7.3.3. Symmetric Cryptography

7.3.3. Symmetric Cryptography

This document allows both asymmetric and symmetric cryptography to be applied to HTTP messages. By their nature, symmetric cryptographic methods require the same key material to be known by both the signer and verifier. This effectively means that a verifier is capable of generating a valid signature, since they have access to the same key material. An attacker that is able to compromise a verifier would be able to then impersonate a signer.

Where possible, asymmetric methods or secure key agreement mechanisms should be used in order to avoid this type of attack. When symmetric methods are used, distribution of the key material needs to be protected by the overall system. One technique for this is the use of separate cryptographic modules that separate the verification process (and therefore the key material) from other code, minimizing the vulnerable attack surface. Another technique is the use of key derivation functions that allow the signer and verifier to agree on unique keys for each message without having to share the key values directly.

Additionally, if symmetric algorithms are allowed within a system, special care must be taken to avoid key downgrade attacks (Section 7.3.6).