The S/MIME for email specification allows for encryption before signing or signing and encrypting. Each has different security properties. PGP specifies that one should sign and then encrypt. Use the Cryptographic protocol shape analyzer(CPSA) tool to determine what attacks are possible against each approach or if either are secure. Is PGP correct to specify only one way to perform authentication and confidentiality? Model the protocols, specified below for your convenience, and identify any attacks against the protocols or if the messages are securely received (authenticated and confidential). Determine if you receive an authentic message, was it intended for you or someone else or if when you receive the message, is it authentic. (For extra credit: If you identify any attacks, can you change the protocol to prevent the attacks?) For the following descriptions, M is the message, K is a freshly chosen symmetric key, PubK(N) is the public key of N, and PrivK(N) is the private key of N. {}K stands for encryption with key K. Hash(X) is hashing of message X. Sign then encrypt: A -> B: {M, A, {Hash(M)}PrivK(A)}K, {K}PubK(B) Encrypt then sign: A -> B: {M}K, A, {Hash({M}K)}Privk(A), {K}PubK(B)
The S/MIME for email specification allows for encryption before signing or signing and encrypting. Each has different security properties. PGP specifies that one should sign and then encrypt. Use the Cryptographic protocol shape analyzer(CPSA) tool to determine what attacks are possible against each approach or if either are secure. Is PGP correct to specify only one way to perform authentication and confidentiality? Model the protocols, specified below for your convenience, and identify any attacks against the protocols or if the messages are securely received (authenticated and confidential). Determine if you receive an authentic message, was it intended for you or someone else or if when you receive the message, is it authentic. (For extra credit: If you identify any attacks, can you change the protocol to prevent the attacks?)
For the following descriptions, M is the message, K is a freshly chosen symmetric key, PubK(N) is the public key of N, and PrivK(N) is the private key of N. {}K stands for encryption with key K. Hash(X) is hashing of message X.
Sign then encrypt:
A -> B: {M, A, {Hash(M)}PrivK(A)}K, {K}PubK(B)
Encrypt then sign:
A -> B: {M}K, A, {Hash({M}K)}Privk(A), {K}PubK(B)
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