IBM Computer Hardware 2 User Manual download pdf (Page 459)

CCA Release 2.54

Formatting Hashes and Keys in Public-Key Cryptography

The Digital_Signature_Generate and Digital_Signature_Verify verbs support several

methods for formatting a hash, and in some cases a descriptor for the hashing

method, into a bit-string to be processed by the cryptographic algorithm. This

section discusses the ANSI X9.31 and PKCS #1 methods. The ISO 9796-1

method can be found in the ISO standard.

This section also describes the PKCS #1, version 1, 1.5, and 2.0, methods for

placing a key in a bit string for RSA ciphering as part of a key exchange.

ANSI X9.31 Hash Format

With ANSI X9.31, the string that is processed by the RSA algorithm is formatted by

the concatenation of a header, padding, the hash and a trailer, from the most

significant bit to the least significant bit, such that the resulting string is the same

length as the modulus of the key. For the CCA implementation, the modulus length

must be a multiple of 8 bits.

 The header consists of X'6B'

 The padding consists of X'BB', repeated as many times as required, and

terminated by X'BA'

 The hash value follows the padding

 The trailer consists of a hashing mechanism specifier and final byte. These

specifiers are defined:

–X'31': RIPEMD-160

–X'32': RIPEMD-128

–X'33': SHA-1

 A final byte of X'CC'.

PKCS #1 Formats

Version 2.0 of the PKCS #1 standard

defines methods for formatting keys and

hashes prior to RSA encryption of the resulting data structures. The earlier

versions of the PKCS #1 standard defined “block types” 0, 1, and 2, but in the

current standard that terminology is dropped.

The CCA products described in this book implement these processes using the

terminology of the Version 2.0 standard:

 For formatting keys for secured transport:

– RSAES-OAEP, the preferred method for key-encipherment

when

exchanging DATA keys between systems. In CCA, keyword PKCSOAEP

is used to invoke this formatting technique. The “P” parameter described in

the standard is not used and its length is set to zero.

– RSAES-PKCS1-v1_5, is an older method for formatting keys. In CCA,

keyword PKCS-1.2 is used to invoke this formatting technique.

 For formatting hashes for digital signatures:

PKCS standards can be retrieved from http://www.rsasecurity.com/rsalabs/pkcs.

The PKA92 method and the method incorporated into the SET standard are other examples of the OAEP technique. The “OAEP”

technique is attributed to Bellare and Rogaway and stands for “Optimal Asymmetric Encryption Padding.”

Appendix D. Algorithms and Processes

D-19

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