Security 1

is one of the most widely used and regarded network services currently message contents are not secure may be inspected either in transit or by suitably privileged users on destination system 2

Security Enhancements confidentiality protection from disclosure authentication of sender of message message integrity protection from modification non-repudiation of origin protection from denial by sender 3

Pretty Good Privacy (PGP) widely used de facto secure developed by Phil Zimmermann selected best available cryptography algorithms integrated into a single program available on Unix, PC, Macintosh and Amiga systems originally free, now have commercial versions available also 4

PGP services messages authentication confidentiality compression compatibility segmentation and reassembly key management generation, distribution, and revocation of public/private keys generation and transport of session keys and IVs 5

Message authentication based on digital signatures supported algorithms: RSA/SHA and DSS/SHA hash enc hash dec compare accept / reject mh  K snd -1 K snd mh  h sender receiver 6

PGP Operation – Authentication 1.sender creates a message 2. SHA-1 used to generate 160-bit hash code of message 3. hash code is encrypted with RSA using the sender's private key, and result is attached to message 4. receiver uses RSA with sender's public key to decrypt and recover hash code 5. receiver generates new hash code for message and compares with decrypted hash code, if match, message is accepted as authentic 6. From the strengths of RSA and SHA-1 the recipient is assured that only the possessor of the private key could generate the signature. 7

Message confidentiality symmetric key encryption in CFB mode with a random session key and IV session key and IV is encrypted with the public key of the receiver supported algorithms: symmetric: CAST, IDEA, 3DES asymmetric: RSA, ElGamal prng s.enc m K rcv sender a.enc k, iv {m} k {k, iv} Krcv 8

PGP Operation – Confidentiality 1.sender generates message and a random 128-bit number to be used as session key for this message only. 2.message is encrypted, using CAST-128 / IDEA/3DES with session key using 64 bit CFB (cipher feedback mode) 3.session key is encrypted using RSA with recipient's public key, then attached to message 4.receiver uses RSA with its private key to decrypt and recover session key 5.session key is used to decrypt message 6.Option to RSA – Diffie-Hellman variant E1Gamal 9

PGP Operation – Confidentiality & Authentication uses both services on same message create signature & attach to message encrypt both message & signature attach RSA encrypted session key 10

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Notation for Figure 5.1 K s session key used in symmetric encryption scheme KR a = private key of user A KU a = public key of user A EP = Encryption Public Key DP = Decryption Public Key EP = Symmetric Encryption DP = Symmetric Decryption H hash function, || concatenation Z compression using ZIP R64 = conversion to radix 64 ASCII format 12

PGP Operation – Compression by default PGP compresses message after signing but before encrypting so can store uncompressed message & signature for later verification uses ZIP compression algorithm 13

PGP Operation – Compatibility when using PGP will have binary data to send (encrypted message etc) however was designed only for text hence PGP must encode raw binary data into printable ASCII characters uses radix-64 algorithm PGP also segments messages if too big 14

PGP Operation – Summary 15

Summary of PGP Services 16

Format of PGP Message 17

PGP message format session key component signature message key ID of K rcv session key k timestamp key ID of K snd leading two octets of hash hash filename timestamp data { } Krcv { } Ksnd -1 { } k ZIP R64 18

Key IDs a user may have several public key – private key pairs which private key to use to decrypt the session key? which public key to use to verify a signature? transmitting the whole public key would be wasteful associating a random ID to a public key would result in management burden PGP key ID: least significant 64 bits of the public key unique within a user with very high probability 19

Private-key ring used to store the public key – private key pairs owned by a given user essentially a table, where each row contains the following entries: timestamp key ID (indexed) public key encrypted private key user ID (indexed) enc passphrase hash private key encrypted private key 20

Public-key ring used to store public keys of other users a table, where each row contains the following entries: timestamp key ID (indexed) public key user ID (indexed) owner trust signature(s) signature trust(s) key legitimacy 21

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PGP Session Keys need a session key for each message of varying sizes: 56-bit DES, 128-bit CAST or IDEA, 168-bit Triple- DES generated using ANSI X12.17 mode uses random inputs taken from previous uses and from keystroke timing of user 25

Random number generation true random numbers used to generate public key – private key pairs provide the initial seed for the pseudo-random number generator (PRNG) provide additional input during pseudo-random number generation pseudo-random numbers used to generate session keys and IVs 26

Pseudo-random numbers based on the ANSI X9.17 PRNG standard 3DES + + DT i ViVi V i+1 K 1, K 2 RiRi 27

PGP Key Management rather than relying on certificate authorities in PGP every user is own CA can sign keys for users they know directly forms a “web of trust” trust keys have signed can trust keys others have signed if have a chain of signatures to them key ring includes trust indicators users can also revoke their keys 28

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The Use of Trust Key legitimacy field Signature trust field Owner trust field 30

Trust management owner trust assigned by the user possible values: unknown user usually not trusted to sign usually trusted to sign always trusted to sign ultimately trusted (own key, present in private key ring) 31

signature trust assigned by the PGP system if the corresponding public key is already in the public- key ring, then its owner trust entry is copied into signature trust otherwise, signature trust is set to unknown user 32

Trust management key legitimacy computed by the PGP system if at least one signature trust is ultimate, then the key legitimacy is 1 (complete) otherwise, a weighted sum of the signature trust values is computed always trusted signatures has a weight of 1/X usually trusted signatures has a weight of 1/Y X, Y are user-configurable parameters 33

example: X=2, Y=4 1 ultimately trusted, or 2 always trusted, or 1 always trusted and 2 usually trusted, or 4 usually trusted signatures are needed to obtain full legitimacy 34

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Public-key revocation why to revoke a public key? suspected to be compromised (private key got known by someone) re-keying the owner issues a revocation certificate … has a similar format to normal public-key certificates contains the public key to be revoked signed with the corresponding private key and disseminates it as widely and quickly as possible if a key is compromised: e.g., Bob knows the private key of Alice Bob can issue a revocation certificate to revoke the public key of Alice even better for Alice 36

Recommended Web Sites PGP home page: MIT distribution site for PGP S/MIME Charter S/MIME Central: RSA Inc.’s Web Site 37