Ciphersuite Memo

I'm sorry if you landed in this keywords soup only to find it not helpful.

  • Key Exchange

    • DH (Diffie-Hellman): \(g^{xy} = (g^x)^y = (g^y)^x\)
    • ECDH (Elliptic-Curve DH)
    • ECDHE (ECDH Ephemeral)
    • DHE (DH Ephemeral)
    • RSA (Encryption): Generate a random bitstream and share it with the peer by encrypting using peer's RSA public key.

    A related concept is PFS (Perfect Forward Secrecy). DH offers PFS while RSA cannot.

  • Authentication
    Also known as key-signing. Commonly used together with the PKI(Public Key Infrastructure).

  • Encryption
    Used for data confidentiality.

    A related concept is mode of operation,which turns a block cipher to a stream cipher. CBC is a commonly used one. When it's used with AES, it's expressed as AES-CBC

  • Message authentication
    Used for data integrity. These algorithms are also called MAC(Message authentication code).

  • Authenticated Encryption (AE)
    Combines confidentiality and integrity. Wikipedia: Authenticated Encryption.

    • EtM (Encrypt-then-MAC): A secure way to combine encryption algorithms with MAC algorithms.
    • GCM (Galois/Counter Mode): A mode of operation, when paired with a block cipher, offers AE (actually AEAD) in one step.

    Some commonly used AE methods:

    • AES-CBC with an HMAC e.g.AES128-CBC-HMAC-SHA256.
    • ChaCha20 with Poly1305.
    • AES-GCM

  • Authenticated Encryption with Associated Data (AEAD)
    Similar to AE, but allows extra unencrypted data (associated data) to be authenticated. Roughly speaking:

    ciphertext = Encrypt(plaintext)
auth_tag = Mac(associated_data + ciphertext)

    A common use case for AEAD is when encrypting a network packet, you want the packet header to stay unencrypted (for network routing purposes) but still authenticated.

  • Note about DH and curves of EC-based algorithms
    DH-based algorithms may have a “Group” option, which specifies a prime field or an elliptic curve. If a prime field is used, such as modp2048, it's normal DH. If an elliptic curve group is used, such as ecp256, it's EC-based DH.
    Some other curves may be used: