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cryptography for developers 2006 phần 2

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Quy tắc mã hóa cơ bản là các thiết lập tự do hầu hết các quy tắc mã hóa, cho phép của các bảng mã khác nhau cho cùng một dữ liệu. Thực tế, bất kỳ mã hóa ASN.1 hoặc là có CER hoặc DER có thể được giải mã BER, nhưng không phải là ngược lại. Tất cả các loại dữ liệu ASN.1 cho phép mô tả đầu tiên về quy tắc BER, và sau đó CER | ASN.1 Encoding Chapter 2 23 Basic encoding rules are the most liberal set of encoding rules allowing a variety of encodings for the same data. Effectively any ASN.1 encoding that is either CER or DER can be BER decoded but not the opposite. All of the data types ASN.1 allows are first described in terms of BER rules and then CER or DER rules can be applied to them. The actual complete ASN.1 specification is far more complex than that required by the average cryptographic tasking. We will only be looking at the DER rules as they are what most cryptographic standards require. It should also be noted that we will not be supporting the constructed encodings even though we will discuss them initially. ASN.1 was originally standardized in 1994 and again in 1997 and 2002. The current standard is ITU-T X.680 also filed as ISO IEC 8824-1 2002 and is freely available on the Internet ASN.1 Reference http asn1.elibel.tm.fr standards .This chapter discusses the implementation of ASN.1 encodings more than the theory behind the ASN.1 design.The reader is encouraged to read the ITU-T documents for a deeper treatment. The standards we are aiming to support by learning ASN.1 are the PKCS 1 7 and ANSI X9.62 X9.63 public key cryptography standards. As it turns out to support these standards your ASN.1 routines have to handle quite a bit of the ASN.1 types in the DER rule-set resulting in encoding routines usable for a broad range of other tasks. While formally ASN.1 was defined for more than just cryptographic tasks it has largely been ignored by cryptographic projects. Often custom headers are encoded in proprietary formats that are not universally decodable such as some of the ANSI X9.63 data making their adoption slower as getting compliant and interoperable software is much harder than it needs be. As we explore at the end of this chapter ASN.1 encoding can be quite easy to use in practical software and very advantageous for the both the developer and the end users. ASN.1 Syntax .