(e, φ) = 1 ➡ d = (d . e) mod φ = 1

(large) prime number (but not too close to p) ➡ n = p . q (bit length of the RSA key) ➡ φ = (p-1) . (q-1) (the φ thingie is called phi) ➡ e = gcd(e, φ) = 1 ➡ d = (d . e) mod φ = 1

φ) = 1 ==> gcd(3, 20) = 1 ‣ P = 11 | Q = 3 | N = 33 | Phi = 20 | e = ? | d = ? 35 Fermat number: 2 + 1 2 n Fermat prime: Fermat that is prime: 3, 5, 17, 257, 65537 Study shows that 98.5% of the time 65537 is used

d p q d mod (p-1) e mod (q-1) (inverse q) mod p Private-Key: (256 bit) modulus: 00:c2:d0:c4:1f:6f:78:16:82:d1:0c:dd:5a:af:de:f2:ff:31:c6: 9b:3b:9f:e8:24:2a:5c:06:56:ea:d7:7c:c6:19 publicExponent: 65537 (0x10001) privateExponent: 22:8f:fd:2b:82:90:30:96:36:d6:6c:73:09:5e:a9:87:73:6e: 2d:d4:d5:78:fc:3b:20:ea:0d:02:e5:2b:cb:3d prime1: 00:f0:49:fd:91:18:01:53:92:8f:87:d7:2b:c8:19:7d:17 prime2: 00:cf:8d:a1:3b:93:af:61:77:8f:c9:8f:1d:aa:8d:b4:4f exponent1: 00:e1:d8:c9:89:bc:84:52:a6:a8:5d:47:32:91:6a:d3:95 exponent2: 5a:88:b1:fa:d5:d9:db:8f:16:a6:5a:0a:1b:ba:42:1b coefficient: 00:99:fa:de:80:d4:ee:f3:69:59:e5:8a:72:ad:e5:30:3d

but before encryption, with random padding bytes shown in green: 0002257F48FD1F1793B7E5E02306F2D3228F5C95ADF5F31566729F132AA12009 E3FC9B2B475CD6944EF191E3F59545E671E474B555799FE3756099F044964038 B16B2148E9A2F9C6F44BB5C52E3C6C8061CF694145FAFDB24402AD1819EACEDF 4A36C6E4D2CD8FC1D62E5A1268F496004E636AF98E40F3ADCFCCB698F4E80B9F After RSA encryption, the output is: 3D2AB25B1EB667A40F504CC4D778EC399A899C8790EDECEF062CD739492C9CE5 8B92B9ECF32AF4AAC7A61EAEC346449891F49A722378E008EFF0B0A8DBC6E621 EDC90CEC64CF34C640F5B36C48EE9322808AF8F4A0212B28715C76F3CB99AC7E 609787ADCE055839829E0142C44B676D218111FFE69F9D41424E177CBA3A435B http://www.di-mgt.com.au/rsa_alg.html#pkcs1schemes 44

server (highest possible encryption used). ➡Symmetric encryption (AES-256, others) ➡But both sides needs the same key, so we have the same problem as before: how do we send over the key? HTTPS 47

to use. ➡Server send certiﬁcate(s). ➡Client sends “session key” encrypted by the public key found in the server certiﬁcate. ➡Server and client uses the “session key” for symmetrical encryption. HTTPS 50

(better!?) encryption. ➡SSL/TLS is a separate talk (it’s way more complex as this) ➡http://www.moserware.com/2009/06/ﬁrst-few- milliseconds-of-https.html HTTPS 51

know for sure that nobody has read this email (before it came to us?) ➡ Do we know for sure that the contents of the message isn’t tampered with? Questions: 54

know for sure that nobody has read this email (before it came to us?) ➡ Do we know for sure that the contents of the message isn’t tampered with? ➡ We use signing! Questions: 54

the validity of a message. ➡ Like md5 or sha1, so when the message changes, so will the signature. ➡ This works on the premise that Alice and only Alice has the private key that can create the signature. Signing a message 55

(or emails). ➡ Try it yourself with Thunderbird’s Enigmail extension. ➡ Public keys can be send / found on PGP- servers so you don’t need to send your keys to everybody all the time. Introduction a pretty-good-privacy 57

‣ Everybody can verify that YOU have send the email and that it is authentic. ‣ Why is this not the standard? ‣ No really, why isn’t it the standard? 58

secure, and it WILL fail. ➡ Encryption is as strong as the weakest link, which 9 out of 10 times will be you. ➡ Encryptions evolve. Do not use today what you used 10 years ago. 64

secure, and it WILL fail. ➡ Encryption is as strong as the weakest link, which 9 out of 10 times will be you. ➡ Encryptions evolve. Do not use today what you used 10 years ago. ➡ Every encryption will become obsolete! 64

secure, and it WILL fail. ➡ Encryption is as strong as the weakest link, which 9 out of 10 times will be you. ➡ Encryptions evolve. Do not use today what you used 10 years ago. ➡ Every encryption will become obsolete! ➡ Always follow the best practices. 64