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D. Implementation Notes
The SSL protocol cannot prevent many common security mistakes. This section provides several recommendations to assist implementors.
D.1 Temporary RSA keys
US Export restrictions limit RSA keys used for encryption to 512 bits, but do not place any limit on lengths of RSA keys used for signing operations. Certificates often need to be larger than 512 bits, since 512-bit RSA keys are not secure enough for high-value transactions or for applications requiring long-term security. Some certificates are also designated signing-only, in which case they cannot be used for key exchange.
When the public key in the certificate cannot be used for encryption, the server signs a temporary RSA key, which is then exchanged. In exportable applications, the temporary RSA key should be the maximum allowable length (i.e., 512 bits). Because 512-bit RSA keys are relatively insecure, they should be changed often. For typical electronic commerce applications, it is suggested that keys be changed daily or every 500 transactions, and more often if possible. Note that while it is acceptable to use the same temporary key for multiple transactions, it must be signed each time it is used.
RSA key generation is a time-consuming process. In many cases, a low-priority process can be assigned the task of key generation. Whenever a new key is completed, the existing temporary key can be replaced with the new one.
D.2 Random Number Generation and Seeding
SSL requires a cryptographically-secure pseudorandom number generator (PRNG). Care must be taken in designing and seeding PRNGs. PRNGs based on secure hash operations, most notably MD5 and/or SHA, are acceptable, but cannot provide more security than the size of the random number generator state. (For example, MD5-based PRNGs usually provide 128 bits of state.)
To estimate the amount of seed material being produced, add the number of bits of unpredictable information in each seed byte. For example, keystroke timing values taken from a PC-compatible's 18.2 Hz timer provide 1 or 2 secure bits each, even though the total size of the counter value is 16 bits or more. To seed a 128-bit PRNG, one would thus require approximately 100 such timer values.
Note: The seeding functions in RSAREF and versions of BSAFE prior to 3.0 are order-independent. For example, if 1000 seed bits are supplied, one at a time, in 1000 separate calls to the seed function, the PRNG will end up in a state which depends only on the number of 0 or 1 seed bits in the seed data (i.e., there are 1001 possible final states). Applications using BSAFE or RSAREF must take extra care to ensure proper seeding.
D.3 Certificates and authentication
Implementations are responsible for verifying the integrity of certificates and should generally support certificate revocation messages. Certificates should always be verified to ensure proper signing by a trusted Certificate Authority (CA). The selection and addition of trusted CAs should be done very carefully. Users should be able to view information about the certificate and root CA.
SSL supports a range of key sizes and security levels, including some which provide no or minimal security. A proper implementation will probably not support many cipher suites. For example, 40-bit encryption is easily broken, so implementations requiring strong security should not allow 40-bit keys. Similarly, anonymous Diffie-Hellman is strongly discouraged because it cannot prevent man-in-the-middle attacks. Applications should also enforce minimum and maximum key sizes. For example, certificate chains containing 512-bit RSA keys or signatures are not appropriate for high-security applications.
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