Forward secrecy
Forward secrecy is the security property that compromising long-term or current keying material does not reveal previously protected communications or session secrets.
Real-World Cryptography
Message-level forward secrecy is the property that compromise of a current message key does not reveal previously protected messages, assuming those messages are no longer available in cleartext.
- How it is used
- Protects previously sent and received messages after a current message key or chain key is compromised.
- How deep it goes
- Explains directional chains, KDF-based key evolution, and Double Ratchet integration, while stopping short of implementation detail.
- Examples
- Uses Signal conversations, Alice-and-Bob message chains, PGP email, and PRNG state compromise as concrete examples.
- What you can do after
- Evaluate whether a messaging design limits historical exposure after current-key compromise.
Hacking Cryptography
Forward secrecy is the property that compromise of long-term or current secret keying material does not reveal previously established session secrets.
- How it is used
- Evaluates whether a pseudorandom generator protects earlier outputs after an attacker observes later output or state.
- How deep it goes
- Explains state mutation, output exposure, cloning, recurrence reversal, and an RC4 key-recovery attack.
- Examples
- Uses a numeric PRNG stream, cloned LCG states, and RC4-based Wi-Fi traffic to demonstrate recovery of earlier secrets.
- What you can do after
- Evaluate/understand whether observed generator output permits reconstruction of past values, state, or seed keys.
Related concepts
- MQV key exchange algorithmproduced by · An authenticated key-agreement protocol that combines static and ephemeral key pairs to derive a shared secret.
- PGPcontrasts with · PGP is a cryptographic standard for protecting messages and files with public-key encryption, digital signatures, and key management.
- RC4contrasts with · RC4 is a stream-cipher algorithm that generates a pseudorandom keystream from a variable-length secret key using a permutation-based internal state.
- symmetric ratchetaddressed by · A symmetric ratchet is a key-evolution mechanism that derives successive chain keys and message-encryption keys from a one-way key update.
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