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In modern cryptography, keys must be kept secret. Depending on the effort that is made to protect the key, keys are classified into the following levels: 

GLYPH<SM590000> A clear key  is a key that is transferred from the application in clear text to the cryptographic function. The key value is stored in the clear (at least briefly) somewhere in unprotected memory areas. Therefore, the key can be made available to someone under certain circumstances who is accessing this memory area. 

This risk must be considered when clear keys are used. However, many applications exist where this risk can be accepted. For example, the transaction security for the (widely used) encryption methods Secure Sockets Layer (SSL) and Transport Layer Security (TLS) is based on clear keys. 

GLYPH<SM590000> The value of a protected key  is stored only in clear in memory areas that cannot be read by applications or users. The key value does not exist outside of the physical hardware, although the hardware might not be tamper-resistant. The principle of protected keys is unique to IBM Z. For more information, see 6.4.2, “CPACF protected key” on page 229. 

GLYPH<SM590000> For a secure key , the key value does not exist in clear format outside of a special hardware device (HSM), which must be secured and tamper-resistant. A secure key is protected from disclosure and misuse, and can be used for the trusted execution of cryptographic algorithms on highly sensitive data. If used and stored outside of the HSM, a secure key must be encrypted with a master key , which is created within the HSM and never leaves the HSM. 

Because a secure key must be handled in a special hardware device, the use of secure keys usually is far slower than the use of clear keys, as shown in Figure 6-1. 

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