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13. Security Considerations As with any technology involving cryptography, you should check the current literature to determine if any algorithms used here have been found to be vulnerable to attack. This specification uses Public Key Cryptography technologies. Possession of the private key portion of a public-private key pair is assumed to be controlled by the proper party or parties. Certain operations in this specification involve the use of random numbers. An appropriate entropy source should be used to generate these numbers. See RFC 1750. The MD5 hash algorithm has been found to have weaknesses (pseudo- collisions in the compress function) that make some people deprecate its use. They consider the SHA-1 algorithm better. Many security protocol designers think that it is a bad idea to use a single key for both privacy (encryption) and integrity (signatures). In fact, this was one of the motivating forces behind the V4 key format with separate signature and encryption keys. If you as an implementor promote dual-use keys, you should at least be aware of this controversy. The DSA algorithm will work with any 160-bit hash, but it is sensitive to the quality of the hash algorithm, if the hash algorithm is broken, it can leak the secret key. The Digital Signature Standard (DSS) specifies that DSA be used with SHA-1. RIPEMD-160 is considered by many cryptographers to be as strong. An implementation should take care which hash algorithms are used with DSA, as a weak hash can not only allow a signature to be forged, but could leak the secret key. These same considerations about the quality of the hash algorithm apply to Elgamal signatures. If you are building an authentication system, the recipient may specify a preferred signing algorithm. However, the signer would be foolish to use a weak algorithm simply because the recipient requests it. Some of the encryption algorithms mentioned in this document have been analyzed less than others. For example, although CAST5 is presently considered strong, it has been analyzed less than Triple- DES. Other algorithms may have other controversies surrounding them. Some technologies mentioned here may be subject to government control in some countries.
Updated: 1999-09-30 wkoch